Search

The expression of elaborated displays provides reliable information to conspecifics about the quality of the signaler. Competition for breeding resources or mates is predicted to affect the expression of signals in both males and females; however, the literature has been typically focused on male behaviors. The spotless starling is an interesting example where both sexes decorate the nest to signal their condition and social status: males add green plants at the beginning of the breeding period, and females place foreign feathers during the incubation period. In this study, we investigate nest decoration by males and females in relation to the intensity of conspecific competition during the breeding period. We distributed nest boxes at either a high (HD) or a low density (LD) and recorded the amount of green plant material added before laying, the number of feathers at hatching, and the breeding productivity under these different density treatments. The amount of green plant material and the number of feathers at hatching were higher in HD than in LD nests. Furthermore, nest boxes at higher density had lower breeding productivity. Our results suggest that manipulating the density of nest boxes increased competition for breeding resources. The aggregation of males competing for territories and mates may stimulate them to add greater amounts of plants, and competition over those males or other resources for reproduction may induce females to add feathers. This study shows that social environment has the potential to influence the expression of signaling behaviors in both males and females., This work was supported the Spanish Ministry of Economy and Competitiveness (project number CGL2014-59670-P). J.G.R. and D.F. were both supported by PhD grants by the Spanish Government.

Understanding the evolution of gregarious breeding in birds necessitates assessing how fitness components vary in relation to conspecific density. So far, the ultimate causes of breeding density remain contentious, owing to the logistic challenges in both obtaining and interpreting long-term effects of density on fledgling survival, recruitment rates or natal dispersal in wild populations. Here, we used observational data from a 13-year study on the reproduction of a facultatively gregarious bird, the Spotless Starling, Sturnus unicolor, to investigate the consequences of breeding density for fitness and offspring natal dispersal. We compared breeding performance, recruitment success and dispersal distances of offspring arising from nest-boxes placed in either a high (HD) or a low density (LD). We found that although neither clutch size nor fledgling survival varied in relation to density, HD nests produced significantly fewer recruits. Moreover, recruits from HD nests settled closer to their natal territories than those from LD nests. These findings suggest that the proximity to neighbours incurs delayed fitness costs in the Spotless Starling. Furthermore, the negative density-dependent dispersal may result from the increased mortality of individuals in worse condition and with lower ability to compete for breeding vacancies in the vicinity of their natal territories. With this study, we provide long-term observational results that may be a basis for further experiments intended to unravel proximate mechanisms underpinning density-dependent patterns in wild breeding colonies., Financial support was provided by the Spanish Ministry of Economy and Competitiveness (project number CGL2014-59670-P). D.F was supported by PhD grant by the Spanish Government.

Aims Climate and human impacts are changing the nitrogen (N) inputs and losses in terrestrial ecosystems. However, it is largely unknown how these two major drivers of global change will simultaneously influence the N cycle in drylands, the largest terrestrial biome on the planet. We conducted a global observational study to evaluate how aridity and human impacts, together with biotic and abiotic factors, affect key soil variables of the N cycle. Location Two hundred and twenty-four dryland sites from all continents except Antarctica widely differing in their environmental conditions and human influence. Methods Using a standardized field survey, we measured aridity, human impacts (i.e. proxies of land uses and air pollution), key biophysical variables (i.e. soil pH and texture and total plant cover) and six important variables related to N cycling in soils: total N, organic N, ammonium, nitrate, dissolved organic:inorganic N and N mineralization rates. We used structural equation modelling to assess the direct and indirect effects of aridity, human impacts and key biophysical variables on the N cycle. Results Human impacts increased the concentration of total N, while aridity reduced it. The effects of aridity and human impacts on the N cycle were spatially disconnected, which may favour scarcity of N in the most arid areas and promote its accumulation in the least arid areas. Main conclusions We found that increasing aridity and anthropogenic pressure are spatially disconnected in drylands. This implies that while places with low aridity and high human impact accumulate N, most arid sites with the lowest human impacts lose N. Our analyses also provide evidence that both increasing aridity and human impacts may enhance the relative dominance of inorganic N in dryland soils, having a negative impact on key functions and services provided by these ecosystems.

Convergence in the use of resources may occur between distantly related organisms. A major ecological resource in which members of various taxa may be interested is a cavity for nesting. A variety of social hymenopterans and vertebrates may nest within tree cavities in tropical ecosystems. We used 241 nest-boxes placed in seven Kenyan localities to investigate the use of nesting cavities by members of distant taxa and discuss whether interaction between them is a potential factor shaping cavity-nester communities in tropical regions. The nest-boxes were occupied by social insects (ants, bees and wasps) (30.1% of nest-boxes in April–May and 33.1% in September–October) and vertebrates (birds and mammals) (20% and 7.7%, respectively). Hymenopterans were more abundant in forest boxes (36.2% of nest-boxes occupied in April–May and 37% in September–October), whereas savannas had lower figures (21.7% and 31.3%, respectively). Among vertebrates, most occupants of nest-boxes in savanna were birds (17.8% of nest-boxes occupied vs. 8% in mammals), while mammals predominated in forests (4.9% of the nest-boxes occupied vs. 0.3% in birds). Spatial and temporal patterns of occupation highlight the potential that interaction between distant taxa may have on the access to nesting cavities. More nest-boxes remained unoccupied in forested areas than in savanna areas suggesting that a shorter supply of nesting sites in the savanna may be a source of competition. The simultaneous occupation of a nest-box by two different taxa was exceptional, also supporting the hypothesized inter-taxon competition.

Patterns of natal dispersal are generally sex‐biased in vertebrates, i.e. female‐biased in birds and male‐biased in mammals. Interphyletic comparisons in mammals suggest that male‐biased dispersal occurs in polygynous and promiscuous species where local mate competition among males exceeds local resource competition among females. However, few studies have analysed sex‐biased patterns of dispersal at the individual level, and facultatively polygynous species might offer this opportunity. In the spotless starling, polygynous males exhibit their mating status during courtship carrying higher amounts of green plants to nests than monogamous males. We experimentally incorporated green plants to nests during four years to analyse long‐term consequences on breeding success and offspring recruitment rates. We unexpectedly found that experimental sons recruited farther than experimental daughters, while control daughters recruited farther than control sons. A similar pattern was found using observational information from eight years. We discuss this result in the context of local competition hypothesis and speculate that sons dispersed farther from nests controlled by polygynous males to avoid competition with relatives. The amount of green plants in nests affects female perception of male attractiveness and degree of polygyny, although little is known about proximate mechanisms linking this process with the offspring dispersal behaviour. Our results support the idea that male‐biased dispersal is related to polygyny in a facultatively polygynous bird., This research has been carried out with the financial support from the Spanish Government, projects: CGL2004‐00126/BOS, CGL2005‐05611‐C02‐01, CGL2008‐02843 and CGL2011‐28095. JGR was supported by a PhD grant from the Spanish Government and the field work was carried out under license of the ‘Consejería de Medio Ambiente y Desarrollo de la Comunidad de Madrid’ and the ‘Ayuntamiento de Manzanares el Real’.

Nest intruders are common in many avian species. In the spotless starling (Sturnus unicolor), a passerine closely related to the European starling (Sturnus vulgaris), it has been recently shown that visits to alien nests were in most cases related to get familiarity and personal information about prospected nest boxes and that collecting public information seems also to be involved in the intruding behaviour. In the present study we investigate whether nest intrusions are related to breeding status in both males and females. Individuals that had bred previously in the colony were detected as frequently as those without previous breeding experience intruding nests and the proportion of intruders that were owners and those that had not a nesting site during the current year was balanced. Males that were actively breeding and those that were not breeding intruded with similar frequencies in alien nests, while most females were not actively breeding when they were observed intruding a nest box. The nests more frequented by male intruders were those showing the highest female nestling feeding rates, but the frequency of female intrusions was not similarly affected by female owner feeding rates. The results suggest that some of the intrusions by males are related to nest acquisition although in general males seem to intrude to obtain other breeding resources, presumably extra-pair matings. Female intruders with a nesting territory but not currently breeding could be sampling alternative nesting sites while female floaters that intruded nests could be searching for nesting sites or attempting to lay parasitic eggs. Birds behaving as intruders at some moment of their stay in the breeding colony raised more fledglings that birds never observed performing this behaviour.

Although sexual selection is typically considered the predominant force driving the evolution of ritualized sexual behaviours, natural selection may also play an important and often underappreciated role. The use of green aromatic plants among nesting birds has been interpreted as a component of extended phenotype that evolved either via natural selection due to potential sanitary functions or via sexual selection as a signal of male attractiveness. Here, we compared both hypotheses using comparative methods in starlings, a group where this behaviour is widespread. We found that the use of green plants was positively related to male-biased size dimorphism and that it was most likely to occur among cavity-nesting species. These results suggest that this behaviour is likely favoured by sexual selection, but also related to its sanitary use in response to higher parasite loads in cavities. We speculate that the use of green plants in starlings may be facilitated by cavity nesting and was subsequently co-opted as a sexual signal by males. Our results represent an example of how an extended phenotypic component of males becomes sexually selected by females. Thus, both natural selection and sexual selection are necessary to fully understand the evolution of ritualized behaviours involved in courtship., This research has been carried out with the financial support from the Spanish Government projects: CGL2004‐00126/BOS, CGL2005‐06611‐C02‐01, CGL2008‐02843, CGL2011‐28095 and CGL2014‐58670‐P. JGR was supported by a PhD grant from the Spanish Government. D.R.R was supported by the US National Science Foundation (IOS‐1121435, IOS‐1257530, IOS‐1439985). This work was partially supported by a Junior Fellow award from the Simons Foundation to R.M.

The prospective behaviour for nests by conspecific intruders may be a strategy to gather public information for future reproduction or to secure resources for immediate reproduction. Although the relationship between the sex, age and breeding experience of intruders and the sex and age of residents may be crucial for understanding the significance of nest-prospecting behaviour, a precise determination of these traits has rarely been addressed in field studies. In a free-ranging population of spotless starlings ( Sturnus unicolor), inexperienced birds were the predominant intruders. The ratio of male-to-female intruders was higher among birds without previous breeding experience, although our results did not allow us to determine whether more inexperienced males or females intruded nests. The average annual rate of intrusion of either sex was strongly correlated with the number of fledglings produced in the study colony the previous year, which seems to indicate that the inexperienced intruders were, in general, yearlings. The proportion of the sexes among the intruders in each study year was correlated with the average age of male owners but not with the age of female owners. This result suggests that the presence of intruding males in the nests was affected by an increased opportunity to find vacancies in nesting territories as resident males became older. This finding supports the hypothesis that, in most cases, birds visited nests to obtain personal information about nesting resources. However, the intrusion rates were higher when there were nestlings than during the incubation period, which suggests that collecting public information was also involved in the intruding behaviour. Both sexes regularly attacked intruders, which implies that the intruders inflicted some kind of cost on the owners. Female floaters were more frequently chased, probably because they regularly lay parasitically in conspecific nests.

Global Ecosystem Analysis The relationship between species richness and the functional properties of their ecosystems has often been studied at small scales in experimental plots. Maestre et al. (p. 214 ; see the Perspective by Midgley ) performed field measurements at 224 dryland sites from six continents and assessed 14 ecosystem functions related to carbon, nitrogen, and phosphorus cycling. Positive relationships were observed between perennial plant species richness and ecosystem functionality. The relative importance of biodiversity was found to be as large as, or larger than, many key abiotic variables. Thus, preservation of plant biodiversity is important to buffer negative effects of climate change and desertification in drylands, which collectively cover 41% of Earth's land surface and support over 38% of the human population.

The males of the European Starling (Sturnus vulgaris) and Spotless Starling (S. unicolor) carry green plants to their nests. However, the consequences of this behaviour for the females are still poorly known. In this study, we evaluated the behavioural responses of female Spotless Starlings to the green plants that they find in their nests. We added green plants to a group of experimental nests (E nests) and quantified the activity of the nesting females in the E nests and in a group of non-manipulated nests (C nests). The females never added green plants to their nests, but they removed the green plants more frequently from the E nests than from the C nests. The number of departures from the nest by females that carried green plants was positively correlated with the number of visits by females that carried nothing. In contrast, the number of departures by females carrying green plants was uncorrelated with the number of visits made by females that carried nest-building materials. These results suggest that the females made a specific additional effort to remove the green plants from the E nests. We propose that this behaviour may be an adaptive response of mated females that serves to counteract the eventual negative effects of green plants on their reproductive success. However, competing hypotheses cannot be rejected, and additional work is needed to better understand the removal of green plants by female Spotless Starlings.

The carrying of feathers to adorn the nest has been recently described as a female behaviour that indicates quality in a passerine bird — the spotless starling (Sturnus unicolor) —, but the consequences that the variability of this trait may have on breeding success are still unknown. The feather-carrying behaviour is a sexual behaviour that is performed in response to a male display: the carrying of green plants. In this paper we explore whether foreign feathers affect male investment on chick rearing or reinforce the pair mating bonds. The experimental addition of feathers to nests caused an increase in clutch size and a reduction of nestling mortality, although it did not affect feeding rates or the removal of faecal sacs by males or females. Nest feathers did not increase the frequency with which females laid a second clutch in the same nest and with the same male. Thus, our results do not support the sexual selection hypothesis for the evolution of the carrying feathers behaviour. An alternative hypothesis that deserves to be addressed in specifically designed studies is that foreign feathers may be taken to nests as a sort of chemical warfare against nest parasites. The sexual selection and the chemical defence hypotheses are, however, not mutually exclusive and future research should evaluate whether the pre-existence of a behavioural trait arising by natural selection may drive the evolution of a signalling trait.

Multiple male traits and displays may act in signalling sexually selected processes during courtship. Spotless starling males (Sturnus unicolor) carry green plants into their nests before egg laying, and recent studies have shown that this behaviour is related to female breeding decisions and the production of male-biased broods. Although the functional implications of this effect on females are not yet clear, data suggest that it could be mediated by female circulating hormones. Additionally, females may show higher androgen levels as a consequence of the increased female‐female competition generated by the increase in male attractiveness. We tested this hypothesis using the same manipulation of green nesting material that has been previously shown to result in an increase of male attractiveness in male spotless starlings. We found that females in experimental nests increased their circulating testosterone levels during the laying period. In addition, there was an increase of social interferences in the experimental nests because of the addition of green plants. We hypothesise that testosterone may allow females to maintain their mating status when competing with other females for the preferred males. Addition of green plants also increased the variance in the levels of circulating testosterone, suggesting plasticity between females in their response to the manipulation. We propose that there is a functional link between high testosterone levels, male-biased sex ratios and female resource-holding potential in intra-sexual competition in this species.

1. The use of behavioural traits by females in signalling condition has been practically ignored in evolutionary theory. However, females may also exhibit ornaments and behavioural displays, although less elaborated than those of males. 2. In this study we suggest that the carrying of feathers by spotless starlings Sturnus unicolor Temminck females to decorate the nest represents an elaborated and costly behaviour that is displayed in response to a courtship male behaviour: the carrying of nest green plants. 3. By experimentally increasing the amount of green plants in the nests, to give the appearance that highly attractive males defended them, we induced females to increase their feather carrying rates. 4. The amount of feathers carried to the nest was correlated to female reproductive experience and laying date, two variables correlated with female body condition. These results suggests that this behaviour may work as an honest indicator of female quality. 5. We conclude that male carrying plants and female carrying feathers can be viewed as two sex-specific functionally related signalling behaviours involved in mutual courtship or status signalling.

Although the presence of feathers in the nest is widespread among birds, it has not been previously suggested that feathers can be used as sexual signals. Females of the spotless starling ( Sturnus unicolor ) regularly carry feathers to their nest, mostly during laying and incubation. We show that the arrangement of these feathers was non-random with respect to the side (obverse or reverse) placed upwards (which can be viewed from the nest entrance). Feathers of the wood pigeon ( Columba palumbus ) and the spotless starling, which exhibit higher ultraviolet and visible reflectance on their reverse side, were predominantly placed with this side upwards. On the contrary, feathers of the jay ( Garrulus glandarius ) were predominantly found exhibiting the obverse side, which possesses higher reflectance in this species. Feathers of the azure-winged magpie ( Cyanopica cyana ), with similar reflectance values on either side, were placed indiscriminately in obverse and reverse positions. The results suggest that feathers are arranged to maximize their conspicuousness within the nest and hence that they might be potentially used as intraspecific signals.

Several studies have shown that inbreeding causes low hatching success. However, it is not always clear under which parental genetic characteristics hatching failure occurs. We examined the effects of parental homozygosity and parental genetic similarity on egg hatchability in the spotless starling. We evaluated whether low hatchability was a consequence of genetic similarity of the parents producing a zygote or because the parents themselves had reduced fertility because of their own genetic characteristics. We found a significant detrimental effect of both highest and lowest female homozygosity on hatchability. Parental genetic similarity was negatively related to egg hatchability. In contrast, we found no significant effect of male homozygosity on hatching success. Our results suggest that effects of homozygosity on hatching success may be relevant even in populations of abundant and colonial wild bird species without geographical barriers that could limit immigration or dispersal. We also highlight the importance of the analyses of male and female homozygosity and parental genetic similarity when investigating related fitness consequences of inbreeding., This study was funded by the MCYT project BOS 2001-0703 given to J.P.V. P.J.C. had a contract of the Ministerio de Ciencia y Tecnologı́a through the project REN 2001-1514.

Although the killing of unrelated young (usually designed as infanticide) has been typically considered a male behavior, recent research has shown that females may commit infanticide even more frequently than do males. In rodents and primates, female infanticide represents a strategy associated to competition for resources or infant exploitation, but little is known about the causes and reproductive consequences of the killing of conspecifics by females in other vertebrates. In the present article, I focus on infanticide committed by females that replace mates of territorial males in a population of the house sparrow. I show that (1) replacement females regularly committed infanticide, (2) experienced females committed infanticide more frequently than did novel females and tended to select polygynous males to take over their nests, and (3) laying date and reproductive success after a territory takeover did not differ between infanticidal and noninfanticidal females. These results seem to indicate that infanticide has not evolved in females because of the short-term reproductive benefits it accrues to the perpetrator. I suggest that the killing of unrelated young by females relates to dominance status among potential female breeders and that this behavior benefits the perpetrators in terms of mate selection. Key words: female-female competition, female infanticide, female replacement, house sparrows, sexual selection. [Behav Ecol 15:219–222 (2004)]

A potential cost of polygyny that may have restrained its evolution in some avian species is the presence of extra-pair offspring in the nests of males mated with several females. However, this relationship is not frequently found and an experimental approach investigating the association between extra-pair offspring and polygyny, while controlling for male traits related to polygyny, has not been undertaken. In this study, we manipulated the testosterone levels of facultatively polygynous spotless starlings, Sturnus unicolor, to establish experimentally different groups of polygyny and analyse its influence on levels of extra-pair offspring in males for which other traits were randomised. During two consecutive breeding seasons, we examined the effects of harem size on the ability of males to assure genetic paternity, assessed by DNA fingerprinting, and on reproductive success in their own nests. The frequency of extra-pair fertilisations varied between 10% (14/140) in 1996 and 20% (39/194) in 1997. Year-to-year analyses of extra-pair paternity variables with treatment as factor and harem size (number of simultaneous mated females) as covariate, were significant only in relation to harem size. Males with few simultaneously mated females were less cuckolded than more polygynous males in 1997 but not in 1996, indicating that mating costs of polygyny occurred, at least in the year with the higher rate of extra-pair paternity. Because polygynous males have more social mates, they may accrue higher reproductive success irrespective of their risk of being cuckolded. However, our results suggest that to be polygynous is costly in terms of paternity loss when we experimentally induce mating status controlling for individual male traits. However, the low values of our power tests for the statistically non-significant results of hormone on cuckoldry do not allow us to discard this hormone effect altogether.

© The Author 2015. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. Although several hypotheses have been proposed to explain the function of fresh green plants in nests of many avian species, the consequences of this behavior on fitness remain poorly understood. In accordance with the nest protection and drug hypotheses, adding greenery to nests should be naturally selected to facilitate positive fitness effects on offspring. Alternatively, the courtship hypothesis postulates that green plants function as a sexually selected signal of male attractiveness that might promote female competition for preferred males. We conducted a long-term study by experimentally increasing the amount of green plants in nests of a wild population of spotless starlings (Sturnus unicolor). We experimentally decoupled the natural carrying behavior of males in half of the population and explored the consequences of this manipulation on offspring condition and their rate of local recruitment. Treatment did not affect the number of fledglings, but it did affect body mass and tarsus in a sexually antagonistic way: a positive effect on female tarsus length and negative on male body mass. The addition of greenery reduced the probability of local recruitment of both males and females. Our results suggest that the addition of green plants induces maternal response with complex short- and long-term consequences on the offspring body condition and recruitment success., This research has been carried out with the financial support from the Spanish Government, projects: CGL2004-00126/BOS, CGL2005-05611-C02-01, CGL2008-02843, and CGL2011-28095.

Female mating status may affect reproductive success in avian polygynous mating systems through a combination of differences in female parental quality and status-dependent male assistance in parental care. Traditionally the literature has emphasized male assistance, neglecting evidence for consistency in female parental quality independent of mating status or repeatability in status. We studied the effects of male assistance on breeding success and its association with female mating status in a population of the polygynous spotless starling, Sturnus unicolor, during 3 years. Nestling provisioning by males improved the fledging success of late (mostly second) but not early (mostly first) broods. Reproductive success of females was affected mainly by female maternal quality: (1) primary and secondary females did not differ in output for early broods and in seasonal output despite a greater male assistance at primary nests; (2) monogamous females were more successful despite receiving no more help than other females; and (3) primary and secondary females had different clutch sizes before male assistance in parental care could operate. Female mating status was not significantly repeatable within seasons or between years. Females who changed status between years were as successful when rearing broods as secondary females as they were as primary or monogamous females. Breeding success (proportion of eggs resulting in fledglings) and clutch size were significantly repeatable between years for late but not for early clutches. Performance showed no strong association with female age or condition. Female breeding success seems to be weakly affected by male assistance in this sedentary, colonial species, and more dependent on inherent differences between females. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Abstract The males' trade-off between caring for the offspring or investing in attracting additional mates is well established in the theory of mating systems. The reproductive consequences for males of alternative strategies adopted by them in response to these conflicting demands should depend on several ecological and social factors that may strongly vary among years. This variation, however, has been rarely addressed in field studies despite it being essential to understand the evolution of parental care in facultatively polygynous species. In the present paper, we examine the reproductive consequences of paternal vs. territorial strategies in the polygynous spotless starling (Sturnus unicolor) during four consecutive breeding seasons (1996-1999) in which the number of male defending nest boxes and several population variables such as nestling starvation rates, nest predation rates and fledgling production varied markedly. In 1996 we manipulated the propensity of males to feed their offspring by means of androgens and antiadrogens, and examined the consequences of male behaviour on reproductive success during the subsequent four breeding seasons. Males implanted in1996 with the antiandrogen cyproterone acetate (Cy-males) fed more frequently than males implanted with testosterone (T-males) or control males (C-males) both during the season in which they were implanted and in the next season. The number of nest boxes defended influenced total breeding success mainly in the years when male feeding frequency was less important for breeding success per nest. A lower proportion of Cy-males than T- or C-males still held at least a nest box in 1999, three years after they were implanted. As a result, the number of fledglings produced over the four years studied was smaller for Cy-males than for T- or C-males. These results suggest that the oscillating selective forces acting on breeding male starlings seem to operate more frequently against the development of parental care strategies, while the polygynous strategy seems to be favoured under a wide array of ecological conditions. Although increased density of potential breeders may constrain territorial expansion and polygyny, the high cost of losing a minimum breeding site may select for the maintenance of aggressive behaviour and low parental investment.

Aim: Geographical, climatic and soil factors are major drivers of plant beta diversity, but their importance for dryland plant communities is poorly known. The aim of this study was to: (1) characterize patterns of beta diversity in global drylands; (2) detect common environmental drivers of beta diversity; and (3) test for thresholds in environmental conditions driving potential shifts in plant species composition. Location: Global. Methods: Beta diversity was quantified in 224 dryland plant communities from 22 geographical regions on all continents except Antarctica using four complementary measures: the percentage of singletons (species occurring at only one site); Whittaker's beta diversity, ?(W); a directional beta diversity metric based on the correlation in species occurrences among spatially contiguous sites, ?(R2); and a multivariate abundance-based metric, ?(MV). We used linear modelling to quantify the relationships between these metrics of beta diversity and geographical, climatic and soil variables. Results: Soil fertility and variability in temperature and rainfall, and to a lesser extent latitude, were the most important environmental predictors of beta diversity. Metrics related to species identity [percentage of singletons and ?(W)] were most sensitive to soil fertility, whereas those metrics related to environmental gradients and abundance [(?(R2) and ?(MV)] were more associated with climate variability. Interactions among soil variables, climatic factors and plant cover were not important determinants of beta diversity. Sites receiving less than 178 mm of annual rainfall differed sharply in species composition from more mesic sites (> 200 mm). Main conclusions: Soil fertility and variability in temperature and rainfall are the most important environmental predictors of variation in plant beta diversity in global drylands. Our results suggest that those sites annually receiving c. 178 mm of rainfall will be especially sensitive to future climate changes. These findings may help to define appropriate conservation strategies for mitigating effects of climate change on dryland vegetation

In male birds that base their acquisition of mates on the defence of resources essential for female reproduc - tion, the size or quality of the territory decisively affects polygyny rate and breeding success. However, there is contra - dictory evidence concerning the importance of resource-holding potential (RHP) versus factors not related to individual quality on between-year consistency in territory occupancy and hence on long-term reproductive success. We manipu- lated the aggressiveness (a major determinant of RHP) of male spotless starlings (Sturnus unicolor) by means of andro- gens and antiandrogens to investigate the importance of RHP in breeding-territory acquisition and maintenance, polygyny rate, and breeding success over 3 years (1996-1998). The number of males that held a territory for the first time was high in 1996 with respect to the year before the experiment began. Most of these males still bred in the col- ony during the third study year, though new owners established territories to the detriment of the former males, espe - cially in 1998. Manipulation of testosterone activity during the first year affected the number of nest boxes defended by males and also the number of mates gained and the number of successful breeding attempts made throughout the 3 study years. The results were in the expected direction: an increase in testosterone activity led to greater success. The results suggest that RHP may be important in the acquisition of territory and mates, at least under conditions of re- cently established interindividual relationships. However, social inertia, i.e., the maintenance of dominance status irre- spective of changes in RHP, seems to be of crucial importance in the long-term maintenance of territory resources and polygyny rates.

We investigated possible pre-hatching mechanisms of sex-differential investment by females that may contribute to offspring sex-ratio adjustment enhancing the fitness return from reproductive effort in the spotless starling (Sturnus unicolor). We found a seasonal shift in sex ratio from daughters to sons as the season advances. Furthermore, the probability of breeding at 1-year old and recruitment into the breeding population in daughters is associated with laying date but not with mass at fledging. The reverse is true for males which rarely bred at 1-year old. We also found that eggs containing female embryos are significantly heavier than those containing males in spite of the slight sexual dimorphism in favour of males. This suggests maternal control of provisioning, favouring daughters that may balance sibling mortality and competition with their brothers. Our results on seasonal variation in sex ratio and differential egg provisioning are consistent with an adaptive tactic in which mothers increase their reproductive return by enhancing the probability that daughters survive and breed in their first year of life.

In this study two experiments were run in parallel. To evaluate the possible influence of moult and age on differential white blood cell (WBC) counts, we captured juvenile and adult house sparrows (Passer domesticus) and housed them in outdoor aviaries. Blood was collected twice, before and after moult. Numbers of basophils, lymphocytes, and monocytes were higher in juveniles than in adults, whereas numbers of eosinophilic cells were similar in the two age groups. Moult induced an increase in basophils and monocytes in both juveniles and adults. This indicates that moult and age impose different immunological challenges on house sparrows. To evaluate the effect of testosterone on differential WBC counts, some house sparrows in aviaries received testosterone during the moult period. Testosterone administration reduced, though not significantly, the number of all WBC types in juveniles, and therefore appeared to have an nonspecific effect. However, the number of lymphocytes increased only in adults, which suggests a specific effect on this cell type in this age group.

Elevation of circulating testosterone levels during the breeding season stimulates the development of breeding coloration and favors traits that maximize reproductive success, but it may have several costs. Retardation of growth by testosterone has been proposed as one of these costs to males of long-lived species. We examine the results of a field experiment on the short-term (within the breeding season) and long-term (1 year) effects of heightening levels of testosterone on the growth of males of the lacertid lizard Psammodromus algirus. Testosterone did not affect the mass gain of males during the reproductive period or between years. The between-years increase in snout-vent length in testosterone-supplemented males tended to be less than in control individuals. Within-season individual mass gain was mainly determined by body mass at emergence: the lower the body mass the higher the mass gain. Our results do not support the idea that the production of ornamental traits limits growth, as the negative correlation between degree of head coloration and mass gain disappeared when body mass was controlled for.

For males of socially polygynous avian spe- cies like the spotless starling, there may exist a trade-off between investing in paternal care and controlling sever- al nests. To determine how the intensity of paternal care affects reproductive success per brood sired or expressed as the total number of young raised in all nests con- trolled by the same male, it is necessary to manipulate paternal care. Testosterone (T) has been shown to de- press the tendency for males to care for their young, and induces them to acquire more mates. The effects of pa- ternal care on reproductive success were studied by treat- ing certain male starlings with exogenous T and others with the antiandrogen cyproterone acetate (CA), and comparing the parental behavior of T- and CA-males throughout the breeding season with that of controls. CA-males fed their chicks more during the first week af- ter hatching than T-males, with controls feeding at inter- mediate rates, both on a per nest basis and as total effort for all nests controlled by the same male. Paternal feed- ing rates during the first week of chick life had a signifi- cant positive effect on the number of fledged young. The hormone treatment significantly affected the number of chicks raised per nest, CA-males having a higher breed- ing success per nest than T-males, and controls showing intermediate levels of success. There was no significant effect of treatment on total reproductive success attained by males throughout the season. In the polygonous spot- less starling, the intensity of paternal care of young af- fects reproductive success per nest positively but not on a seasonal basis.

The theory that fluctuating asymmetry is sensitive to both environmental and genetic stress is gaining acceptance among evolutionary biologists. Most empirical work has focused on ornamental traits on the assumption that they are more susceptible to stress. In tegumentary coloration is a common ornament in nature, and frequently has a hormonal basis. Earlier studies in the lizard Psammodromus algirus indicate that testosterone induces the development of head nuptial coloration in large males and, at the same time, produces an increase of the ectoparasite load and higher mortality. Hence, the manipulation of testosterone levels may be a way to increase ornament expression and simultaneously create conditions that may make symmetrical development difficult. This positive covariation between character size and symmetry is opposite to that expected in theory for sexually selected traits, so the predicted elevation of asymmetry due to the treatment cannot be confounded by any intrinsic association between symmetry and character size. We firstly consider the effect that testosterone supplementation has on two variables that reflect the symmetry of bilateral throat nuptial coloration in large P. algirus males. Also, we examine whether ornament symmetry is positively associated with reproductive success, a prediction of theory of sexually selected symmetry. Testosterone treatment did not increase the fluctuating asymmetry of throat coloration. Size asymmetry increased with character size in individuals with a fragmented colour pattern, but changed suddenly to a highly symmetric pattern in individuals with non-fragmented coloration. Mirror asymmetry decreased steadily with character size. These results suggest that the development of coloration on both sides of the throat midline follows a random pattern. Asymmetry did not correlate with variables that estimated reproductive success, suggesting that asymmetry is not affected by physiological stress and that this trait is not a sexually selected signal in P. algirus.

According to the immunocompetence hypothesis, testosterone stimulates the expression of male sexually selected traits while decreasing immunocompetence. This proposed trade-off was studied by experimental supplementation of testosterone to small, subordinate, dull-colored male lizards, Piammodromus algirus. Experimental males showed a tendency to overlap their home range with fewer small males than did control males and tended to be more aggressive. However, control males were observed more frequently attending females than experimental males. The area of patches of breeding coloration, the number of ticks, and the frequency of recoveries of testosterone-supplemented and control males did not differ significantly. The results suggest that small adult males with high levels of testosterone behave more aggressively, which may be advantageous to securing a breeding territory in the next season. However, the hormone did not apparently affect ornamentation or parasite load. We argue that, whatever the mechanisms involved, blocking effects of testosterone may be adaptive because being cryptic facilitates a sneaking strategy, and low ectoparasite load may improve survival.

In many bird species the cryptic winter plumage is due to the presence of light feather tips that conceal conspicuous colorations. The gradual abrasion of these tips that makes sexual traits visible has been interpreted as a strategy to improve mating success (here referred as the permanent exhibition hypothesis). However, under some circumstances, the maintenance of a full plumage that facultatively enables the bird to exhibit or cover aggression-inducing traits has proved to be advantageous (the coverable badge hypothesis). In a population of house sparrows where black throat patches (here called badges) are used in intrasexual competition, the degree of abrasion of dull feather tips that conceal bright colour early in the breeding season correlated neither with badge size nor with traits indicating morphology and body condition. These results are only in accordance with predictions of the coverable badge hypothesis. In 1992 experimental clipping of badge feather tips, which forced permanent exposure of badges, negatively affected birds with the largest badges in terms of nest acquisition, but an opposite trend was indicated for birds with the smallest badges. In 1993, when more novel birds in the study area were competing for nesting sites than in 1992, only badge size, but not the experimental manipulation, affected nesting success. These results suggest that the loss of the ability to conceal badges was disadvantageous, and more so if most competing individuals had already been resident in the colony in previous years and the larger their badges were. There is a striking contrast between the results reported for this study population, where badges mainly signal fighting ability, and those reported for another house sparrow population, where badges are mainly used in mate choice. In this last population, tip abrasion is advantageous for dominant individuals showing the largest badges. This suggests that the trade-off between conserving versus wearing off the feathers that conceal ornaments could have different optimal resolutions depending on the relative importance of inter and intrasexual selection on indicator traits.

Many birds have distinctive patches of feathers that in some cases have been shown to function as reliable signals of phenotypic quality. It has generally been assumed that these called `badges' are inexpensive to produce, so other kind of costs have been invoked to explain how they are maintained as honest signals. In the present study we addressed the hypothesis that the black throat badge of male house sparrows functions as an honest signal of condition because it is nutritionally costly to produce, a possibility not yet evaluated for badges showing status. While under field conditions adult birds exhibited larger badges than juveniles, individuals of both age classes developed badges of similar size when moulting in an aviary with ad libitum food. Fat reserves, blood proteins and body mass records of free ranging birds at the start of moult suggest that juveniles were in poorer condition than adults. The reduction of blood proteins during moult negatively correlated with badge size after moult in juveniles but not in adults, and only the variation of blood proteins, but not fat reserves, correlated with the size of the badge when using multiple regression analysis. The results suggest that: (i) a limited access to food sources during moult may account for the smaller badge size of juveniles as previously reported in the same study population; and (ii) the use of more blood proteins, that seems to depend on a worse nutritional condition, constrains the production of a larger badge. The results give support for the hypothesis that badges are costly to produce, though the magnitude of the costs may be age dependent.

Alfredo Salvador, Jose P. Veiga," Jose Martin,*'' Pilar Lopez,* Maria Abelenda, and Marisa Puerta Departamento de Ecologia Evolutiva, Museo Nacional de Ciencias Naturales, C.S.I.C., J. Gutierrez Abascal 2, 28006 Madrid, Spain, Department of Zoology, Uppsala University, Villavagen 9, S-75236 Uppsala, Sweden, and Departamento de Biologia Animal II (Fisiologia Animal), Facultad de Ciencias Biologicas, Universidad Complutense, 28040 Madrid, Spain

This study deals with a general description of haematology and plasma chemistry of free-living males of the lacertid lizards Psammodromus algirus subjected to an experiment of testosterone supplementation. In control lizards the number of red blood cells — 1700000 cells/µl — was smaller than those published for birds and mammals. Haematocrit and haemoglobin content were 33% and 8.4 g/dl. However, they were greater than those reported for other lizards. Testosterone treatment reduced the number of red blood cells and haemoglobin content but did not affect red cell indexes. White blood cell number was 27 000 cells/µl, a value higher than those previously published for other lizards. Lymphocytes, heterophils and azurophils counts were 22300, 3700 and 2000 cells/µl. Monocytes, eosinophils and basophils showed proportions lower than 500 cell/µl. Testosterone treatment reduced the total number of leucocytes (17000 cells/µl) and the number of lymphocytes (10000 cell/µl). Glucose plasma level was in the range published for birds — 250 mg/dl — in both control and treated lizards. Plasma proteins (3.6 g/dl), uric acid and urea (4.5 and 5.4 mg/dl, respectively) were similar to previously published values in lizards. Testosterone treatment increased only protein levels (8.2 g/dl).

The aim of this study was to measure the rates of sperum utilization and production in male house sparrows, passer domesticus: this is the first attempt to do this for any wild bird. The number of spermatozoa in the seminal glomera of males on days -1 to +3 (i.e. during their partner9s fertile period) declined significantly throughout the course of the day. Males had about 12.13 $\times $ 10$^{6}$ spermatozoa in their seminal glomera at the start of the day and during 16 copulations each day used 10.31 $\times $ 10$^{6}$ spermatozoa, which is probably all that was available for ejaculation. Mean ejaculate size was estimated to be 0.64 $\times $ 10$^{6}$ spermatozoa. Seminal glomera occur only in passerines and their function is unclear, but our results suggest a new hypothesis: if spermatozoa production and spermatozoa transport to the seminal glomera are limited to a short period during the night, the seminal glomera serve as a daily spermatozoa store.

Abstract. Replacement male house sparrows, Passer domesticus , invariably committed infanticide when they took over a nest after a female's fertile period had recently ended ( N = 8). Males that took over a nest during egg laying destroyed eggs in 58% of the cases ( N = 12), whereas those arriving before clutch initiation never committed infanticide ( N = 32). Seven out of 12 nests in which the replacement male arrived during laying suffered egg losses (58%). In six of them the female owner stopped laying, while the remaining female left her nest. Two more females whose original males were replaced during the laying period (17%) and two out of seven females whose males were replaced during the 4 days prior to clutch initiation (29%) stopped laying even though they had not suffered egg losses. In contrast, females that initiated laying 5 days or more after the arrival of the replacement male did not stop laying. Nine out of 11 females initiated a new clutch with the replacement male on average 7·3 days after they stopped laying. The results suggest that male prospective infanticide functions to ensure genetic paternity, and that females use strategic ovulation retardation to cope with the risks of infanticide.

Observational and experimental evidence collected in a 4-year study of house sparrows, Passer domesticus, was used to test hypotheses about the evolution of monogamy. Reproductive success increased for males and decreased for females with increasing harem size. This rejects hypotheses that state that monogamy is advantageous for both sexes. Only 9·6% of 197 control males were polygynous. However, after removal of 30 males, between 36·7 and 73·3% of potential bigamous males acquired an additional female; 43–65% of the experimental widows mated with already mated males while only 23·3% rejected polygynous bonds. This contradicts the prediction, derived from the hypothesis based upon the polygynythreshold model, that widowed females prefer to remate with unmated males. Of 86 males, 27·9%, more than the percentage of polygynous males, remated after a first breeding attempt with a female that had not bred in the colony that season. Hence, the limitation of the polygyny level cannot be explained by a lack of females. When nestboxes were put closer together, males held more of them, but they did not increase their harem size. This suggests that the limitation of nest sites is not a proximate determinant of monogamy. Resident females were very aggressive towards a caged female placed close to their nest. The way in which the aggression rate varied with breeding phase and intruder persistence favours the hypothesis that females protect their monogamous status. However, further direct tests are required to confirm that monogamy in this population is maintained by aggression between females.

Male birds often face a loss of paternity in the broods they attend because of promiscuous behaviour by their mates. Parental investment theory predicts that caring for the offspring of rival males incurs costs, selecting against males that care for unrelated offspring. We examined the relationship between male parental care and paternity in the facultatively polygynous spotless starling, Sturnus unicolor, during three consecutive breeding seasons. We found no evidence that male spotless starlings reduce their feeding rates to their nestlings in relation to their loss of paternity. Unexpectedly, cuckolded males provisioned nestlings more than noncuckolded males in relation to the presence of extrapair offspring. Although this result deserves further investigation, we argue that spotless starling males do not reduce their feeding rate when they lose paternity mostly because variance in male feeding rate is not very costly in a species in which polygamy is related to male fitness, the occurrence of extrapair offspring is unpredictable and reliable cues to assess paternity are unlikely. Several hypotheses are presented to interpret an increase in feeding rates of cuckolded males over non-cuckolded males in terms of parental quality with feeding rates either envisaged as a form of mating effort or related to kin selection and depending on begging level, sibling competition and genetic parentage within broods., During this work E.G-V. was supported by a predoctoral fellowship from the Ministerio de Educación y Ciencia. This research was funded by project CGL-2005-05611-C02-01 and 02/BOS of the Ministerio de Educación y Ciencia and PCI08-0130 of the JCCM.

The role of testosterone in female fitness is poorly known in most vertebrates. It has been proposed that the dynamics of female testosterone may represent a correlated response to selection acting on male testosterone. However, several costs and benefits of enhancing the circulating levels of testosterone in females have been documented in a number of species, suggesting that female testosterone may be the product of direct selection. Despite this, no study has tried to quantify the long-term fitness consequences of manipulating female testosterone. We report the results of an experiment in which we implanted females with testosterone (T-females) and recorded the consequences of this manipulation on investment and reproductive success throughout their lifetime in relation to control females (C-females). Our results show that T-females bred more years in the colony than C-females. The onset of laying was delayed in T-females compared with C-females, but this effect decreased in the years following manipulation. The T-females laid fewer eggs per year, but eggs were heavier than those laid by C-females. The T-females fed nestlings at a lower rate than C-females in the 3 years in which feeding rates were measured and raised fewer fledglings than C-females in the manipulation year. However, since T-females accomplished more breeding attempts than C-females, as a result of their longer stay in the colony, there were no significant differences between the lifetime reproductive success of T- and C-females. There were no overall differences in body size of nestlings raised by T- and C-females, but the body mass of fledglings reached a peak 2-3 years after manipulation for C-females, a tendency not recorded in offspring of T-females. Also, young that were fledged in broods of C-females tended to recruit into the study colony more frequently than those fledged in T-females' nests. All these results indicate that our testosterone manipulation improved the ability of females to acquire and maintain a breeding site but interfered with their reproductive biology and parental investment. Thus, our main conclusion is that under the ecological conditions of our experiment, the addition of exogenous testosterone reduced female fitness. However, differences in population density and female-female competition are likely to alter the fitness landscape of this trait and change the adaptiveness of increased testosterone as a component of female reproductive strategies.

The effects of exogenous testosterone on the breeding biology of avian females are largely unknown. One unanswered question is whether testosterone has a role in female mating strategies affecting, for example, the rate of extrapair offspring (EPO) obtained. We experimentally increased circulating testosterone in females of a free-ranging population of the spotless starling, Sturnus unicolor, and measured rates of mixed paternity in their broods. Females treated with exogenous testosterone had a significantly reduced percentage of extrapair offspring in relation to controls and this difference persisted in the years after treatment. In addition, female age was negatively, and the number of nestlings was positively, related to EPO. To our knowledge, this is the first study on females in which EPO has been analysed in relation to an experimental testosterone manipulation. We suggest that females treated with testosterone are less attractive to males seeking extrapair mates and that there are delayed effects of testosterone on female mating that persist for years., During this work E.G-V. was supported by a predoctoral fellowship from the Ministerio de Educación y Ciencia. This research was funded by project CGL-2005-05611-C02-01 and 02/BOS of the Ministerio de Educación y Ciencia.

Theoretical and empirical evidence suggests that avian females are able to manipulate the offspring sex ratio at birth. Although mating with an attractive male may induce females to skew the sex ratio toward males, the balance between the benefits of producing attractive sons and the costs of competing with other females for mates could vary with female age, a possibility that had not been previously explored. In this paper we increment experimentally the attractiveness of males of the polygynous spotless starling (Sturnus unicolor) by adding green plants to their nests, a trait involved in courtship, and look for female age-differential effects on offspring primary sex ratio. Young and middle aged females produced more sons in experimental than in control nests, as expected, but old females showed the opposite tendency. To explain this novel result, we speculate that older females are limited to produce the most costly sex because the physiological drawbacks imposed by ageing reduce their ability to compete with younger ones for the non-shareable resources offered by males. We discuss that this evolutionary scenario may be widespread in avian polygynous systems., This work was supported by projects BOS2001-0703 of the Ministerio de Ciencia y Tecnología, CGL2004-00126/BOS and CGL2005-05611-C02-01/BOS of the Ministerio de Educación y Ciencia.

Many species of lizards have colour spots on the flanks that may function as ornaments. We investigate the between-years stability of the blue patch colour saturation and the relationships between colour saturation, mating success, and survival in males of the lizard Psammodromus algirus. Saturation values of the anterior blue patch of males were significantly repeatable across years. Survivors had more saturated patches than non-survivors among paired males but not among non-paired males. The positive relationship between blue patch saturation and survival suggests that this morphological trait acts as a reliable signal of male quality measured as adult survival., The study was supported by Ministerio de Ciencia y Tecnología (PB94-0067 and BOS2001-0533). Permission for this study was granted by Agencia de Medio Ambiente, Comunidad de Madrid (10/015574.0/97).

We followed a field population of the alpine lizard Iberolacerta cyreni over 2 consecutive breeding seasons and assigned paternity to the offpring using 8 microsatellite markers. Paternity data, combined with observations of the behavior, morphology, and spacing patterns of lizards, allowed us to document the extent of polygamy, the phenotypic correlates of the number of offspring sired, and the correlation between male reproductive success (RS) and probability of recapture the second year of our study. Multiple paternity was observed in nearly 50% of clutches, and the mating system was highly polygynandrous. In the first year of our study, male RS increased with body size, activity, tail length, and color saturation of the dorsum. In the second year, male RS increased with activity and body condition. Overall, increased male activity, a trait that is expected to decrease survivorship, was the explanatory variable that had the greatest effect on RS. However, the residents of our first study year that were recaptured in the second year had longer tails, were more active, and sired more offspring than their conspecifics that were not recaptured. Thus, contrary to expectations, no negative correlation between present reproduction and future survival was found, which suggests that male investment in reproduction is condition dependent and positively correlated with the ability to pay the underlying costs of increased activity., Ministerio de Educación y Ciencia (BOS2001-0533, CGL2004-01151).

The reproductive success of female house sparrows mated with polygynous males depended to some extent on the aid received from their mates. Polygynous males fed nestlings at the same rate as monogamous males although polygynous males gave aid almost exclusively to one of their mates (the “preferred”). As a consequence, the number and quality of young raised by “preferred” females were similar to those of monogamous females, whereas nonaided females experienced a reduction of clutch size, hatching success, and fledgling quality. Males aided the harem female that hatched her clutch earliest (in 11 out of 12 cases). However, harem females that laid eggs earliest (i.e., the potential “preferred”) lost eggs and/or nestlings more frequently than harem females that delayed laying. This resulted from competing females' infanticide suited for redirecting males' aid. All polygynously mated females delayed the start of a new clutch and reproduced fewer times per season than monogamous females. Consequently, monogamy seems to be the optimum mating situation for females; whereas polygynous males raised more young per year than monogamous males, interference between harem females reduced their maximum expected success.

1. The use of behavioural traits by females in signalling condition has been practically ignored in evolutionary theory. However, females may also exhibit ornaments and behavioural displays, although less elaborated than those of males. 2. In this study we suggest that the carrying of feathers by spotless starlings Sturnus unicolor Temminck females to decorate the nest represents an elaborated and costly behaviour that is displayed in response to a courtship male behaviour: the carrying of nest green plants. 3. By experimentally increasing the amount of green plants in the nests, to give the appearance that highly attractive males defended them, we induced females to increase their feather carrying rates. 4. The amount of feathers carried to the nest was correlated to female reproductive experience and laying date, two variables correlated with female body condition. These results suggests that this behaviour may work as an honest indicator of female quality. 5. We conclude that male carrying plants and female carrying feathers can be viewed as two sex-specific functionally related signalling behaviours involved in mutual courtship or status signalling.

The use of green nest plants by birds has received much attention in recent years. Although many hypotheses have been put forward to explain the functional significance of this trait, three of them, the courtship hypothesis, the nest protection hypothesis and the drug hypothesis, have particularly captured the interest of researchers. In this paper we demonstrate that in the spotless starling, Sturnus unicolor (1) most of the green plants were carried to nests during the 10 d that preceded the start of egg laying, (2) males that controlled a greater number of nest boxes, so that they mated with more females, also carried a larger amount of green material to each of their nests, (3) males carried larger amounts of green plants in second than in first clutches, and (4) monogamous males did not exhibit this last tendency when they mated with inexperienced females. Thus, plants seem to exert a function during the female fertile period but not during incubation or nestling stages. The amount of green plants added by a male may be seen as a honest signal of attractiveness and status. We suggest that males uses green plants to stimulate females to breed in it so that they carry more plants in second clutches, when females might be more reluctant to breed. This strategy was apparently constrained in monogamous males mated with inexperienced females probably because parental duties, rarely recorded in polygynous males, limits the time available to carry plants to nests. We also discuss to what extent other hypotheses may further explain the evolution of green plants carrying in starlings., This study was funded by projects PB97-1249 (DGICYT), BOS2001-0703 (MCYT) and CGL-2004-00126/BOS. The present research has been conducted under licence of the Consejería de Medio Ambiente of the Comunidad of Madrid.