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According to the good genes and genetic compatibility hypotheses, females of socially monogamous species obtain genetic benefits for their offspring by performing extrapair copulations with males of higher quality than their social mates or males with whom they are more genetically compatible. If extrapair offspring do receive genetic benefits in the form of advantageous alleles or more compatible allele combinations, they should outperform their within-pair half-siblings' survival and/or reproductive output. Here, we followed 52 extrapair and 737 within-pair blue-footed booby, Sula nebouxii , offspring during their first 10 years of life (excluding the embryonic period and the first 10 days after hatching) to assess whether they differed in fledging probability, fledgling body condition, recruitment probability, age at first reproduction, number of breeding events or accumulated breeding success. Extrapair and within-pair offspring did not differ in any of these proxies of fitness. Furthermore, we found that extrapair offspring were equally likely to occur in any hatching position. However, differences in fledgling production over the lifetime could not be ruled out, and because only within-pair production of eggs and fledglings was tallied, the possibility remains that extrapair offspring could produce more extrapair offspring later in life than do within-pair offspring. Furthermore, the possibility of context-dependent genetic benefits occurring only under stressful conditions cannot be discounted because our sample of offspring was obtained in a single exceptionally favourable reproductive season. • Pair-bonded females of most avian species copulate with extrapair males. • If extrapair mates have high-quality genes, then they may improve offspring quality. • To examine this, we followed extrapair and within-pair booby offspring for 10 years. • Extrapair offspring did not show superior survival or reproductive output. [ABSTRACT FROM AUTHOR]

We describe a new disjunct population of Blue-footed Booby Sula nebouxii at Islote González in the Islas Desventuradas archipelago. Though likely much larger, the population size at the time of our survey was a minimum of 16 breeding pairs. On 14-15 December 2020, two pairs of boobies with nestlings and four pairs with empty nests were found. Likely the species is a recent arrival to the archipelago, perhaps during one of the strong El Niño events between 1970 and 2001 (especially 1982/83, when the species was first recorded in Chile). Blue-footed Booby might also nest on different areas at San Félix Island and some smaller islets around the archipelago, where they would have no nesting-site competition from the larger Masked Booby S. dactylatra. [ABSTRACT FROM AUTHOR]

Blue-footed boobies (Sula nebouxii) are socially monogamous, colonial seabirds exhibiting intra-specific nest parasitism and extra-pair copulations. Prior DNA fingerprinting assays failed to detect extra-pair offspring in the nests of congeners, and the rate of intra-specific nest parasitism has not been estimated using molecular techniques. We describe the development and characterization of 11 microsatellite DNA loci, tested using 31 individuals collected on Isla Isabel, Nayarit, México. The number of alleles per locus ranged from three to 22, averaging seven; total exclusionary power of the microsatellite panel was 0.99; no loci deviated from Hardy-Weinberg equilibrium; and we did not detect linkage disequilibrium following Bonferroni correction. This microsatellite panel will facilitate future studies of nest parasitism and extra-pair paternity in blue-footed boobies.

The probability of Blue‐footed Booby Sula nebouxii fledglings becoming reproductive adults is maximal when one parent is old and the other young, and minimal when both are old or young. No mechanism has been identified to explain this pattern, but here we showed that nestlings with different‐aged parents are the least infested with ticks. This result constitutes preliminary confirmation of the hypothesis that the effect of combined parental ages on probability of recruitment is mediated by offspring immunocompetence. The contribution of immunocompetence and parental care to these parental age effects needs to be unravelled. [ABSTRACT FROM AUTHOR]

Understanding the role of diet in the physiological condition of adults during reproduction and hence its effect on reproductive performance is fundamental to understand reproductive strategies in long-lived animals. In birds, little is known about the influence of the quality of food consumed at the beginning of the reproductive period and its short-term effects on reproductive performance. To assess the role of diet in the physiological condition of female blue-footed booby, Sula nebouxii (BFBO), during reproduction we evaluated whether individual differences in diet (assessed by using δ13C and δ15N values of whole blood from female birds and muscle tissue of the principal prey species) prior to egg laying and during incubation influenced their lipid metabolic profile (measured as triglyceride levels and C:N ratio) and their reproductive performance (defined by laying date, clutch size and hatching success). Females with higher δ15N values in their blood during the courtship and incubation periods had a higher lipid metabolic profile, earlier laying date, greater clutch size (2–3 eggs) and higher hatching success. Females that laid earlier and more eggs (2–3 eggs) consumed more Pacific anchoveta (Cetengraulis mysticetus) and Pacific thread herring (Opisthonema libertate) than did other females. These two prey species also had high amounts of lipids (C:N ratio) and caloric content (Kcal/g fresh weight). The quality of food consumed by females at the beginning of reproduction affected their physiological condition, as well as their short-term reproductive performance. Our work emphasizes the importance of determining the influence of food quality during reproduction to understand the reproductive decisions and consequences in long-lived animals. [ABSTRACT FROM AUTHOR]

It is common for marine animals to be selective in the amount and quality of foods during reproduction due to the higher energetic demands. As central place foragers, seabirds are often selective with the prey species they deliver to their offspring. We evaluated trophic segregation between parents and their offspring in blue-footed booby ( Sula nebouxii) by measuring stable isotope ratios (δN and δC) in blood samples during two breeding seasons and at different reproductive stages (incubation, early and late chick rearing). Additionally, we examined if δN values from chicks were correlated with their condition as reflected in blood alkaline phosphatase (ALP) levels. δN values increased and δC values decreased in adults as the breeding season progressed, indicating differences in foraging regions or the assimilated prey. δN values differed significantly between sexes; males had higher δN values than females during all reproductive stages sampled during the 2011 season, whereas in 2012 the difference between sexes was only observed during incubation. Offspring had higher δN values and lower δC values than adults, suggesting that, when feeding their chicks, parents feed them with prey from higher trophic levels and possibly from more oceanic environments. ALP levels, a proxy for bone growth and condition, measured in the offspring were positively correlated with δN values during the chick-rearing period. Although the diet of parents differed between reproductive stages, a multi-source Bayesian mixing model indicated that their primary prey were Pacific anchovy and halfbeaks, with a combined modal fractional contribution of 50-70% of the diet. The diet of the offspring was dominated by Pacific anchovy (≈36%), which was also the prey with the highest lipid content (C:N ratio 5.57), whereas the contributions of other fish species (or groups) were similar (17-24% each). Our findings suggest that parents actively feed their young with high-quality prey, which seems to increase some of the individual fitness components associated with better growth. [ABSTRACT FROM AUTHOR]

The function of female birds' extra-pair (EP) behavior has remained an unresolved question in ornithology and behavioral ecology for > 30 yr. The genetic compatibility hypothesis (GCH) proposes that females benefit by acquiring biological sires that yield more heterozygous, and therefore fitter, offspring than their social mates. We used ten polymorphic microsatellite loci to test GCH predictions and its assumption that fitness increases with heterozygosity in blue-footed boobies Sula nebouxii, a long-lived tropical seabird. Our predictions were not supported. Heterozygosity was uncorrelated with quality indicators (fledging probability, fledgling or adult body size or mass, adult ornamentation, mean breeding success). Females were no more likely to have EP behavior or chicks when their social mates were less heterozygous or compatible, nor were EP males more heterozygous or compatible than the males they cuckolded. Finally, EP chicks were no more heterozygous than within-pair chicks overall or in half-sib comparisons, nor were within-pair chicks from all-within-pair nests more heterozygous that those with EP nest-mates. There are both methodological and biological explanations for these consistently negative results. Inadequate sample size is possible but unlikely, since our samples were comparable or larger than those of similar studies with significant findings. Lack of identity disequilibrium (within-individual heterozygosity correlation) among our marker loci could be responsible, and suggests either insufficient marker coverage or lack of inbreeding, bottleneck, and/or admixture. An independent social pedigree revealed infrequent inbreeding, suggesting that pressure on females to select sires that maximize offspring heterozygosity may be genuinely lax. Alternatively, it is possible that the GCH is only upheld when selection on young is strongest; this would not be detected in our sample, which was taken during an extremely productive year. Whatever their cause, our results expand the taxonomic breadth of avian EP behavior analyses and should be considered in future evaluations of the GCH. [ABSTRACT FROM AUTHOR]

We used blood samples of the Blue-footed Booby, considering sex (female and male) and age-class (adult and chick) of individuals at different breeding stages during two breeding seasons (2010–2011 and 2011–2012) in Isla El Rancho, Sinaloa, to determine lead, cadmium, and mercury concentrations. Lead and cadmium concentrations were below our detection limit (0.05 and 0.36 ppm, respectively). A higher concentration of mercury was found in early stages of breeding, likely related to changes in mercury environmental availability. Mercury concentrations in adults did not relate with their breeding output. Males and adults had higher mercury concentration than females and chicks. We provide information of temporal, sex and age-related variations in the concentrations of mercury in blood of the Blue-footed Booby. [ABSTRACT FROM AUTHOR]

Populations of blue-footed boobies in the Galápagos Islands have declined by at least 50% compared with numbers recorded during the 1960s. Recently, concern has been expressed about potential adverse effects of introduced blood parasites; in particular, malarial parasites (i.e. Plasmodium sp.), which are frequently pathogenic and can cause mortality and morbidity in birds, and blood parasites of the genus Haemoproteus, which can reduce fitness. We screened blue-footed boobies at six breeding colonies within Galápagos for both genera of parasite. At two sites, we also investigated whether there was any relationship between infection status and physiological condition or breeding success. We found a high prevalence of Haemoproteus spp. in adults and chicks using PCR, although none was found on blood smears. We found no evidence of Plasmodium. Effects of parasitism on the condition of birds were limited. Also, there was no evidence that infection with Haemoproteus adversely affected breeding success or chick growth. Our results suggest that poor success of breeding birds was not strongly associated with Haemoproteus infection. However, regular colony monitoring and examination of dead birds are necessary to assess whether acute Haemoproteus infection might be responsible for some mortality, particularly of chicks. [ABSTRACT FROM AUTHOR]

Explaining the remarkable variation in socially monogamous females' extrapair ( EP) behaviour revealed by decades of molecular paternity testing remains an important challenge. One hypothesis proposes that restrictive environmental conditions (e.g. extreme weather, food scarcity) limit females' resources and increase EP behaviour costs, forcing females to reduce EP reproductive behaviours., For the first time, we tested this hypothesis by directly quantifying within-pair and EP behaviours rather than inferring behaviour from paternity. We evaluated whether warmer sea surface temperatures depress total pre-laying reproductive behaviours, and particularly EP behaviours, in socially paired female blue-footed boobies ( Sula nebouxii). Warm waters in the Eastern Pacific are associated with El Niño Southern Oscillation and lead to decreased food availability and reproductive success in this and other marine predators., With warmer waters, females decreased their neighbourhood attendance, total copulation frequency and laying probability, suggesting that they contend with restricted resources by prioritizing self-maintenance and committing less to reproduction, sometimes abandoning the attempt altogether. Females were also less likely to participate in EP courtship and copulations, but when they did, rates of these behaviours were unaffected by water temperature., Females' neighbourhood attendance, total copulation frequency and EP courtship probability responded to temperature differences at the between-season scale, and neighbourhood attendance and EP copulation probability were affected by within-season fluctuations. Path analysis indicated that decreased EP participation was not attributable to reduced female time available for EP activities. Together, our results suggest that immediate time and energy constraints were not the main factors limiting females' infidelity., Our study shows that El Niño conditions depress female boobies' EP participation and total reproductive activity. In addition to increasing general self-maintenance and reproductive costs, warm waters may increase costs specific to EP behaviours including divorce, reduced male parental care, or pathogen exposure. Our results suggest that female boobies strategically refrained from EP behaviours to avoid these or other longer-term costs, rather than being compelled by immediate constraints. This study demonstrates that current environmental conditions affect females' mating decisions, contributing to variation in EP behaviours, even in a long-lived, iteroparous species that can buffer against temporary adversity. [ABSTRACT FROM AUTHOR]

Copyright of Animal Biodiversity & Conservation is the property of Museu de Ciencies Naturals de Barcelona and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Parental care in long-lived bird species involves a trade-off between the benefits of increasing the effort expended on current offspring and the costs that this represents for future reproductive output. Under regimes of high environmental variability, long-lived seabirds can adjust their breeding effort to buffer the negative effects of this variability on their offspring. However, the potential impacts of variation in breeding effort on offspring physiology in the short term and on longer-term survival are poorly understood. In this study, we manipulated brood age through a cross-fostering experiment to assess whether increasing or decreasing parental reproductive expenditure led to costs in Blue-footed Booby Sula nebouxii chicks. Specifically, we tested the consequences of altered parental reproductive expenditure on the offspring's physiological condition (plasma metabolites, heterophil to lymphocyte ratio (H/L) and body condition index ( BCI)) and survival. Offspring from broods in which parental investment was experimentally increased showed a lower BCI and lower alkaline phosphatase levels and higher H/L ratios than controls. Conversely, offspring showed the opposite pattern when reproductive expenditure was experimentally decreased. We observed no effects of manipulation of parental investment on triglyceride levels or on survival rates. Although our findings suggest that Blue-footed Booby parents have the ability to adjust their breeding effort according to the demands of their offspring, parental effort could influence the effect of hatching order by suppressing the aggressive tendency of the senior chick. [ABSTRACT FROM AUTHOR]

The evolution of remating and prolonged pair bonds in animals has generally been explained in terms of improved coordination and cooperation between familiar individuals, but costs of mate familiarity have rarely been considered. A possible cost for males is increased risk of losing paternity if familiarity enables females to detect when alternative sires are desirable, evade mate guarding or invest more in infidelity. To test whether this familiarity cost exists, we examined whether extrapair paternity increases with bond length in the socially monogamous blue-footed booby, Sula nebouxii, using microsatellite-based analysis of 384 broods. Extrapair paternity increased from 9.4% of broods in first pairings to 21.6% in second pairings, then declined to 7.5% in third through eighth pairings. On their first remating with a female, males faced enhanced risk of losing paternity, but thereafter the risk was no greater than on first matings. However, after loss of paternity, males were no more likely to divorce. Effects of familiarity on extrapair paternity could influence the evolution and taxonomic distribution of remating and prolonged pair bonding in socially monogamous animals. [ABSTRACT FROM AUTHOR]

When animals potentially occupy diverse microhabitats, how can camouflage be achieved? Here we combine descriptive and experimental methods to uncover a novel form of phenotypic plasticity in the camouflage of bird eggs that may be present in other avian taxa. Soil from the bare substrate adheres to the blue-footed booby's(Sula nebouxii's) pale eggs, which parents manipulate both under and on top of their webs. Analysis of digital images confirmed that dirtiness increases progressively during the first 16daysofthe incubation period, making eggs more similar to the nest substrate. Observations of 3,668 single-egg clutches showed that the probability of egg loss declines progressively over the same time frame and then remains low for the rest of the 41-day incubation period. An experiment showed that when chicken eggs are soiled and exposed in artificial booby nests, they are less likely to be taken by Heermann'sgulls (Larus heermanni) than clean eggs. [ABSTRACT FROM AUTHOR]

In the short-term, reproductive expenditure has repercussions for survival and future reproductive output. In long-lived seabirds, parents are expected to adjust their reproductive investment based on their physiological and immune status and then respond to the needs of their offspring without incurring additional costs. However, the impacts of parental expenditures on physiological and immune status have not been well explored. We compared the foraging effort (number and duration of foraging trips), time at nest and physiological status (plasma metabolites, heterophil/lymphocyte (H/L) ratio, creatine kinase (CK) activity) and body condition index (BCI) of blue-footed boobies subjected to experimentally increased or decreased breeding demands. When parental reproductive demands were increased, adults made more foraging trips and decreased their time at the nest. The increases in adult physical activity were reflected in higher levels of creatine kinase/total protein (CK/TP, U/g) and higher H/L ratios, indicating that adjustments in foraging effort were reflected in their physiological status. When parental reproductive demands were reduced, parents adjusted their level of effort according to the lower needs of their offspring; as a result, their overall physiological status was greater than that of the controls, showing lower CK/TP levels and H/L ratios. Our findings indicate that parents modified their foraging effort in response to variation in the food demands of the brood to maintain reproductive value, but this adjustment had consequences for physiological status in both costs and benefits. Blue-footed boobies inhabit upwelling systems where they experience high environmental variability throughout their life span. Thus, the ability to adjust breeding effort may buffer breeding success under different scenarios, with short-term physiological expenditures possibly related to long-term survival. [ABSTRACT FROM AUTHOR]

Variation in the age at first reproduction may have important implications for growth and dynamics of populations, but these potential impacts remain virtually unexplored. By using 26 years of data from a marked colony of blue-footed boobies Sula nebouxii (Milne-Edwards), we tested whether survival of early recruits is lower in comparison to late recruits, and investigate how different recruiting ages contribute to the persistence of the population. In addition, we simulated changes in the age at recruitment and estimated their impact on population fitness. Our analyses of 1310 capture-recapture histories revealed that the younger males and females initiated reproduction within the first 6 years of life, the lower were their mean annual survival probabilities. Survival did not differ between the sexes and the impact of recruiting age on survival was similar for males and females. Each additional year that male and female boobies delayed the start of reproduction meant an increase of roughly 2 % in their annual survival rates. Male and female recruits 2-5 years old contributed the same to the rate of population growth (λ) regardless of their particular age, but the contribution to population growth started to decline from age 6 years. According to our simulations, delaying reproduction beyond 5 years in the case of females, and beyond 6 years in the case of males, would negatively affect this booby population. Therefore, we suggest that there is selection against recruiting beyond 5-6 years old. This study adds to the nascent empirical literature on long-term fitness consequences of variation in the age at first reproduction in long-lived species, and provides insights into evolution of early and late reproduction and trade-offs from a demographic perspective. [ABSTRACT FROM AUTHOR]

It is widely expected that the quality of offspring will vary with the age of their parents and that this variation should influence animals’ choice of mates. However, theoretical predictions for age effects are contradictory and, to our knowledge, we do not know for any wild animal how the quality of offspring is affected by both parents’ ages across their lifespans, or whether mothers’ and fathers’ ages interact. We tackled this question using long-term data on a highly philopatric, insular population of the blue-footed booby (Sula nebouxii). In this species extra-pair paternity is most common in pairs of two young parents or two old parents, implying that these age combinations might prejudice offspring quality. Analysis of the viability of 3,361 offspring of parents up to 21 years old revealed that fledglings with two young parents or two old parents were least likely to become breeders, whereas fledglings with one young parent and one old parent were most likely to do so. For young parents of either sex, offspring viability increased with age of the other parent; for very old parents, it decreased. These effects could be mediated by parents flexibly modifying their investment in offspring in response to their own and their partners´ ages, but evidence for this was lacking. In 5,343 breeding attempts, although mothers’ and fathers’ ages independently affected four heavily care-dependent breeding traits at the clutch and nestling stages, their interaction did not affect any trait. The effects of parental age combinations on viability could also be mediated by genes: fledglings with one young parent and one old parent could benefit from greater heterozygosity or better genes. [ABSTRACT FROM AUTHOR]

Females sometimes obtain older sires for their offspring through extra-pair interactions, but how female age influences paternity is largely unexplored and interactive effects across the age span of both sexes have not been analyzed. To test whether female choice of sire age varies with female age in the blue-footed booby ( Sula nebouxii), we examined associations between ages of both partners and the probability of extra-pair paternity (EPP) in 350 broods of parents up to 22 years old in a single breeding season. Extra-pair paternity enables a female to select an alternative sire for her offspring and could function to avoid or achieve particular combinations of parental ages. A male age × female age interaction revealed that in young females (≤4 years), EPP decreased with increasing age of the social partner, whereas in old females (≥8 years), it increased. Moreover, sires of extra-pair (EP) chicks of young females paired to young males were on average 6.33 years older than the females' social partners. Since female boobies control copulatory access, this pattern could imply that young females choose old sires for their proven genetic quality and that old females avoid very old males because matings with them may risk infertility or genetic defects in offspring. Taking female age into account and observing across the whole age span may be necessary for understanding female age-based mate choice. [ABSTRACT FROM AUTHOR]

Individual variation in sexual fidelity and extrapair paternity (EPP) is widely attributed to environmental heterogeneity, but the only variables known to be influential are food abundance and density of conspecific breeders (potential extrapair partners). Habitat structure is thought to impact EPP but is rarely measured and, when considered, is usually confounded with food abundance and predation pressure. To sidestep these confounds, we tested whether EPP is associated with habitat structure variables and with local conspecific density in a species whose nesting habitat is not used for feeding and lacks predators. In a blue-footed booby, Sula nebouxii , colony, the probability of EPP in a female's nest was highest in parts of the study plot where there were few obstacles to locomotion, and was quadratically related to local density of sexually active males, even though local males did not sire the EP chicks. The probability of a male breeder siring EP (extrapair) chicks elsewhere was quadratically related to local density of sexually active males around his nest. From these patterns we infer that both sexes may foray for EP interactions, that males and females nesting at intermediate density are most likely to be accessed by forayers, and that obstacles in the vicinity of a female's nest constrain access of foraying males. To our knowledge, this is the first demonstration that individual variation in EPP is associated with habitat structure in the absence of confounding variation in food availability, predation pressure or breeder quality, and the first evidence that EPP opportunities of female and male breeders are reduced by high density of conspecific breeders above a particular threshold. [ABSTRACT FROM AUTHOR]

The good genes hypothesis has long been a major focus of research on the function of extrapair (EP) behaviour by socially monogamous females. It predicts that females should be less faithful when paired to low-quality males and, as EP males become increasingly superior to their social mates, that EP males should be of higher quality than the males they cuckold. Evaluations have yielded mixed results, perhaps in part due to an overly strict interpretation of these predictions. A recent theoretical model demonstrated that while females with high-quality mates should be selective, those with low-quality males may gain good genes benefits by random EP mating, provided EP choosiness has costs, phenotypic indicators are imperfect and sperm competition favours genetic quality. We tested these classic and modified behavioural predictions in the blue-footed booby, Sula nebouxii , at two critical steps of EP interactions: courtship and copulation. Support for the classic good genes predictions was limited: EP courtship was more common in females with younger mates, but EP copulation following courtship was not predicted by social mate or relative male age, body size, condition, foot colour (a dynamic ornament) or breeding success, and these characteristics did not differ consistently between social and EP males. However, EP copulation increased with relative attractiveness (greener feet) of the EP male only when the social male was attractive, supporting the model's prediction that female choosiness should increase with social male attractiveness. Moreover, EP copulation probability decreased as EP males became increasingly superior to unattractive females, suggesting that EP males can also be choosy and may constrain female behaviour. Our results confirm that EP interactions involve more complexity than originally envisioned, warranting further studies that focus on behaviour and include all three main players: female, social mate and EP male. [ABSTRACT FROM AUTHOR]

Copyright of Revista Científica Juyyaaania is the property of Universidad Autonoma Indigena de Mexico and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The article discusses the molecular characterization of hybridization between brown Sula leucogaster and blue-footed Boobies S. nebouxii. The researchers collected blood, feather or tissue samples from 30 brown and 15 blue-footed boobies from the Eastern Tropical Pacific. The findings of molecular sexing demonstrated that both putative hybrids were male.

Meraz, J., Ancona, S., Rodríguez, C., and Drummond, H. 2013. Reproduction of the blue-footed booby predicts commercial fish abundance in the eastern tropical Pacific. – ICES Journal of Marine Science, 70: 1263–1272.To establish whether reproduction in a colony of the blue-footed booby in the eastern tropical Pacific predicts local abundance of prey fish several months later, 13 years of data were analysed. Eight reproductive variables assessed during the period of January–May, grouped in two factors, were related to commercial catches in the surrounding 6600 km2 area during the following June–December. The first factor explained 33% of interannual variance in fish captures per unit effort during June–December (future FCUE), and 63% when only El Niño years were considered. Also, the proportion of large clutches present on three single-day censuses in the spring explained 51, 46 and 35% of variance in future FCUE among all years, and 78, 85 and 82% of variance among El Niño years. In contrast, sea surface temperatures in March did not explain variance in future FCUE. Proportion of large clutches is a moderately good predictor of the abundance of commercial fish during the subsequent seven months and can be satisfactorily and cheaply measured on a single day. [ABSTRACT FROM PUBLISHER]

Experiments on birds, fish and mammals have shown that adverse conditions during infancy can produce diverse long-term and delayed deficits during adulthood, prejudicing both the individual and its offspring. Natural selection should prepare animals to cope with adversity of the type, magnitude and timing that commonly occur in their natural habitat, but very little is known about such evolved developmental buffering against natural 'poor starts' in life. In two-chick broods of the blue-footed booby Sula nebouxii, the junior (younger) chick usually experiences aggressive subordination, reduced nutrition and growth and elevated circulating corticosterone. To test whether this poor start produces long-term, delayed or intergenerational deficits in body size, body condition or cell-mediated immune response, we measured 3-8 yr old female breeders banded as chicks, and their infant offspring. Results failed to support our predictions. Compared to former seniors and former singletons (solitary nestlings), former juniors showed no deficit in cell-mediated immune response at any age. They showed an 8.04% deficit in body condition at age 4-6 yr but this deficit disappeared completely by age 7-8 yr. Furthermore, their offspring showed no deficits in body size, body condition or immune response. Junior chicks are affected by their poor start, but their developmental resilience, also confirmed by studies of post-fledging survivorship, recruitment, natal dispersal, aggressive nest defense and reproduction, is evidence of evolved developmental buffering against predictable adversity during infancy. [ABSTRACT FROM AUTHOR]

The behaviour that mediates divorce and partner change in socially monogamous species is largely unstudied and unknown, although roles and adaptive functions in some birds have been inferred from breeding outcomes, partial behavioural records and captive studies. Here, roles and functions of natural within-season mate switching of a colonial bird were characterized by describing interactions over many days during the whole daylight period. Switching occurred in 5.9% of bluefooted booby pairs and was initiated by desertion of males or, less commonly, females. Three male desertions were consistent with either the Errors of Mate Choice or Incompatibility hypotheses but were better explained by our new Unfaithful Mate hypothesis because infidelity of the mate preceded desertion. Another two male desertions were more consistent with the Better Options hypothesis because the males switched to their ongoing extra-pair (EP) partners. One female desertion was consistent with the Errors of Mate choice or Incompatibility hypotheses, and another with the Better Options hypothesis. All five deserted females switched promptly to their EP partners. Thus, most switches consisted of (1) males replacing a possibly unsatisfactory partner with a new (already identified or to be identified) partner, or (2) deserted females pairing with their EP partners. [ABSTRACT FROM AUTHOR]

In breeding birds, acoustic signalling is often an important cue for reunion between sexual partners. However, in spite of its potential interest, mate recognition has rarely been examined by comparing the two sexes. We studied the blue-footed booby, Sula nebouxii, a socially monogamous seabird, with a dramatic call sexual dimorphism suggesting two different strategies for identity coding: the female call is an amplitude-modulated sound with a harmonic series slowly modulated in frequency, while the male call is a noisy whistle strongly modulated in frequency. To compare acoustic strategies between the sexes, we (1) recorded calls of both males and females, (2) searched for an individual signature and characterized it, and (3) tested, using a playback experiment, whether the calls of males and females were equally efficient for mate recognition. Results showed that an individual signature was present in the calls of both sexes. However, the acoustic parameters involved differed: female individual identification was principally achieved by a spectral analysis of the call whereas males'' identity relied mostly on temporal cues. More than 70% of both females and males tested in playback experiments successfully recognized their mate. This suggests that the coding strategies are equally efficient in terms of individual recognition between mates. From a broader point of view, our results underline the importance of assessing both males and females within the same investigative framework. [Copyright &y& Elsevier]

Hybrid zones represent natural laboratories in which the processes of divergence and genetic isolation can be examined. The generation and maintenance of a hybrid zone requires mispairing and successful reproduction between organisms that differ in one or more heritable traits. Understanding the dynamics of hybridization between two species requires an understanding of the extent to which they have diverged genetically, the frequency of misparing and hybrid production, and the extent of introgression. Three hundred and twenty one blue-footed Sula nebouxii and Peruvian S. variegata boobies from the eastern tropical Pacific Ocean were analyzed using 19 putatively neutral genetic markers to evaluate interspecific differentiation, to classify morphological hybrids using Bayesian assignments, and to characterize hybridization using cline theory and Bayesian assignments. The species were well differentiated at mitochondrial and nuclear microsatellites, the hybrid zone was bimodal (contained a high frequency of each parental species but a low frequency of hybrids), and morphologically intermediate individuals were most likely F1 hybrids resulting from mating between female Peruvian boobies and male blue-footed boobies. Clines in allele frequency could be constrained to share a common geographic centre but could not be constrained to share a common width. Peruvian and blue-footed boobies hybridize infrequently, potentially due to strong premating reproductive isolation; however, backcrossing appears to facilitate introgression from blue-footed to Peruvian boobies in this hybrid system. [ABSTRACT FROM AUTHOR]

In evolutionary biology, whether parents should enhance or reduce parental care according to mate ornamentation is a subject of great debate. However, the evolution of female ornaments can shed light on this question. In theory, female ornamentation should be traded off against fecundity and thus cannot be wholly informative to males without a direct indication of fecundity. Hence, direct cues of offspring quality should affect the relationship between male investment and female ornamentation. Under this hypothesis, we manipulated two direct cues of offspring quality (egg size and color) after first egg laying in the blue-footed booby and registered male incubation patterns. In this species, foot color is a dynamic signal of current condition and in females is traded off with egg size. We found that males spent more time incubating when paired with dull females but only in nests with large eggs. Males also spent less time incubating small dull eggs. Results indicate that egg size, a direct cue of reproductive value, affected the relationship between male effort and female ornamentation. Males may be willing to help females that have invested in offspring at the expense of ornamentation, which suggests compensation when females are in low condition. Another possibility is that males relax their effort when paired with highly ornamented and fecund females because they have high parenting abilities. Our findings suggest that the information conveyed by female ornaments may depend on direct cues of fecundity. Results also highlight that parental decisions are complex, modulated by a combination of information sources. [ABSTRACT FROM AUTHOR]

Recent studies of wild populations provide compelling evidence that survival and reproduction decrease with age because of senescence, a decline in functional capacities at old ages. However, in the wild, little is known about effects of parental senescence on offspring quality. We used data from a 21-year study to examine the role of parental age on offspring probability of recruitment in a long-lived bird, the blue-footed booby (Sula nebouxii). Offspring probability of recruiting into the breeding population varied over the life of parents and effects age were similar in mothers and fathers. Offspring recruitment was high when parents were roughly 6-12 years old and low before and after then. Effects of parental age on offspring recruitment varied with lifespan (parental age at last reproduction) and previous breeding experience. Offspring recruitment from young and old parents with long reproductive lifespans was greater than that of offspring from parents with short lifespans at young and old ages. For parents with little previous breeding experience recruitment of offspring decreased with their hatch date, but experienced parents were no similarly affected. We found evidence of terminal effects on offspring recruitment in young parents but not in older parents, suggesting that senescence is more likely a gradual process of deterioration than a process of terminal illness. Failure to recruit probably reflects mortality during the first years after independence but also during the fledgling transition to full independence. Our results show effects of parental age and quality on offspring viability in a long-lived wild vertebrate and support the idea that wild populations are composed of individuals of different quality, and that this individual heterogeneity can influence the dynamics of age-structured populations. [ABSTRACT FROM AUTHOR]

A survey of the helminth fauna of the blue-footed booby, Sula nebouxii, on Isla Isabel, off the Pacific coast of Mexico, is presented. Eight parasite species were found: 4 digeneans (Galactosomum puffini, Mesostephanus microbursa, Opisthometra planicollis, and Renicola thapari), 3 nematodes (Contracaecum sp., Porrocaecum sp., and Tetrameres sp.), and 1 cestode {Tetrabothrius sp). All these species are reported for the first time in the blue-footed booby, and they represent 8 new locality records as well. Species accumulation curves suggest these 8 parasite species comprise the total helminth fauna of this population of boobies. The most frequent and abundant parasite was R. thapari, inhabiting the kidney of its host. In male boobies, a significant negative correlation was found between abundance of R. thapari and host body condition. [ABSTRACT FROM AUTHOR]

There is increasing interest in the impacts of El Niño Southern Oscillation (ENSO) on reproduction of apical predators such as seabirds and marine mammals. Long-term studies documenting ENSO effects on reproduction of seabirds in the warm tropics are scarce, and differential sensitivity of breeding parameters to ENSO has rarely been explored. Analysis of 18 years of breeding data from a colony of the blue-footed booby Sula nebouxii (Milne-Edwards) showed a delay in onset of breeding when the global Southern Oscillation Index was negative; each unit of the atmospheric pressure differential (hPa) across the Pacific Ocean meant a delay of 7 days. ENSO conditions also produced declines in breeding participation, clutch size, brood size, hatching success and fledging success, especially when surface waters surrounding the colony were warmer during winter and spring. Each additional degree (°C) of water temperature produced a reduction of 0·45 fledglings per nest. Different breeding parameters were sensitive to ENSO indices in different blocks of months. Warming of local waters during the winter was associated with decline in ocean productivity in the current year and the following year, consistent with ENSO impacts on breeding parameters being mediated by effects on local productivity and prey availability. However, there was no evidence of lagged effects of ENSO on any breeding parameter. Comparison of 5 years revealed that when local surface waters were warm, chicks grew more slowly, but no effects of ENSO on weight and size of eggs were evident in data of 9 and 7 years, respectively. Our findings extend evidence of impacts of ENSO on seabird reproduction to the eastern tropical Pacific and indicate that several breeding parameters of blue-footed boobies (but not egg size) are affected in the short term by ENSO conditions, particularly by local anomalies in sea surface temperature associated with decline in ocean productivity. [ABSTRACT FROM AUTHOR]

Theories of ageing predict that early reproduction should be associated with accelerated reproductive senescence and reduced longevity. Here, the influence of age of first reproduction on reproductive senescence and lifespan, and consequences for lifetime reproductive success (LRS), were examined using longitudinal reproductive records of male and female blue-footed boobies ( Sula nebouxii) from two cohorts (1989 and 1991). The two sexes showed different relationships between age of first reproduction and rate of senescent decline: the earlier males recruited, the faster they experienced senescence in brood size and breeding success, whereas in females, recruiting age was unrelated to age-specific patterns of reproductive performance. Effects of recruiting age on lifespan, number of reproductive events and LRS were cohort- and/or sex-specific. Late-recruiting males of the 1989 cohort lived longer but performed as well over the lifetime as early recruits, suggesting the existence of a trade-off between early recruitment and long lifespan. In males of the 1991 cohort and females of both cohorts, recruiting age was apparently unrelated to lifespan, but early recruits reproduced more frequently and fledged more chicks over their lifetime than late recruits. Male boobies may be more likely than females to incur long-term costs of early reproduction, such as early reproductive senescence and diminished lifespan, because they probably invest more heavily than females. In the 1991 cohort, which faced the severe environmental challenge of an El Niño event in the first year of life, life-history trade-offs of males may have been masked by effects of individual quality. [ABSTRACT FROM AUTHOR]

We assessed population differentiation and gene flow across the range of the blue-footed booby ( Sula nebouxii) (1) to test the generality of the hypothesis that tropical seabirds exhibit higher levels of population genetic differentiation than their northern temperate counterparts, and (2) to determine if specialization to cold-water upwelling systems increases dispersal, and thus gene flow, in blue-footed boobies compared with other tropical sulids. Work was carried out on islands in the eastern tropical Pacific Ocean from Mexico to northern Peru. We collected samples from 173 juvenile blue-footed boobies from nine colonies spanning their breeding distribution and used molecular markers (540 base pairs of the mitochondrial control region and seven microsatellite loci) to estimate population genetic differentiation and gene flow. Our analyses included classic population genetic estimation of pairwise population differentiation, population growth, isolation by distance, associations between haplotypes and geographic locations, and analysis of molecular variance, as well as Bayesian analyses of gene flow and population differentiation. We compared our results with those for other tropical seabirds that are not specialized to cold-water upwellings, including brown ( Sula leucogaster), red-footed ( S. sula) and masked ( S. dactylatra) boobies. Blue-footed boobies exhibited weak global population differentiation at both mitochondrial and nuclear loci compared with all other tropical sulids. We found evidence of high levels of gene flow between colonies within Mexico and between colonies within the southern portion of the range, but reduced gene flow between these regions. We also found evidence for population growth, isolation by distance and weak phylogeographic structure. Tropical seabirds can exhibit weak genetic differentiation across large geographic distances, and blue-footed boobies exhibit the weakest population differentiation of any tropical sulid studied thus far. The weak population genetic structure that we detected in blue-footed boobies may be caused by increased dispersal, and subsequently increased gene flow, compared with other sulids. Increased dispersal by blue-footed boobies may be the result of the selective pressures associated with cold-water upwelling systems, to which blue-footed boobies appear specialized. Consideration of foraging environment may be particularly important in future studies of marine biogeography. [ABSTRACT FROM AUTHOR]

As organisms age, DNA of somatic cells deteriorates, but it is believed that germ cells are protected from DNA-damaging agents. In recent years, this vision has been challenged by studies on humans indicating that genomic instability in germ cells increases with age. However, nothing is known about germ line senescence in wild animals. Here, we examine DNA damage in sperm of a wild vertebrate, the blue-footed booby Sula nebouxii. One of the major types of premutagenic DNA damage generated by oxidative stress (a proximal cause of ageing) is loss of single bases resulting in apurinic/apyrimidinic sites (AP sites). We examined AP sites in the sperm of known-age males sampled during courtship on Isla Isabel, Mexico. We show that damage to the DNA of sperm increases with age of male blue-footed boobies. Moreover, we found that sexual attractiveness (foot colour) declines with age and is correlated with germ line damage of senescent males. By choosing attractive males, females might reduce the probability of their progeny bearing damaged DNA. This study reports the first evidence of senescence in the germ line of a wild vertebrate and future studies should investigate whether this burden of senescence is sidestepped by potential sexual partners. [ABSTRACT FROM AUTHOR]

vian eggshell color is remarkably variable among and within species and its possible function has long been controversial. Female birds use biliverdin pigment to color their eggs blue and green. Although evidence is accumulating that blue-green eggshell coloration is costly to produce, the exact mechanisms underlying its expression are little studied. Biliverdin pigment is involved in important maintenance functions such as immune and antioxidant defenses and shows similar properties to carotenoid pigments. Carotenoids play a role as immunoenhancers and have the potential to ameliorate the impact of oxidative processes, although their antioxidant function could be minor for some bird species. Important life-history components can be limited by carotenoid availability, including sexual displays and fecundity. Here we explored if biliverdin-based eggshell pigmentation was affected by carotenoid availability in a seabird, by performing a carotenoid supplementation experiment in female blue-footed boobies ( Sula nebouxii) after laying their first egg. In this species, blue eggshell color is associated with incubation patterns, and carotenoid availability affects the crucial trade-off between female foot ornamentation and fecundity. We found that under natural conditions, there was a decline in eggshell color with laying order, suggesting pigment limitation for females. However, carotenoid-supplemented females had enhanced second egg coloration compared to controls, and the color increase was independent of their previous pigment allocation to the first egg. Our results suggest that biliverdin-based eggshell coloration is costly to produce and can be alleviated by current carotenoid availability. [ABSTRACT FROM AUTHOR]

The article discusses a study which investigated the hybridization between Blue-footed (Sula nebouxii) and Peruvian (Sula variegata) Boobies on Lobos de Tierra Island and Lobos de Afuera islands in Northern Peru. The researchers took morphological measurements, feather and blood samples over three field seasons. It was found that there was a rare incident of hybridization between the three species.

In marine ecosystems climatic fluctuation and other physical variables greatly influence population dynamics, but differential effects of physical variables on the demographic parameters of the two sexes and different age classes are largely unexplored. We analyzed the effects of climate on the survival and recruitment of both sexes and several age classes of a long-lived tropical seabird, the Blue-footed Booby (Sula nebouxii), using long-term observations on marked individuals. Results demonstrated a complex interaction between yearly fluctuations in climate (both local and global indexes, during both winter and breeding season) and the sex and age of individuals. Youngest birds' survival and. recruitment were commonly affected by local climate, whereas oldest birds' parameters tended to be constant and less influenced by environmental variables. These results confirm the theoretical prediction that sex- and age-related variation in life-history demographic traits is greater under poor environmental conditions, and they highlight the importance of including variability in fitness components in demographic and evolutionary models. Males and females showed similar variation in survival but different recruitment patterns, in relation to both age and the spatial scale of climatic influence (local or global). Results indicate different life-history tactics for each sex and different ages, with birds likely trying to maximize their fitness by responding to the environmental contingencies of each year. [ABSTRACT FROM AUTHOR]

Ten microsatellite loci were isolated and characterized from Blue-footed ( Sula nebouxii) and Peruvian Boobies ( S. variegata). The loci were screened in 24 Blue-footed Boobies and 27 Peruvian Boobies: 8 were polymorphic in Blue-footed Boobies with between 2 and 10 alleles per locus and 9 were polymorphic in Peruvian Boobies with between 2 and 12 alleles per locus. Observed heterozygosity ranged from 0.29 to 0.84. These loci were also tested in Brown Boobies ( S. leucogaster) and were variably polymorphic. These new loci are currently being used to assess population genetic structure in Blue-footed and Peruvian Boobies and will also be used to examine hybridization between the species. [ABSTRACT FROM AUTHOR]

Senescence could depress prenatal and postnatal capacities of mothers to invest in offspring. Longitudinal observations on the blue-footed booby (Sula nebouxii) revealed a quadratic effect of female age on fledgling production and cohort differences in rate of reproductive decline. By swapping clutches between females of different ages, we tested whether reproductive senescence is due to decline in egg quality or capacity to care. As laying mothers aged, egg size, ulna length of 5-day-old chicks, and ulna growth of second chicks up to age 30 days declined, and as rearing mothers aged, ulna growth and cellular mediated immune response of second chicks diminished. Oddly, senescent females (>11 years) produced more fledglings when rearing offspring of middle-aged females (8-11 years) than when rearing offspring of senescent or young females. Thus, senescence reduced egg quality and rearing capacities, and reproductive success of senescent mothers depended on prenatal effects associated with the age of the laying mother. Reproductive senescence of boobies may involve constraints on resources allocated to reproduction as well as adaptive adjustment of provision and care according to offspring value, implying that negative effects of senescence on offspring survival can be ameliorated by plasticity in postlaying or postnatal care. [ABSTRACT FROM AUTHOR]

Contradictory patterns of density-dependent animal dispersal can potentially be reconciled by integrating the conspecific attraction hypothesis with the traditional competition hypothesis. We propose a hypothesis that predicts a U-shaped relationship between density and both natal and breeding dispersal distance. Using 10 years of observations on a breeding colony of the Blue-footed Booby (Sula nebouxii), the hypothesis was confirmed by documenting simultaneous positive and negative density-dependent dispersal distances in natal and breeding dispersal of males and breeding dispersal of females within the colony. Point-pattern analyses demonstrated that the breeding sites of Blue-looted Boobies were highly aggregated in all years within a large study area. and aggregation presumably resulted in heterogeneity in patch density throughout the colony. As predicted, at moderate to high densities, dispersal distances showed positive density dependence, with individuals moving to lower density patches. In contrast, at low to moderate densities, dispersal distances showed negative density dependence, with individuals moving to higher density patches. In both sexes of the 1994 cohort, the higher the mean density in patches used by an individual over the long term (up to age I 1 years), the fewer fledglings it produced. A positive effect of density on long- term reproductive success was not detected, possibly because birds that failed during pair formation or incubation were not sampled. Density of conspecifics may be an important influence on habitat selection of breeders, and dispersal may tend to carry individuals to patches where pair formation opportunities are better and negative effects of competition on reproductive success are reduced. [ABSTRACT FROM AUTHOR]

Females are expected to vary investment in offspring according to variables that may influence the offspring fitness in a way that optimises her inclusive fitness for a particular context. Thus, when sexual ornaments signal the quality of the male, females might invest in reproduction as a function of the attractiveness of their mate. We tested whether breeding conditions and male feet colour influence reproductive decisions of blue-footed booby females. In the blue-footed booby, male feet colour is a dynamic condition-dependent sexually selected trait that is related to paternal effort. During two consecutive years, an El Niño year (poor breeding conditions) and a year with good breeding conditions, we experimentally reduced male attractiveness by modifying their feet colour after the first egg was laid and recorded female investment in the second egg. We found that, relative to the first egg in the clutch, females laid heavier second eggs during the poor year than during the good year. Females paired with males with duller feet colour reduced second-egg mass and volume and delayed the laying of the second egg, independently of the year. Absolute yolk androstenedione (A4) concentration (but not testosterone, T) in second eggs was higher during a poor year than during a good year. Only during a year with poor breeding conditions, females paired with experimental males decreased the relative A4 concentration (but not T) in the second egg compared to control females. Thus, blue-footed booby females probably favour brood reduction by decreasing egg quality and increasing size asymmetry between chicks when the breeding and the mate conditions are poor. [ABSTRACT FROM AUTHOR]

The association between the distribution of blue-footed boobies Sula nebouxii (BFB) and marine productivity around their breeding grounds is unclear. In Peru, they breed in a region seasonally influenced by the cold, nutrient rich Humboldt Current, and by warm, nutrient poor oceanic waters. The foraging range of BFB in Peru is unknown, although some evidence suggest that they may feed in warm offshore waters. During the austral winter 2002 and summer 2003, we tracked 26 BFBs breeding on Isla Lobos de Tierra, Peru (located approximately 15 km west of the mainland), during 59 feeding trips using small direction recorders to determine their marine habitat use. BFBs preferentially moved to the east of the island and foraged over the continental shelf in cold water masses where marine productivity and the abundance of the main prey, the Peruvian anchovy Engraulis ringens, are usually higher than in other areas. Overall, the median maximum foraging distance from the colony was 39 km (range 3–109 km), with 90% of all trips located within 30 km from the mainland, and enclosed in an area of 13,113 km2. These results reveal that BFBs exploit the productive waters of the Peruvian Coastal Upwelling System, rather than open oceanic waters as previously suggested. BFBs travelled a greater distance, dispersed in a broader area, and visited more offshore waters during the winter 2002, than during the summer 2003, following the typical seasonal horizontal dispersion of the Peruvian anchovy. [ABSTRACT FROM AUTHOR]

Background: Exposure to androgens early in development, while activating adaptive aggressive behavior, may also exert long-lasting effects on non-target components of phenotype. Here we compare these organizational effects of perinatal androgens in closely related Nazca (Sula granti) and blue-footed (S. nebouxii) boobies that differ in neonatal social system. The older of two Nazca booby hatchlings unconditionally attacks and ejects the younger from the nest within days of hatching, while blue-footed booby neonates lack lethal aggression. Both Nazca booby chicks facultatively upregulate testosterone (T) during fights, motivating the prediction that baseline androgen levels differ between obligately siblicidal and other species. Methodology/Principal Findings: We show that obligately siblicidal Nazca boobies hatch with higher circulating androgen levels than do facultatively siblicidal blue-footed boobies, providing comparative evidence of the role of androgens in sociality. Although androgens confer a short-term benefit of increased aggression to Nazca booby neonates, exposure to elevated androgen levels during this sensitive period in development can also induce long-term organizational effects on behavior or morphology. Adult Nazca boobies show evidence of organizational effects of early androgen exposure in aberrant adult behavior: they visit unattended non-familial chicks in the colony and direct mixtures of aggression, affiliative, and sexual behavior toward them. In a longitudinal analysis, we found that the most active Non-parental Adult Visitors (NAVs) were those with a history of siblicidal behavior as a neonate, suggesting that the tendency to show social interest in chicks is programmed, in part, by the high perinatal androgens associated with obligate siblicide. Data from closely related blue-footed boobies provide comparative support for this interpretation. Lacking obligate siblicide, they hatch with a corresponding low androgen level, and blue-footed booby adults show a much lower frequency of NAV behavior and a lower probability of behaving aggressively during NAV interactions. This species difference in adult social behavior appears to have roots in both pleiotropic and experiential effects of nestling social system. Conclusions/Significance: Our results indicate that Nazca boobies experience life-long consequences of androgenic preparation for an early battle to the death. [ABSTRACT FROM AUTHOR]

Theoreticians propose that trained winning and losing are important processes in creating linear animal dominance hierarchies, and experiments have shown that both processes can occur in animals, but their actual roles in creating natural hierarchies are unknown. We described agonism in 18 broods of three blue-footed boobies, Sula nebouxii, a species for which trained winning and losing have been demonstrated, to infer how these processes generate and maintain a natural hierarchy. Ranks in the linear hierarchy that emerged in every brood were initially assigned by asymmetries in age, size and maturity, which led to differences between broodmates in levels of expressed and received aggression and, consequently, to differences in the training of their aggressiveness and submissiveness. Later, ranks appeared to be maintained by the chicks'' acquired aggressive and submissive tendencies combined with ongoing effects of persisting differences in size and maturity. Our results suggest that trained winning and trained losing are important in the construction of booby hierarchies but that these two axes of learning are largely independent. Increase in submissiveness occurs over a period of about 10–20 days, and the level of submissiveness reached varies with the amount of aggression received. After training, submissiveness is apparently maintained by a lower level of aggression and increasing use of threats. Threats become increasingly effective as chicks age, but are never as effective as attacks. [Copyright &y& Elsevier]

We tested the hypothesis that stability of a linear dominance hierarchy is due to each individual being trained to a different level of general aggressiveness. This mechanism, combined with assessment of aggressiveness, is particularly likely when individual recognition is absent, as may be the case in young broods of altricial chicks. Dominance relationships in broods of blue-footed boobies involve trained winning and trained losing, and broods of three chicks form stable linear dominance hierarchies. To test the prediction that chicks of intermediate rank in broods of three express an intermediate level of general aggressiveness, we pitted intermediates against similar sized bottom rank chicks from broods of two (equivalent to bottom rank chicks from broods of three). Contrary to expectation, intermediates failed to outperform bottom rank chicks by attacking first, attacking more, or submitting less. This failure suggests that general aggressiveness of intermediate chicks is not enhanced by the experience of attacking their younger broodmates and does not determine their intermediate rank in the brood hierarchy. Differential training of general aggressiveness may well be important in linear hierarchies of broods, but in dyads that do not differ in their training effects, stable dominance may depend on responsiveness to relative size. [ABSTRACT FROM AUTHOR]