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Understanding egg-laying behaviour of brood parasites in space and time can improve our knowledge of interactions between hosts and parasites. However, no studies have combined information on the laying activity of an obligate brood parasite with detailed information on the distribution of host nests within an area and time period. Here, we used molecular methods and analysis of egg phenotypes to determine maternal identity of common cuckoo, Cuculus canorus, eggs and chicks found in the nests of four species of Acrocephalus warblers in consecutive years. The median size of a cuckoo female laying area (calculated as a minimum convex polygon) was correlated negatively with the density of host nests and positively with the number of eggs assigned to a particular female. Cuckoo female laying areas overlapped to a large extent and their size and location did not change between years. Cuckoo females preferentially parasitized host nests located close to their previously parasitized nests and were mostly host specific except for two that parasitized two host species. Future studies should focus on sympatric host and parasite communities with variable densities across different brood-parasitic systems to investigate how population density of hosts affects fitness and evolution of brood parasites. For instance, it remains unknown whether female parasites moving to new sites need to meet a threshold density of a potential host. In addition, young females may be more limited in their egg laying, particularly with respect to the activity of other parasites and hosts, than older females.

Egg rejection behaviour is a host strategy adopted by birds against brood parasitism. The optimal acceptance threshold hypothesis predicts that, when the frequency or risk of parasitism increases, host acceptance thresholds should become more restrictive. Here, we investigated egg rejection behaviour in the Daurian redstart, a cavity-nesting host of the common cuckoo, Cuculus canorus. In a Daurian redstart population located in the northern cold-temperate zone of China where redstarts have two peaks of egg laying during the breeding season, cuckoos can only parasitize redstart nests during the second egg-laying peak due to their late arrival on the breeding grounds. We artificially simulated brood parasitism by introducing a model cuckoo egg into a redstart nest during both egg-laying peaks to test the prediction that redstarts should reject eggs at a greater rate during the second than the first egg-laying peak. In accordance with the optimal acceptance threshold hypothesis, Daurian redstarts showed significantly higher egg rejection rates during the second egg-laying peak. This result indicates that temporal variation in egg rejection behaviour by Daurian redstarts may be an adaptive response to seasonal change in risk of cuckoo parasitism. To our knowledge, this is the first study showing that the egg rejection rate of a host population increases with increasing risk of cuckoo parasitism within a breeding seas.

Coevolutionary arms races between brood parasites and hosts can evolve along several distinct trajectories. To understand why, host–parasite interactions must be examined at every stage of the breeding cycle. While there have been numerous studies of interactions at the egg and nestling stages, studies of preparasitism ‘frontline’ interactions have received less attention. Frontline defences in hosts should be widespread, both because they can preserve the entire host clutch (while postparasitism defences, such as egg or chick rejection, do not) and because they tend to overlap with defences against nest predation. We investigated whether parasitism by the little bronze-cuckoo, Chalcites minutillus, has selected for frontline defences in large-billed gerygones, Gerygone magnirostris, a host that rejects cuckoo chicks. We considered three possible defences: (1) mobbing of adult cuckoos near the nest, (2) ‘decoy nest clustering’ of active nests alongside old nests and flood debris, and (3) cryptic nest architecture. Gerygones were more likely to mob a mount of a cuckoo near the nest than that of a hawk or harmless sympatric passerine. The role of nest traits in repelling parasitism was equivocal; gerygones built their nests alongside previously used nests more often than expected by chance. The experimental addition of decoy nests did not reduce parasitism rates but did lead to a significant delay in latency to predation. Although large-billed gerygones build large nests compared to other gerygones species, we found that larger nests were more likely to be parasitized than smaller nests. We conclude that parasitism has selected for a portfolio of defences in large-billed gerygones, comprising both low-cost, but mostly ineffective, frontline defences (mobbing, some nest traits), and a high-cost, but highly effective, chick stage defence (chick rejection). Thus, the relatively low effectiveness of frontline and egg stage defences may explain why some hosts evolve the rare defence of chick rejection.

Brood-parasitic cuckoos lay their eggs in the nests of other birds and thereafter abandon their young to the care of the host. Thus, all maternal investment is restricted to investment in the egg. Optimal investment at this stage is likely to have a large impact on maternal reproductive success. Many bird species optimize the size of their eggs to suit both the prevailing environmental conditions and the number of individuals that will provide care to the chicks. However, relatively few cues are available to avian brood parasites to facilitate optimal investment in their eggs. Moreover, optimization of egg size to suit environmental conditions or the social structure of the host group may be constrained by stronger selection for egg mimicry, which reduces the likelihood of detection and rejection of foreign eggs by the host. We aimed to test how the conflicting selection pressures of (1) selection for large eggs in harsh environmental conditions versus (2) rejection of large eggs by hosts interact to influence the size and shape of Horsfield's bronze-cuckoo, Chalcites basalis, eggs. Using a sliding window approach to investigate periods of climate sensitivity, we show that climatic variables did not predict egg size. Nor did cuckoos modify their egg size or shape in relation to the group size of the host (superb fairy-wren, Malurus cyaneus). Conversely, host defences did appear to have influenced cuckoo egg morphology: eggs that were relatively short and round had a higher probability of being abandoned by the host than eggs that were long and narrow. This suggests that host defences are the overriding selection pressure on cuckoo egg morphology, and, unlike their hosts, cuckoos do not adapt their eggs to the prevailing climate or host group size.

Nestlings of the obligate brood-parasitic common cuckoo, Cuculus canorus, use diverse signals to manipulate host parents into feeding them. Begging calls, one of the most prominent parent–offspring communication signals, have been suggested to differ between cuckoos parasitizing different host species but the call characteristics involved differ between studies. We studied nestling begging calls of the cuckoo and two closely related cuckoo hosts breeding in sympatry, the great reed warbler, Acrocephalus arundinaceus, and the reed warbler, Acrocephalus scirpaceus, to determine whether cuckoo nestlings adjust their begging calls according to the host species and if these calls vary with cuckoo age and sex. We found that begging calls of host nestlings differed markedly between species but there was no difference in the begging calls of cuckoo nestlings raised by great reed warblers and those raised by reed warblers. These results suggest that cuckoo nestlings do not adjust begging vocalizations to match host species but rather use general begging call features to solicit food. However, their call frequency band narrowed, syllable duration shortened and call rate tended to increase with increasing age. None of the begging call characteristics differed between the sexes. The rapid development of cuckoo begging call parameters during ontogeny suggests that any comparisons of begging calls of cuckoo nestlings raised by different host species must control for nestling age. Finally, some discordant conclusions of this and other studies emphasize how little we understand parasite–host communication.

One of the most effective defensive strategies of hosts against brood parasites is rejection, commonly achieved by ejection of the parasitic egg or desertion of the parasitized nest. Nest desertion should be a costlier strategy than egg ejection, because birds must thesn spend additional time and energy renesting, and therefore we still cannot explain why some individuals desert their nests rather than eject parasitic eggs and continue a given breeding attempt. The great reed warbler, Acrocephalus arundinaceus, is a frequent host of the common cuckoo, Cuculus canorus, and is known to use both types of rejection response. By measuring cuckoo egg mimicry, we investigated the hypothesis that the hosts desert if they cannot reliably recognize the cuckoo egg in their nest. We predicted that we would find better mimicry when hosts deserted rather than ejected. However, we did not find a difference in mimicry between these two groups of nests, implying that host females do not desert because they cannot reliably recognize the parasitic egg. We also showed that neither the date in the season nor the age of the host females influenced the type of rejection. Other factors potentially eliciting nest desertion, including host personality, host, inability to eject, excessive clutch reduction and visibility of the cuckoo female at the host nest, are discussed. Finally, we suggest that desertion may persist as a host defensive strategy against brood parasitism because it is not as costly as previously assumed and/or it is beneficial for host females in good physical condition.

Collective signalling occurs in diverse animal groups. A particularly well-studied form of this sophisticated communication behaviour is vocal duetting in birds, in which members of a mated pair coordinate their songs on short temporal scales. Many hypotheses have been proposed to explain the function of bird duets, but the experimental evidence for them is still somewhat conflicting. Studies on songbirds often provide support for the joint resource defence or mate-guarding hypotheses. We investigated these hypotheses for a nesting cuckoo, the white-browed coucal, Centropus superciliosus, by presenting territorial pairs with four types of playback treatment: male–female duets, female solos, male solos, and a heterospecific control. The observed birds reacted aggressively to the conspecific playback by approaching the playback loudspeaker and singing. Pairs responded with duets more often to duet playbacks than to playbacks of female and male solo song. Also, neither sex treated simulated unmated intruders (solo playback) as more threatening than mated intruders (duet playback). Taken together, these findings argue against the mate-guarding hypothesis and instead suggest that duetting in white-browed coucals functions primarily in joint territory defence. In almost all cases, males were the first sex to sing in response to playbacks and, overall, they sang more than females. This suggests that males take a primary role in territorial defence. However, females also responded strongly, especially to the simulation of mated or unmated female intruders, hinting at a sex-specific division of labour in territorial defence.

Common cuckoos, Cuculus canorus, are brood parasites: they lay their eggs in the nests of other bird species, and let these hosts incubate their eggs and feed and rear the nestlings. Although cuckoos do not show parental care, they demonstrate complex social interactions, including territorial behaviours and male–male aggression. Cuckoos have a well-known and simple two-phrase call (‘cu’ and ‘coo’), uttered by males during their breeding season. Previous studies suggested that the ‘cu-coo’ call of males is individually unique, potentially allowing discrimination between different classes of males. Using playback experiments in a dense population of radiotagged cuckoos, we tested whether neighbouring males are tolerated more than unfamiliar intruders: the classic ‘dear enemy’ phenomenon. Focal birds responded more aggressively to the calls of unfamiliar simulated intruders (strangers) than to the calls of conspecifics with whom they shared territorial boundaries (familiar neighbours). Cuckoos responded quickly, within, on average, less than half a minute; they often approached the loudspeaker to within 5–10 m, even from up to 80 m away, and used their ‘cu-coo’ calls in response. Our results showed that cuckoos used their simple call for the discrimination of familiar versus unfamiliar individuals, and did so specifically to defend their own territories. In turn, cuckoos showed tolerance to nearby conspecifics, for example neighbours with overlapping territories, and did not respond to control playbacks. Finally, as typically more than one cuckoo was interested in the playbacks, this study confirmed the opportunity for brood-parasitic birds to socialize during the breeding season.

Recent studies have suggested that parasitic cuckoos have evolved laying behaviour resulting in matching of host and cuckoo eggs by choosing to lay eggs in host nests with host eggs that match the cuckoo eggs as an adaptation against egg recognition by the hosts. However, previous studies provided weak and indirect evidence with mixed results, leaving this question unresolved. Here, for the first time, we developed a robust methodology to provide unambiguous evidence that egg recognition in the host does not select for optimal egg matching during laying by the cuckoo. By using experiments that attracted parasitism, we showed that cuckoos, Cuculus canorus, indiscriminately laid eggs in oriental reed warbler, Acrocephalus orientalis, host nests containing real host eggs, egg-shaped models, stick models or coin models without any preference. Furthermore, cuckoos only selected to lay their eggs in nests with active hosts. These experiments provide evidence of cuckoos being indiscriminate in their choice of host nests, implying that coevolution of the egg phenotype of host and cuckoo eggs must have arisen from mechanisms other than matching of host eggs and those of the parasite.

Avian brood parasites lay their eggs in the nests of other species, imposing high costs on their hosts. In theory, this should select for the evolution of defences against parasitism in hosts, yet eviction of parasite eggs or chicks is absent in many host species. One such host is the yellow-rumped thornbill, Acanthiza chrysorrhoa, the primary host of the shining bronze-cuckoo, Chalcites lucidus, in Australia. Here we tested whether the lack of egg and chick rejection in yellow-rumped thornbills has led to the evolution of alternative defences against brood parasitism. We provide evidence that this host has evolved two types of frontline defences. First, yellow-rumped thornbills responded to the presence of a cuckoo mount near the nest with mobbing behaviour and by occupying their dome-shaped nests, potentially as a means of blocking the small entrance hole. Second, we show that brood parasitism imposes directional selection for early breeding on the yellow-rumped thornbill and that yellow-rumped thornbills show a concomitant shift in their breeding phenology, breeding earlier than both congeneric and sympatric species. Our results highlight the importance of studying apparently defenceless hosts in order to identify alternative defence strategies.

Fitness costs associated with brood parasitism have led host species to evolve several lines of defence. The first two lines of defence, aggression against adult parasites and egg rejection, are present at varying levels in almost all hosts. However, it remains unclear how these two fundamental defences covary at host individual level, with previous studies suggesting both positive and negative correlations. A theoretically critical yet empirically untested scenario is that variation in host antiparasite behaviour may relate to individual variation in host behavioural types or personalities. Here we examined whether host aggression against adult brood parasites and egg rejection behaviour were correlated with host behaviours displayed outside the context of brood parasitism. We selected the great reed warbler, Acrocephalus arundinaceus, a favourite cuckoo, Cuculus canorus, host as a suitable model. Only females reject foreign eggs and show high individual repeatability of both aggression towards cuckoos and nest guarding. We found that female behaviours in different situations (nest guarding, nest defence, handling in the net) were strongly correlated with each other. This is the first empirical evidence on correlation between individually consistent antiparasite adaptation (female nest defence) and behaviours that are not directly related to brood parasitism. In contrast, egg rejection/acceptance responses and latency to these responses did not correlate with any of the female defence/guarding behaviours and behaviour during handling. Proximately, this may be because nest defence and egg recognition represent cognitively and behaviourally completely different tasks. These patterns were not affected by female mating status in this polygynous cuckoo host. We hypothesize that differences in host behavioural types, rather than host egg discrimination ability, may predict host nest defence behaviour against adult brood parasites in general.

A fundamental principle of Batesian mimicry is that it pays to look like a local harmful species that is recognizable to other local species (receivers). Mimicking an allopatric species confers no benefit, as it is not recognizable to local species. It is thought that the common cuckoo, Cuculus canorus, is a Batesian mimic of the Eurasian sparrowhawk, Accipiter nisus, predominantly via its barred plumage, which facilitates access to host nests to deposit eggs. Barring is widespread in five genera of Old World cuckoos, unlike nonparasitic cuckoos, and evolved after the evolution of parasitism. Although barred plumage is predominant in parasitic cuckoos, it is unclear whether it may have a widespread function in cuckoo–hawk mimicry. If widespread, there should be a visual similarity between all five genera of Old World parasitic cuckoos and sympatric raptors. In addition, given that it pays to look like a local harmful species, sympatry should predict the degree of similarity. We compared barred plumage from all five genera of parasitic Old World cuckoos and up to eight sympatric raptors using digital image analysis. Cuckoos predominantly matched most raptors for at least one pattern attribute. In addition, three out of five cuckoos closely resembled a sympatric raptor for all barred pattern attributes examined, and potential model species were not confined to sparrowhawks. Habitat did not appear to influence plumage pattern similarity in most species studied. Finally, the barred plumage of sympatric species was more similar in appearance than those in allopatry. Together this demonstrates that cuckoos look like a local harmful species, which is congruous with Batesian mimicry.

Polygyny typically has negative fitness consequences for secondary females, but may equally impose costs on primary females or even on polygynous males. We investigated how polygynous and monogamous great reed warbler, Acrocephalus arundinaceus , males assist their mates with aggressive nest defence against the common cuckoo, Cuculus canorus , and whether the females adjust their nest defence intensity according to male investment in aggression. Additionally, we investigated whether host social mating status affects host vulnerability to parasitism. We presented taxidermic cuckoo mounts at nests of primary, secondary and monogamous females, and recorded aggressive responses of nest owners. We found that monogamous males defended their nests most aggressively while polygynous males allocated their nest protection effort unevenly between their two mates, responding more vigorously on the primary than secondary nests. In contrast, nest defence intensity of females did not differ with respect to their social status, indicating that females of polygynous males did not compensate for low levels of male aggression. Similarly, we found no differences in natural cuckoo parasitism rates between monogamous, primary and secondary nests. Our results thus suggest that while monogamous females receive more assistance with nest defence than females of polygynous males, this has no effect on the probability of parasitism.

In communally breeding animal societies, theory predicts that a male's investment in parental care should be correlated with his share of paternity in the mixed brood. Here I test this hypothesis in the greater ani, Crotophaga major, a Neotropical cuckoo that nests in groups composed of two to three unrelated, behaviourally monogamous pairs. Each group constructs a single nest in which all of the females lay eggs, and all group members participate in rearing the joint clutch. Previous work has shown that parental investment among males is unequal: just one male in the group performs all nocturnal incubation, and the same male also plays a greater role in diurnal incubation and nest defence. I used parentage and sibling analysis of 357 greater ani nestlings in 53 clutches to investigate genetic mating patterns and the distribution of reproduction within communal groups. Contrary to predictions, male reproductive skew was negligible and nocturnally incubating males did not sire significantly more nestlings than did non-nocturnal incubators. Approximately 75–80% of nestlings were produced by socially monogamous pairs, 12–18% by extrapair fertilizations within the same breeding group, and 3–5% by extrapair fertilizations outside the breeding group. There was no difference in the frequency of extrapair paternity between nocturnally incubating and non-nocturnally incubating males. Compared to other cooperative avian societies, the division of reproduction in greater ani breeding groups is unusually egalitarian, despite marked inequalities in male parental care.

Fitness costs imposed by the common cuckoo, Cuculus canorus, on its hosts select for host defences such as nest defence and egg discrimination. The efficiency of both types of defence varies among host species, host populations and even seasonally within a host population. We examined changes in egg recognition as defence against cuckoo parasitism in the rufous-tailed scrub robin, Cercotrichas galactotes, both seasonally and over a decade. The rejection rate decreased from 64.7% to zero within 10 years following the cuckoos' disappearance from the area. In this host species, egg recognition without rejection had previously been reported and in this study, using nonmimetic model eggs, we found that, in the absence of cuckoos, females lost their motivation to reject despite maintaining their discrimination capacity. During presentations of dummy cuckoos and other species, rufous-tailed scrub robins did not recognize the female cuckoo dummy as an enemy and its presentation near the host nest did not increase egg rejection, the rejection rate remaining very similar regardless of the dummy species placed near the nest. We suggest that other cues such as the sight of cuckoos flying across the breeding area or directly to the nest are responsible for a fine-tuned assessment of the risk of parasitism in this host species. The sharp decrease in rejection rates according to perceived risk of parasitism reported here reflects phenotypic plasticity, an adaptive mechanism that enables animals to adjust their behaviour to a rapidly changing environment.

Understanding the causes and consequences of avian dispersal is important since dispersal movements may influence individuals’ survival and reproductive success. Although obligate interspecific brood parasitism has obvious detrimental effects for its avian host, few empirical studies have addressed the question of how brood parasitism may influence host dispersal. We studied factors affecting, and consequences of, natal and breeding dispersal movements in a population of magpies, Pica pica, parasitized by the great spotted cuckoo, Clamator glandarius, by monitoring dispersal of individuals and cuckoo parasitism over 6 years. Recruitment probability increased with fledging weight and was higher for individuals hatching early in the breeding season. Heaviest recruits at fledging settled closer to their natal nests and bred in more saturated sites within the study area. Natal dispersal distance did not predict the magpie’s productivity or risk of cuckoo parasitism in its first reproductive attempt. Females and parasitized males decreased their breeding dispersal distance when breeding close to other pairs, whereas nonparasitized males breeding close to other pairs dispersed more in subsequent years. Females, but not males, breeding closer to their previous breeding places had larger clutches than those breeding further away. Dispersing longer or shorter distances between breeding attempts had no consequences in terms of cuckoo parasitism avoidance for magpies. Our results suggest a minor role of great spotted cuckoo parasitism on magpie dispersal movements at the spatial scale analysed. Instead, our study revealed density-dependent dispersal in our parasitized magpie population.

It is generally assumed that begging signals provide parents with reliable information on the state of their young, allowing them to decide accordingly how much to invest in their nestlings. Avian brood-parasitic nestlings exhibit begging displays that are highly exaggerated. In most species, they also increase their begging level in relation to their level of hunger, as predicted by models of honest signalling. However, it has been suggested that great spotted cuckoo, Clamator glandarius, nestlings are an exception to this general rule because they spit out food when satiated and continue begging at a high intensity. We tested this assertion by using both natural observations of food delivery by foster parents and a laboratory experiment in which we hand-fed both cuckoo and host magpie, Pica pica, and carrion crow, Corvus corone, nestlings while controlling for the degree of food deprivation. First, we found that cuckoo nestlings did not throw out food because they are satiated. Second, as in host magpie nestlings, during a period of food deprivation the cuckoo nestlings begged at a significantly higher frequency and intensity than when fed ad libitum (a period of food abundance) and begging signals increased with an increased level of hunger in both species. These results indicate that in this cuckoo species the frequency and intensity of begging are in accordance with food requirements, and, thus, that begging is not dishonest in the sense of being unreliable.

The close resemblance between the common cuckoo, Cuculus canorus, and the Eurasian sparrowhawk, Accipiter nisus, is often viewed as Batesian mimicry evolved by the cuckoo in response to the aggression of its host. However, the effectiveness of such mimicry is poorly known. We examined cuckoo–hawk discrimination ability in the great reed warbler, Acrocephalus arundinaceus, a well-known and aggressive cuckoo host. We measured the responses of birds to three combinations of simultaneously presented taxidermic mounts of the cuckoo, sparrowhawk and turtle dove, as a harmless control, placed near their nests. Great reed warblers clearly discriminated the two enemies from the innocuous species. They always attacked cuckoos and sparrowhawks more often than turtle doves suggesting they considered both a danger to their broods. However, when we confronted the tested birds with the simultaneously presented mounts of these species, the parents attacked the cuckoo more frequently than the sparrowhawk. The results revealed that although great reed warblers attacked both cuckoo and sparrowhawk mounts near their nest, they were able to discriminate between them. This may suggest that if cuckoos had evolved the visual mimicry of a sparrowhawk to avoid host attacks, this mimicry may be unsuccessful not only because it is imperfect and hosts can learn to discriminate but also because of the generalized nest defence of more aggressive hosts.

Avian brood parasitism reduces the reproductive success of hosts. Thus, hosts have evolved antiparasite strategies such as rejection of parasitic eggs and aggressive nest defence against brood parasites. Japanese bush warblers, Cettia diphone , do not reject the parasitic eggs of little cuckoos, Cuculus poliocephalus , because their eggs mimic warblers’ eggs. To examine whether Japanese bush warblers show nest defence behaviour against their parasite, I investigated the responses of the warblers to a little cuckoo dummy. Japanese bush warblers are residents, while little cuckoos are summer visitors; thus, the warblers start breeding earlier than the cuckoos. Accordingly, the risk of being parasitized increases as the season progresses. Therefore, I also examined whether the intensity of nest defence differed before and after cuckoo arrival. Japanese bush warblers showed remarkably aggressive responses to the cuckoo dummy, and levels of aggression were higher after the cuckoo arrived. These results suggest that Japanese bush warblers adaptively adjust their nest defence behaviour in response to increased risk of being parasitized. This study indicates that differences in migratory habits between hosts and brood parasites produce seasonal changes in antiparasite behaviour.

To aid discrimination of mimetic cuckoo eggs, hosts of avian brood parasites would theoretically benefit by laying clutches that are more uniform in appearance and by laying clutches that differ from those of other females in the population. Support for these hypotheses is equivocal, particularly amongst studies that have utilized reflectance spectrophotometry; quantitative studies comparing clutch variation between parasitized and unparasitized host populations have supported these hypotheses, whereas experimental studies comparing clutch variation between rejector and acceptor individuals within a population generally have not. Here we extend the approach of comparing population differences in clutch variation by asking whether parasitized populations that suffer relatively higher parasitism rates, and are therefore subject to more intense selection for host defences, show lower within-clutch variation and higher between-clutch variation than populations with low parasitism rates. We used reflectance spectrophotometry to compare clutch variation in two hosts of the pallid cuckoo, Cuculus pallidus, each of which exhibits two geographically distinct subspecies that differ in parasitism rates (red wattlebirds, Anthochaera carunculata, and yellow-throated miners, Manorina flavigula). A third host species, the yellow-tufted honeyeater, Lichenostomus melanops, was included as a control. Our results do not support either hypothesis, and are consistent with experimental studies that also failed to support the hypothesis, casting doubt on the generality of these processes.

The coevolutionary arms race between cuckoos and their hosts has famously yielded cuckoo eggs that evade host recognition and rejection by mimicking the appearance of the host's own clutch. But not all coevolutionary interactions between cuckoos and hosts have followed the same pathway. Several host species do not show egg rejection even when the cuckoo's egg looks entirely unlike their own. For example, hosts of some Australian bronze-cuckoos, Chalcites spp., routinely accept olive-brown cuckoo eggs that look very different from the speckled white eggs of their own clutch. Here we investigate the hypothesis that these cuckoo eggs are cryptic, which might explain why hosts do not remove them from their clutch. First, we use a phylogenetic analysis to show that dark bronze-cuckoo eggs are not ancestral, but instead have evolved in a group that parasitizes hosts with dark nests exclusively. Second, we show that dark bronze-cuckoo eggs are laid by two species that parasitize hosts with relatively dark nests, whereas a congeneric bronze-cuckoo species parasitizing host nests with greater ambient light levels lays a mimetic egg. Finally, we use a model of avian visual processing to show that the dark eggs of Gould's bronze-cuckoo C. russatus are cryptic in dark host nests. Our results support the hypothesis that some bronze-cuckoo species and their hosts have pursued an alternative coevolutionary trajectory, which has resulted in the evolution of cryptic, rather than mimetic, cuckoo eggs.

Interactions between brood parasites and hosts can lead to coevolutionary arms races, whereby hosts evolve defences such as ejection of alien eggs and nest defence, and parasites respond with mimetic eggs and shorter laying times. However, little is known about interactions between brood parasites and cooperatively breeding hosts. The presence of several caregivers may either benefit the brood parasite by increasing reproductive success or be a disadvantage, if helpers improve nest defence. We addressed these hypotheses in cooperative carrion crows, Corvus corone corone, and their brood parasite, the great spotted cuckoo, Clamator glandarius, whose chicks are raised together. Parasitism rate by great spotted cuckoos was inversely correlated with crow group size and unassisted pairs were most likely be parasitized, in spite of being relatively rare. Moreover, in the nests of unassisted pairs, cuckoo chicks enjoyed a greater age advantage with respect to crow chicks compared to nests of larger groups, suggesting that parasitic females could not optimize laying date in nests with helpers. Although multiple caregivers increased cuckoo chick survival, this merely compensated for the smaller age advantage that parasitic chicks enjoyed in nests with helpers compared to those of unassisted pairs. Our results indicate that the presence of helpers reduces the cuckoo's laying performance, providing the first indication that cooperative breeding may protect against brood parasitism. This might occur in crows because the presence of helpers allowed incubating females to increase the time spent at the nest, reducing the parasite's opportunities to lay eggs.

In birds, multiple parasitism is the laying of two or more eggs by one or more parasitic females in a single host nest. Several cognitive mechanisms may explain how multiple parasitism could affect parasite egg discrimination by hosts. Rejection based on discordance predicts that multiple parasitism provides a perceptually more error-prone way for hosts to reject parasitism because more foreign eggs decrease the chance that any one egg is perceived as most dissimilar and recognized as foreign, unless parasite eggs are all similarly highly nonmimetic. In contrast, rejection based on clutch uniformity predicts that in multiple parasitism egg rejection is more error-proof if mimicry by parasite eggs is variable, because increased variation in egg appearance makes for easier egg rejection for hosts. Finally, true egg recognition, that is, rejection based on memory of the host's own eggs, predicts no differences in rejection rates from nests with single or multiple parasitism. We studied common cuckoos, Cuculus canorus, parasitizing a population of great reed warblers, Acrocephalus arundinaceus, in Hungary where multiple parasitism was frequent. Hosts rejected parasite eggs less often in nests with multiple parasitism than in nests with single parasitism. These observations were confirmed by experimental parasitism and support the rejection based on discordance hypothesis. As hosts were more likely to tolerate cuckoo eggs in nests with multiple parasitism, we found that multiple parasitism more than doubled cuckoos' reproductive output per host nest compared to single parasitism.

Brood-parasitic birds such as cuckoos and cowbirds lay eggs with unusual by strong shells. Three main hypotheses have been proposed to explain the adaptive significance of this trait. The present study focused on the puncture resistance hypothesis and tested its critical prediction that increased eggshell strength enhances the probability that the parasitic egg is accepted by the host. To address the problem, we experimentally parasitized marsh warbler, Acrocephalus palustris, nests with three types of real eggs having similar size but differing in eggshell strength and/or mimicry: (1) great reed warbler, Acrocephalus arundinaceus, eggs painted to be nonmimetic; (2) common cuckoo, Cuculus canorus, eggs painted in the same way; and (3) unmanipulated cuckoo eggs. When we controlled for mimicry, ejection of strong-shelled (cuckoo) eggs was considerably more costly than ejection of weak-shelled (great reed warbler) eggs. However, nonmimetic cuckoo eggs were not more likely to be accepted than nonmimetic great reed warbler eggs, suggesting no effect of eggshell strength alone on rejection decisions. Mimetic cuckoo eggs were accepted more often than the eggs painted to be nonmimetic suggesting that mimicry primarily determines the probability of rejection. Thus, we found no support for the puncture resistance hypothesis in marsh warblers, which is a host with well-developed defence mechanisms against cuckoo parasitism.

The common cuckoo, Cuculus canorus, is a brood parasite that monopolizes parental care of its host species: soon after hatching, the chicks remove the host offspring. Although cuckoo chicks trick their foster parents into providing enough food, it is unknown whether cuckoo begging behaviour represents an advantage over that of the host chicks in a hypothetical competitive scenario. We studied the feeding behaviour of rufous bush robins, Cercotrichas galactotes, when rearing their own and parasitic chicks in natural and in experimental nests where a cuckoo and a host brood were presented simultaneously to parents. In natural parasitized and nonparasitized nests, the feeding rate for cuckoo chicks did not differ from that of a single host chick of the same age, but cuckoos were fed with a different diet and with larger prey. Thus, cuckoo chicks received a similar amount of food to that received by a whole host brood. Cuckoo chicks in experimental nests did not receive a diet, prey size or feeding rate different to that received by a single host chick and thus received considerably less food than cuckoo chicks in unmanipulated nests. These results suggest that cuckoo chicks could not outcompete host chicks, at least when parasitizing this host species, and thus that their eviction behaviour is beneficial. We discuss various explanations for the inability of cuckoos to outcompete host chicks.

We examined how different facets of guira cuckoo sociality influence reproductive investment in eggs (mass, volume, albumen, yolk, shell weight and thickness). The highly variable egg size in this species suggests the possibility of reproductive manipulation by females. Our data support the hypothesis that investment in eggs and clutches reflects a social context. The lowest egg ejection probabilities occurred in very small communal clutches where there were fewer laying females. Eggs retained in the nest had larger yolks than those ejected, a possible mechanism for minimizing energy expenditure in eggs with high chances of failure. Results showed that eggs laid in smaller communal clutches had, on average, less investment in all nutrient measures, except yolk, compared with eggs from larger communal clutches. It may be important for eggs to be of high quality in larger communal clutches, where competition between hatchlings will most likely be intense. Larger guira cuckoo eggs in this study produced heavier chicks. We found that chicks that were smaller and lighter at hatching were victimized by infanticide significantly more than chicks that were heavier and bigger at hatching.

Brood-parasitic young are reared in the nests of different species and can derive no genetic benefit from the survival of host offspring. However, although the nestlings of many parasitic cuckoo and honeyguide species routinely kill host young soon after hatching, nestling brown-headed cowbirds, Molothrus ater, tolerate host offspring and are commonly reared alongside them for at least part of the nestling period. I used comparative analyses of data from the literature to investigate whether brown-headed cowbird nestlings gain direct benefits by allowing host young to live. The brown-headed cowbird (44 g) parasitizes many passerines (adult mass range about 5–90 g) and the likelihood that host young survive to fledge from parasitized nests varies between species. In common with previous work, I found that host offspring mortality was highest in species whose offspring were relatively small compared with the cowbird nestling. Furthermore, cowbird nestlings were most likely to fledge when reared alongside host young of intermediate size. In these nests, one or two host young typically fledged as well. I suggest that cowbirds, and other host-tolerant brood parasites, could benefit from the presence of host nestlings through the assistance that host chicks offer in soliciting a higher provisioning rate, and that such benefits might outweigh the costs of having competition for food at each nest visit. Variation in this cost–benefit ratio could explain differences between brood parasite species in their tolerance of host young.

We compared nestling begging calls of four hosts (reed warbler, Acrocephalus scirpaceus; great reed warbler, A. arundinaceus; dunnock, Prunella modularis; and meadow pipit, Anthus pratensis) and the respective host-races of the common cuckoo. Note structure varied between host species, but not between cuckoo host-races, so cuckoos did not vary their call note structure to match that of their hosts' chicks. Call rate increased with age, but there were marked differences between both host species and cuckoo host-races. Dunnock-cuckoos called more rapidly than reed warbler-cuckoos despite growing at the same rate. We suggest this difference reflects how cuckoos tune into the way these host species respond to begging signals from their own young, because dunnock chicks called much more rapidly than reed warbler chicks. Great reed warbler-cuckoos called at a lower rate than reed warbler-cuckoos when young, but at a greater rate when older than 8 days. This could also result from the cuckoo chicks tuning into differences in the way these hosts respond to begging signals. However, great reed warbler-cuckoos grew at a faster rate than the other cuckoo host-races, so they may also call faster to demand higher provisioning rates from this larger host. To test these hypotheses critically, data are needed on how the different host species integrate visual and vocal begging signals from their own broods. We discuss how differences in cuckoo begging might develop, given that cuckoo host-races are restricted to female cuckoo lineages. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Previous studies have shown that reed warblers, Acrocephalus scirpaceus , are more likely to reject a cuckoo, Cuculus canorus , egg if they have seen a cuckoo at their nest. This suggests that they would benefit from watching out for cuckoos. We tested whether presentations of a cuckoo mount near the nest (to simulate nest inspection) led to increased nest attendance by the warblers. Cuckoo presentations at completed nests before laying, when males guarded their females closely, led to desertion at 40% of nests before any eggs were laid (there were no desertions after presentations of a jay, Garrulus glandarius , a nest predator). In the remaining cases, there was no effect of the cuckoo on nest attendance before laying began, but a marked increase in male nest attendance (compared with jay and no-presentation controls) on the days the first and second eggs were laid. Cuckoo presentations at the one-egg stage led to the same increase in male nest attendance as did the prelaying presentations. Increased male nest attendance at the one–two-egg stage was not at the expense of mate guarding, because this declined anyway when laying began, and it did not lead to increased paternity loss compared with controls. Overall, 15% of broods had one or two extrapair young (6% of all young extrapair). We conclude that male reed warblers do increase nest guarding in response to cuckoos, but only after their females have begun egg laying, when there are less likely to be costs in lost paternity. Females did not increase nest guarding, perhaps because they need to spend more time foraging during the egg-laying period. Our results suggest that cuckoos should be secretive not only when they lay but also when they monitor host nests beforehand. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Egg mimicry is an important adaptation of common cuckoos, Cuculus canorus, against rejection of eggs by their respective hosts. A precondition for the maintenance of egg mimicry is that female cuckoos find hosts with a matching egg type. Experimental evidence indicated that habitat imprinting may be important for host selection. We tested whether the spacing and laying patterns of female cuckoos in the field are compatible with the supposed habitat-imprinting mechanism. We observed 16 females, with the help of radiotelemetry; of seven females, we observed directly 26 egg layings and 27 nest visits without laying. As expected if females were imprinted on different vegetation types, (1) the distribution of vegetation types differed between female home ranges, (2) female habitat use differed from average habitat availability within the egg-laying area (habitat preference), (3) females visited nests and deposited their eggs in the habitat they preferred, and (4) females laid their eggs consistently in a particular habitat type, irrespective of the host species. These results indicate that cuckoo females show habitat preference when searching for suitable host nests. Hence our data are compatible with the habitat-imprinting hypothesis, but owing to the habitat specificity of hosts the data cannot disprove a potential role of host specificity in cuckoo females.

The village weaverbird, Ploceus cucullatus, lays eggs of an extremely broad range of appearance between individuals. This variation is thought to have evolved as a counteradaptation to brood parasitism by the diederik cuckoo Chrysococcyx caprius. The primary objective of our study was to characterize the relationship between egg appearance and egg rejection in the African village weaverbird. We test predictions of three hypotheses in a study in The Gambia, West Africa: (1) interindividual egg variability permits individuals to discriminate between own and foreign eggs by rejecting eggs in proportion to the difference in appearance from their own; (2) village weavers remember the appearance of their own eggs and do not require a discordancy within their clutch, nor even the presence of one of their own eggs, in order to distinguish a foreign egg as such; and (3) colour and speckling contain the signature information by which village weavers can distinguish their eggs from foreign ones; whereas shape and mass, being less reliable, do not. We analysed rejection behaviour according to egg appearance differences by logistic regression. Results supported all three hypotheses. We estimated the predictive efficacy of our model, the amount of explained variation and the relative contribution of various egg appearance factors to discrimination by the host. These results are consistent with the hypothesis that interindividual egg variation in this species facilitates offspring recognition and is a counteradaptation to either interspecific or intraspecific brood parasitism.

Most previous studies of brood parasitism have stressed that host defences, such as egg recognition, are lost in the absence of parasitism. Such losses could result in coevolutionary cycles in which parasites shift away from well-defended hosts only to switch back to them later at a time when these hosts have lost much or all of their defences and the parasite's current hosts have built up effective defences. However, the alternative 'single trajectory' model predicts that parasites rarely switch back to old hosts because ex-hosts retain egg recognition for long periods in the absence of parasitism. If true, egg recognition by the host may be a 'relic behaviour', because in the absence of parasitism its adaptive value is close to neutral. Using artificial nonmimetic eggs, I tested for egg recognition in two populations that are currently unparasitized but that are descended from lineages likely to have been parasitized in the past: the grey catbird, Dumetella carolinensis, on Bermuda and the loggerhead shrike, Lanius ludovicianus, in California. Both of these populations showed long-term retention, ejecting nonmimetic eggs at rates of nearly 100%. Because potential present-day selection pressures, such as conspecific parasitism, do not explain this egg recognition, Bermuda catbirds apparently retain recognition from North American conspecifics that were cowbird hosts before colonizing Bermuda and shrikes retain recognition from Old World congeners that were hosts of cuckoos. Retention is also indicated by passerines in California and the Caribbean that had high rejection rates of nonmimetic eggs before coming into contact with cowbirds. These new data suggest that both the coevolutionary cycles and single trajectory models have importance and that rejection behaviour can have insignificant costs, which is consistent with evolutionary lag explanations for the acceptance of parasitic eggs shown by some cuckoo and many cowbird hosts. Copyright 2001 The Association for the Study of Animal Behaviour.

Avian brood parasites reduce the reproductive success of their hosts, selecting for the evolution of egg discrimination by the host, and potentially creating a coevolutionary arms race between host and parasite. Host egg discrimination ability is crucial in determining whether the arms race results in extinction (of the parasite on a particular host) or stable coevolutionary equilibrium of the host-parasite pair. I examined egg discrimination behaviour in the yellow-browed leaf warbler, Phylloscopus humei, a presumed former host of parasitic cuckoos, to show how discrimination ability has become very strong. Field experiments using model eggs demonstrate that rejection decisions are based on the relative size of eggs in the clutch. Individuals do not learn the particular size of their own eggs, but will accept both large and small eggs as long as all eggs in the clutch are of similar size. Host rejection decisions are continuously modified based on assessment of variation in egg sizes currently in the clutch, making it a difficult strategy for a cuckoo to defeat. Copyright 2000 The Association for the Study of Animal Behaviour.

Rufous bush robin, Cercotrichas galactotes, nests were experimentally parasitized in order to test which sex is responsible for egg ejection. We tested nests belonging to the same individuals (male, female or pair) in 2 consecutive years. Whether the eggs were rejected did not depend on environmental variables such as laying date and clutch size, and did not change between tests, when the two consecutive nests tested belonged to the same pair members, supporting the idea that some individuals reject odd eggs, while others accept them. The expected frequencies for rejection and acceptance, if females were the sex responsible for ejection behaviour, did not differ from the observed frequencies when the same female owned both the nests tested. However, the expected frequencies calculated for males as if they were the sex responsible differed from those observed, when the same male owned both the nests tested. A lower percentage of females than males changed ejection behaviour. These results show that female rufous bush robins are responsible for egg ejection. Copyright 1998 The Association for the Study of Animal Behaviour.

Adult great spotted cuckoos, Clamator glandariusdamage the eggs of their magpie, Pica picahost without removing or eating them. The number of damaged magpie eggs was recorded in 360 parasitized nests of which 62.2% contained between one and eight damaged magpie eggs. Egg-destroying behaviour may be adaptive if it reduces nestling competition and/or enhances the hatching success of the cuckoo. To clarify the role of egg destruction for the reproductive success of great spotted cuckoos, unparasitized magpie nests were experimentally parasitized (without egg damage) by introducing cuckoo eggs or chicks. Egg damage was common in parasitized nests but the eggs were not damaged by the hosts. Egg damage increased the breeding success of the cuckoos, by both reducing the number of competing host chicks in the nest and increasing the likelihood that late-laid cuckoo eggs would hatch.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

The observed rates of interspecific brood parasitism by diederik cuckoos, Chrysococcyx caprius , on red bishops, Euplectes orix , are reported for four colonies. Experimental manipulations were used to examine how tolerant red bishops were towards foreign eggs. Red bishops did not discriminate mimetic cuckoo-sized models on the basis of egg size, an increase in clutch size or the stage at which the parasite egg appeared in the nest. Only eggs that were heavily maculated or had a darker ground colour and were quite unlike the host's light-blue eggs, were significantly rejected. Low rates of rejection (11.1%) of cuckoo-egg models may be a consequence of evolutionary lag in a coevolutionary arms race between host and parasite. Since the cuckoo egg is a good mimic the cost to a red bishop of making a mistake in the closed nest and rejecting an ‘own’ egg may constrain rates of egg rejection. The parasite may be unable to exploit this constraint because corporate vigilance and defence of large colonies probably prevents any significant parasitism from taking place in a population; rates of interspecific parasitism were negatively correlated with colony size. The potential exists for an evolutionary equilibrium between host and parasite in which the parasite can exploit breeders only in small, low density and poorly defended colonies.

To understand the co-existence of rejection and acceptance of cuckoo eggs within a host population, the mechanism of egg discrimination and the cost-benefit balance of rejection behaviour were investigated. At a study site in central Japan, rejection rate of cuckoo, Cuculus canorus, eggs by great reed warblers, Acrocephalus arundinaceus, was 61·5%. An analysis of host response to natural and experimental parasitism with real cuckoo eggs, cuckoo egg models and painted host eggs indicated that: (1) hosts are more likely to reject eggs that look diVerent from their own; (2) almost all individuals (94%) can reject highly non-mimetic eggs, suggesting that there are few, if any, true accepter genotypes in the host population; (3) hosts usually reject by egg ejection; (4) during the host-laying period, the day of parasitism does not aVect host response; (5) egg types that were rejected at lower rates also took longer to be rejected; (6) acceptance was more likely to occur among mid-season breeders which consist of a higher proportion of younger females in the host population. Two experiments indicated that previous exposure of a host to its own eggs aVects its rejection behaviour, suggesting that a learning mechanism (an imprinting-like process) is involved. Parasitized nests from which the cuckoo egg was experimentally removed, or ejected by hosts, fledged more host young than nests in which the cuckoo egg was accepted. Hosts that deserted parasitized nests were likely to re-nest, and the success of re-nests was high. Costs due to breakage of host eggs occurred in only 3·5% of successful cuckoo egg ejections. A cost-benefit model of egg rejection suggests that under some circumstances, the cost of recognition errors may exceed that of parasitism. Egg variability within a clutch was higher among younger females. Some hosts rejected painted eggs and conspecific eggs based on diVerences that may occur naturally within variable clutches of other individuals. It is suggested that host egg variability is a major constraint on the learning mechanism of egg recognition. Accordingly, the cost of mistakenly rejecting an odd egg from the nest selects for greater tolerance towards divergent eggs in young breeders, and justifies a prolonged learning mechanism in which a host can learn to recognize the variation range of its own eggs. The co-existence of rejection and acceptance within the host population can therefore be explained as a compromise between the cost of parasitism and the cost of recognition errors, rather than as an evolutionary lag. This explanation is particularly pertinent where the cuckoo has evolved mimetic eggs and where the parasitism rate is low.

Abstract. Cooperatively breeding birds may be more susceptible to interspecific brood parasitism by cuckoos than solitarily breeding birds because the additional activity of helpers around the nest could attract the attention of cuckoos. Alternatively, the helpers-at-the-nest may provide additional vigilance and therefore some degree of protection from cuckoos. These possibilities were investigated in a comparative analysis of the frequency of cuckoo parasitism and the breeding system of potential and actual hosts among the Australian avifauna. There was no statistically significant association between passerine breeding system and brood parasitism by cuckoos.

A population of sexually monomorphic, communally breeding guira cuckoos, Guira guira, located near Brasilia, Brazil, was studied during three breeding seasons. Previous studies indicated considerable reproductive conflict among adults over contribution to the incubated clutch. In this study parentage and relatedness among nestlings were evaluated by DNA fingerprinting of samples from four nesting groups. The hypotheses tested were: (1) that guira cuckoo breeding groups consisted of monogamous pairs: and (2) that a single female contributed most of the incubated eggs in each group. The data indicate that nestmates were offspring of different adults. Some adults did not breed. The monogamy hypothesis was not supported by the data. Most related nestlings (78%) appeared to be half-, but not full-siblings. Parentage analyses directly suggested polygynandry in one breeding group and polygamy in another. Only 41% of pairs of nestlings selected at random from the same nest were found to share statistically significant numbers of bands. A single female was not responsible for the majority of young in a nest; however, further research is necessary to evaluate the possibility that dominant females contribute more incubated eggs than do subordinates. The data exemplify a breeding system in which nestling rivalries and reproductive conflict among adults would be expected.

A coevolutionary arms-race between the common cuckoo, Cuculus canorus, and its hosts includes adaptations such as mimetic cuckoo eggs and counter-adaptations such as rejection of the parasitic eggs and aggression towards the parasite by the host. To investigate the existence of defence mechanisms in two species not so far reported as being cuckoo hosts in Norway, but considered to be suitable as hosts, experiments were carried out on bramblings, Fringilla montifringilla, and chaffinches, F. coelebs. Since there were no statistically significant differences between the two species in the way they treated foreign eggs, the data were pooled. The birds rejected about 80% of both non-mimetic and mimetic, artificial, cuckoo eggs, and many even ejected some conspecific eggs. Reports of cuckoo eggs mimicking brambling eggs, in Finland early in the present century, suggest a rapid spread of rejecter alleles. Although never observed in either bramblings or chaffinches, intraspecific nest parasitism is proposed as a possible factor to account for the rejection of conspecific eggs. The parent bramblings attacked a stuffed dummy cuckoo aggressively, but the presence of a stuffed cuckoo led to no increase in the rejection rate of conspecific eggs.

We studied the begging display of nestling cuckoos, Cuculus canorus, reared by reed warbler, Acrocephalus scirpaceus, hosts, to test various hypotheses for the cuckoo's unusually rapid begging call. The hypotheses are not mutually exclusive but focus on different parts of the chain: chick need-begging signals-provisioning by hosts. We reject two hypotheses. (1) Cuckoo chicks do not use their exaggerated begging to counteract host rejection: begging displays varied with hunger and functioned entirely to solicit food. (2) Cuckoos also do not exaggerate their begging calls simply because they need more food than a host brood. Single cuckoos grew at a similar rate to a brood of four reed warblers, and more slowly than a blackbird, Turdus merula, chick (a nonparasitic chick of similar size). Our data support two other hypotheses. (3) To elicit sufficient care in reed warbler nests, the cuckoo must exaggerate the vocal component of its display to compensate for its deficient visual signal (a single gape) compared with a host brood. Thus rapid calling reflects the way the cuckoo exploits the provisioning rules that hosts use to feed their own young. (4) In theory, cuckoos should be more selfish than host young because their greed is unconstrained by kinship. Our data are equivocal; compared with host broods, cuckoos solicited a higher provisioning rate in relation to one measure of need but not for another. We discuss whether cuckoos are likely to have gens-specific begging displays. Copyright 1999 The Association for the Study of Animal Behaviour.

Kleptoparasitic Nomada bees (‘cuckoo bees’) are guided by olfactory cues in their local topographic orientation to Andrena host nests. Decisions regarding (1) the availability and suitability of a partly provisioned nest cell, (2) the threat of maternal host presence, and (3) prior visitation by another kleptoparasite, are all assessed olfactorily by the Nomada female while she is safely poised at the entrance to the host's ground nest. The influences of visual, tactile, acoustical and other sensory clues were controlled experimentally by presenting stimuli in artificial host nests. The nest approach behaviours of the kleptoparasites are comparable to non-parasitic bees. Although the host and parasite encounter the same odour cues for individual nest orientation, striking differences in their subsequent approach behaviours apparently result from the degree to which each generalizes the olfactory information available.

At study sites in Cambridgeshire, England, the percentage of reed warbler, Acrocephalus scirpaceus , nests parasitized by cuckoos, Cuculus canorus , in 2 years was 22·5% and 9·1%. The warblers rejected cuckoo eggs at 19% of parasitized nests. Parasitized clutches suffered less predation than unparasitized clutches, suggesting that the cuckoo itself was the major predator, plundering nests too advanced for parasitism so that the hosts would re-lay. The cuckoos laid a mimetic egg, parasitized nests in the afternoons during the host laying period, usually removed one host egg, laid a remarkably small egg and laid very quickly. Nests were experimentally parasitized with model eggs to study the significance of this procedure. Experiments showed that host discrimination selects for: (1) egg mimicry by cuckoos (poorer matching model eggs were more likely to be rejected); (2) parasitism during the laying period (mimetic eggs put in nests before host laying began were rejected); (3) afternoon laying (mimetic eggs were less likely to be accepted in the early morning than in the afternoon, when hosts were more often absent from the nest); (4) a small egg (large eggs, typical of non-parasitic cuckoos, were more likely to be rejected); (5) rapid laying (a stuffed cuckoo on the nest stimulated increased rejection of model eggs), and (6) sets a limit to host egg removal by cuckoos (if more than one or two are removed desertion may occur). Mimicry may also be selected for because it reduced the chance that second cuckoos can discriminate the first cuckoo's egg from the host's clutch. Predation did not select for mimicry; nests with a non-mimetic egg did not suffer greater predation than those with a mimetic egg. Host rejection of model eggs did not depend on: (1) stage of parasitism once host egg laying had begun (nevertheless cuckoos were more likely to lay early in the host laying period probably to increase the chance the cuckoo chick hatched); (2) removal of a host egg (however, this reduced the incidence of unhatched eggs so cuckoos may remove a host egg so as not to exceed the host incubation limit). There were two costs of rejection, an ‘ejection’ cost (own eggs ejected as well as the cuckoo egg) and, with mimetic eggs, a ‘recognition’ cost (own eggs ejected instead of the cuckoo egg). Reed warblers did not discriminate against unlike chicks (another species) and did not favour either a cuckoo chick or their own chicks when these were placed in two nests side by side. Possible reasons why the hosts discriminate against unlike eggs but not unlike chicks are discussed.