Egg mimicry by the Pacific koel: mimicry of one host facilitates exploitation of other hosts with similar egg types

When brood parasites exploit multiple host species, egg rejection by hosts may select for the evolution of host‐specific races, where each race mimics a particular host's egg type. However, some brood parasites that exploit multiple hosts with the ability to reject foreign eggs appear to have only a single egg type. In these cases, it is unclear how the parasite egg escapes detection by its hosts. Three possible explanations are: 1) host‐specific races are present, but differences in egg morphology are difficult for the human eye to detect; 2) the brood parasite evolves a single egg type that is intermediate in appearance between the eggs of its hosts; 3) or the parasite evolves mimicry of one of its hosts, which subsequently allows it to exploit other species with similar egg morphology. Here we test these possibilities by quantifying parameters of egg appearance of the brood‐parasitic Pacific koel Eudynamys orientalis and seven of its hosts. Koel eggs laid in the nests of different hosts did not show significant differences in colour or pattern, suggesting that koels have not evolved host‐specific races. Koel eggs were similar in colour, luminance and pattern to the majority of hosts, but were significantly more similar in colour and luminance to one of the major hosts than to two other major hosts, supporting hypothesis 3. Our findings suggest that mimicry of one host can allow a brood parasite to exploit new hosts with similar egg morphologies, which could inhibit the evolution of host defences in naïve hosts.     

Egg Discrimination and Sex-Specific Pecking Behaviour in Parasitic Cowbirds

We studied egg‐pecking behaviour in males and females of three cowbird species: the shiny cowbird (Molothrus bonariensis), a host generalist brood parasite, the screaming cowbird (M. rufoaxillaris), a host specialist brood parasite, and the bay‐winged cowbird (Agelaioides badius), a non‐parasitic species. We conducted three experiments in which we offered each bird an artificial nest with two plaster eggs and recorded whether egg pecking occurred and the number of pecks on each egg. In expt 1, we tested if there were species and sex differences in egg‐pecking behaviour by offering the birds two spotted eggs of similar pattern. Shiny and screaming cowbirds responded in 40.3% and 44% of the trials, respectively, with females and males presenting similar levels of response. In contrast, bay‐winged cowbirds did not show any response. In expt 2, we tested if shiny cowbirds responded differentially when they faced a choice between one host and one shiny cowbird egg, while in expt 3, we tested if screaming cowbirds responded differentially when they faced a choice between one shiny and one screaming cowbird egg. Shiny cowbirds pecked preferentially host eggs while screaming cowbirds pecked more frequently shiny cowbird eggs. Our results show that egg‐pecking behaviour is present in both sexes of parasitic cowbirds, but not in non‐parasitic birds, and that parasitic cowbirds can discriminate between eggs of their own species and the eggs of their hosts or other brood parasites.     

The sight of an adult brood parasite near the nest is an insufficient cue for a honeyguide host to reject foreign eggs

Hosts of brood‐parasitic birds typically evolve anti‐parasitism defences, including mobbing of parasitic intruders at the nest and the ability to recognize and reject foreign eggs from their clutches. The Greater Honeyguide Indicator indicator is a virulent brood parasite that punctures host eggs and kills host young, and accordingly, a common host, the Little Bee‐eater Merops pusillus frequently rejects entire clutches that have been parasitized. We predicted that given the high costs of accidentally rejecting an entire clutch, and that the experimental addition of a foreign egg is insufficient to induce this defence, Bee‐eaters require the sight of an adult parasite near the nest as an additional cue for parasitism before they reject a clutch. We found that many Little Bee‐eater parents mobbed Greater Honeyguide dummies while ignoring barbet control dummies, showing that they recognized them as a threat. Surprisingly, however, neither a dummy Honeyguide nor the presence of a foreign egg, either separately or in combination, was sufficient to stimulate egg rejection.     

Do Nest Light Conditions Affect Rejection of Parasitic Eggs? A Test of the Light Environment Hypothesis

Discrimination of foreign eggs is one of the most studied aspects of host defences against avian brood parasites. Although many factors affecting host egg‐recognition processes have already been evaluated, only a few attempts have been made to test the importance of light conditions in microhabitats of host nests. Here, we examined whether the objectively measured nest light environment affects great reed warbler (Acrocephalus arundinaceus) responses towards real common cuckoo (Cuculus canorus) eggs. More specifically, we predicted that parasitic eggs will be rejected with a lower frequency from nests placed in darker conditions than those in lighter conditions. However, we found no effect of the ambient light on egg‐rejection behaviour alone, but the photosynthetically active radiation exhibited a positive interactive effect with chromatic contrast between cuckoo and host eggs. Most rejection events were accomplished when cuckoo eggs of poor mimicry were laid in well‐lit nests. Our study suggests that this phenomenon may have important implications for the evolution of egg mimicry and host egg discrimination. We encourage further testing of the light environment hypothesis in other host species breeding in variable nest microhabitats and light conditions.     

Experimental Shifts in Intraclutch Egg Color Variation Do Not Affect Egg Rejection in a Host of a Non-Egg-Mimetic Avian Brood Parasite

Avian brood parasites lay their eggs in the nests of other birds, and impose the costs associated with rearing parasitic young onto these hosts. Many hosts of brood parasites defend against parasitism by removing foreign eggs from the nest. In systems where parasitic eggs mimic host eggs in coloration and patterning, extensive intraclutch variation in egg appearances may impair the host’s ability to recognize and reject parasitic eggs, but experimental investigation of this effect has produced conflicting results. The cognitive mechanism by which hosts recognize parasitic eggs may vary across brood parasite hosts, and this may explain variation in experimental outcome across studies investigating egg rejection in hosts of egg-mimicking brood parasites. In contrast, for hosts of non-egg-mimetic parasites, intraclutch egg color variation is not predicted to co-vary with foreign egg rejection, irrespective of cognitive mechanism. Here we tested for effects of intraclutch egg color variation in a host of nonmimetic brood parasite by manipulating egg color in American robins (Turdus migratorius), hosts of brown-headed cowbirds (Molothrus ater). We recorded robins’ behavioral responses to simulated cowbird parasitism in nests where color variation was artificially enhanced or reduced. We also quantified egg color variation within and between unmanipulated robin clutches as perceived by robins themselves using spectrophotometric measures and avian visual modeling. In unmanipulated nests, egg color varied more between than within robin clutches. As predicted, however, manipulation of color variation did not affect rejection rates. Overall, our results best support the scenario wherein egg rejection is the outcome of selective pressure by a nonmimetic brood parasite, because robins are efficient rejecters of foreign eggs, irrespective of the color variation within their own clutch.     

Blunt egg pole holds cues for alien egg discrimination: experimental evidence

Eggshell colour patterns play a crucial role in avian host–parasite coevolution. In contrast to many experiments investigating general host egg discrimination abilities, studies testing where specific recognition cues are located on the eggshells (on blunt, sharp or both egg poles) are lacking. Previous studies suggested that discrimination cues might be located at the blunt egg pole, where the shell patterning is typically concentrated. We tested this hypothesis experimentally in species subject to interspecific (great reed warblers Acrocephalus arundinaceus, reed warblers A. scirpaceus), and also intraspecific parasitism (song thrushes Turdus philomelos, blackbirds T. merula). We examined host responses towards two types of intraspecific eggs painted non‐mimetic immaculate blue either at blunt or sharp poles. All four species rejected eggs manipulated at the blunt pole at significantly higher rates, indicating that they perceive the critical recognition cues in the same egg part. Thus, the presence of egg recognition cues at the blunt egg pole may be a general phenomenon in birds parasitized by both intraspecific and interspecific parasites.     

Egg Recognition and Social Parasitism in Formica Ants

Nests of social insects are an attractive resource in terms of nutrition and shelter and therefore targeted by a variety of pathogens and parasites that harness the resources of a host colony in their own reproductive interests. Colonies of the ants Formica fusca and F. lemani serve as hosts for mound‐building Formica species, the queens of which use host colonies during colony founding. Here, we investigate whether workers of the host species can mitigate the costs imposed on them by invading parasite queens by recognizing and selectively removing eggs laid by these queens. We used behavioural assays, allowing host workers to choose between con‐colonial eggs and eggs laid by the parasite species F. truncorum. We show that workers of both host species discriminate between the two types of eggs in favour of con‐colonial eggs. Moreover, workers of F. fusca rejected more con‐colonial eggs than F. lemani. This higher rate of error in F. fusca may reflect a greater selectivity or a greater difficulty in discriminating between the two egg types. Nevertheless, both host species removed parasite eggs at a similar rate, when these were artificially introduced into the colonies, although some eggs remained after 10 d. In addition, upon receiving parasite eggs, host workers started to lay unfertilized male‐destined eggs within 6 d, thus employing an alternative pathway to gain direct fitness when the resident queen is no longer present and the colony is parasitized.     

Egg ejection cost can limit defence strategies against brood parasitism

A cost associated with the evolution of antiparasite strategies is the failure to recognize parasitic eggs, leading the host to evict its own eggs. However, there is evidence that birds recognize their own eggs through imprinting. This leads to the question of why birds accept parasitic eggs if such eggs can be identified. Here, we tested whether egg ejection per se can be costly due to increased predation risk to the remaining clutch and whether olfactory or visual cues of egg ejection increase predation. We carried out three field experiments to answer the following questions: (a) Does ejecting an egg increase nest predation risk? (b) Does the presence of olfactory cues, such as the smell of a broken egg, increase nest predation risk? And (c) Does the presence of visual cues, such as an egg shell below the nest, increase nest predation risk? We found evidence that egg ejection increases nest predation and that olfactory cues alone also increase nest predation. The presence of visual cues did not change predation rates. These data indicate that egg ejection is costly for both host and parasitic eggs that may remain in the nest. Our results suggest why host and parasite eggs are commonly found within the same nests, despite the possibility that hosts recognize and could possibly eject the parasite’s egg.     

Host‐specific parasitism in the Central American striped cuckoo,Tapera naevia

Coevolutionary theories of brood parasite strategy and host defense have been informed by research on egg mimicry and host recognition. However, there is no information on the strategies of New World parasitic cuckoos and their hosts. The striped cuckoo Tapera naevia is a New World cuckoo that uses multiple host species and maintains an egg color polymorphism. To investigate if color‐matching influenced rejection behavior in hosts, I conducted an egg rejection experiment on a host that lays blue‐green eggs, the rufous‐and‐white wren Thryophilus rufalbus and a host that lays white eggs, the plain wren Cantorchilus modestus. I used spectrophotometric analysis of egg color to determine the degree of egg color‐matching. I found that at the field site the striped cuckoo lays highly mimetic eggs for the rufous‐and‐white wren, in both color and brightness. The rufous‐and‐white wren was more likely to accept mimetic artificial eggs than non‐mimetic eggs. The plain wren exhibited low rejection rates for both mimetic and non‐mimetic artificial eggs. The evidence from this study indicates that the striped cuckoo lays eggs that are closely color‐matched to those of its preferred host, the rufous‐and‐white wren, and that this mimicry improves acceptance.     

Size and material of model parasitic eggs affect the rejection response of Western Bonelli's Warbler Phylloscopus bonelli

Given the high costs of brood parasitism, avian hosts have adopted different defences to counteract parasites by ejecting the foreign egg or by deserting the parasitized nest. These responses depend mainly on the relative size of the host compared with the parasitic egg. Small hosts must deal with an egg considerably larger than their own, so nest desertion becomes the only possible method of egg rejection in these cases. The use of artificial model eggs made of hard material in egg‐recognition experiments has been criticized because hard eggs underestimate the frequency of egg ejection. However, no available studies have investigated the effect of softer material. Here, we test the potential effect of size of dummy parasitic eggs in relation to egg‐rejection behaviour (egg ejection and nest desertion rates) in Western Bonelli's Warbler Phylloscopus bonelli, a small host, using plasticine non‐mimetic eggs of three different sizes. In addition, we tested the potential effect of material, comparing ejection and desertion responses between real and plasticine eggs. As predicted, small eggs were always ejected, whereas nest desertion occurred more frequently with large eggs, thus suggesting that nest desertion occurs because of the constraints imposed by the large eggs. We found that plasticine may misrepresent the responses to experimental parasitism, at least in small host species, because this material facilitates egg ejection, provoking a decrease in nest desertion rate. Thus, particular caution is needed in the interpretation of the results in egg‐rejection experiments performed using dummy eggs made of soft materials.     

Selective Defecation and Selective Foraging: Antiparasite Behavior in Wild Ungulates?

Selective defecation and selective foraging are two potential antiparasite behaviors used by grazing ungulates to reduce infection by fecal–oral transmitted parasites. While there is some evidence that domestic species use these strategies, less is known about the occurrence and efficacy of these behaviors in wild ungulates. In this study, I examined whether wild antelope use selective defecation and selective foraging strategies to reduce exposure to gastrointestinal nematode parasites. By quantifying parasite levels in the environment in relation to the defecation patterns of three species, dik‐dik (Madoqua kirkii), Grant's gazelle (Gazella granti), and impala (Aepyceros melampus), I found that nematode larval concentrations in pasture were higher in the vicinity of clusters of feces (dung middens) compared to single fecal pellet groups or dung‐free areas. In addition, experimental feeding trials in free‐ranging dik‐dik showed that individuals selectively avoided feeding near concentrations of feces. Given that increased parasite contamination was found in the immediate vicinity of fecal clusters, fecal avoidance could help reduce host consumption of parasites and may therefore be an effective antiparasite behavior for certain species. On the other hand, while the concentration of parasite larvae in the vicinity of middens coupled with host avoidance of these areas during grazing could reduce host contact with parasites, results showing a positive correlation between the number of middens in a habitat and larval abundance at control sites suggest that dung middens might increase and not decrease overall host exposure to parasites. If this is the case, dung midden formation may not be a viable antiparasite strategy.     

To eject or to abandon? Life history traits of hosts and parasites interact to influence the fitness payoffs of alternative anti‐parasite strategies

Hosts either tolerate avian brood parasitism or reject it by ejecting parasitic eggs, as seen in most rejecter hosts of common cuckoos, Cuculus canorus, or by abandoning parasitized clutches, as seen in most rejecter hosts of brown‐headed cowbirds, Molothrus ater. What explains consistent variation between alternative rejection behaviours of hosts within the same species and across species when exposed to different types of parasites? Life history theory predicts that when parasites decrease the fitness of host offspring, but not the future reproductive success of host adults, optimal clutch size should decrease. Consistent with this prediction, evolutionarily old cowbird hosts, but not cuckoo hosts, have lower clutch sizes than related rarely‐ or newly parasitized species. We constructed a mathematical model to calculate the fitness payoffs of egg ejector vs. nest abandoner hosts to determine if various aspects of host life history traits and brood parasites’ virulence on adult and young host fitness differentially influence the payoffs of alternative host defences. These calculations showed that in general egg ejection was a superior anti‐parasite strategy to nest abandonment. Yet, increasing parasitism rates and increasing fitness values of hosts’ eggs in both currently parasitized and future replacement nests led to switch points in fitness payoffs in favour of nest abandonment. Nonetheless, nest abandonment became selectively more favourable only at lower clutch sizes and only when hosts faced parasitism by a cowbird‐ rather than a cuckoo‐type brood parasite. We suggest that, in addition to evolutionary lag and gape‐size limitation, our estimated fitness differences based on life history trait variation provide new insights for the consistent differences observed in the anti‐parasite rejection strategies between many cuckoo‐ and cowbird‐hosts.     

Host and brood parasite coevolutionary interactions covary with comparative patterns of the avian visual system

In coevolutionary arms-races, reciprocal ecological interactions and their fitness impacts shape the course of phenotypic evolution. The classic example of avian host–brood parasite interactions selects for host recognition and rejection of increasingly mimetic foreign eggs. An essential component of perceptual mimicry is that parasitic eggs escape detection by host sensory systems, yet there is no direct evidence that the avian visual system covaries with parasitic egg recognition or mimicry. Here, we used eye size measurements collected from preserved museum specimens as a metric of the avian visual system for species involved in host–brood parasite interactions. We discovered that (i) hosts had smaller eyes compared with non-hosts, (ii) parasites had larger eyes compared with hosts before but not after phylogenetic corrections, perhaps owing to the limited number of independent evolutionary origins of obligate brood parasitism, (iii) egg rejection in hosts with non-mimetic parasitic eggs positively correlated with eye size, and (iv) eye size was positively associated with increased avian-perceived host–parasite eggshell similarity. These results imply that both host-use by parasites and anti-parasitic responses by hosts covary with a metric of the visual system across relevant bird species, providing comparative evidence for coevolutionary patterns of host and brood parasite sensory systems.     

Coevolution between the large hawk‐cuckoo (Cuculus sparverioides) and its two sympatric Leiothrichidae hosts: evidence for recent expansion and switch in host use?

Obligate brood parasites only account for 1% of birds in the world, but utilize a great variety of avian species as hosts. Host switch theory predicts that parasites should shift from one host to another during the long‐term arms race with hosts whenever such a shift would be facilitated by similarity in ecology and distribution. However, few studies have been conducted to address this puzzle because it is extremely difficult for humans to witness such host shifts during the long‐lasting process of evolution. Here we adopted an alternative way to understand host switch behaviour of brood parasites by comparing egg colour variation, cuckoo egg mimicry and egg recognition capacity between two sympatric hosts, the Chinese babax (Babax lanceolatus) and the white‐browed laughing thrush (Garrulax sannio), which are both parasitized by the large hawk‐cuckoo (Cuculus sparverioides). The babax lays dark blue eggs whilst the laughing thrush lays white to pale blue eggs, and the large hawk‐cuckoo parasitizes them by laying eggs that optimally match laughing thrush eggs according to avian vision. The laughing thrush possesses a greater capacity of egg recognition than the babax because it rejected all non‐mimetic eggs while the babax is an intermediate rejecter. Furthermore, all the nest characteristics measured were similar in these two host species with no statistical significant differences. These results are consistent with the hypothesis that the white‐browed laughing thrush is the original and main host species that has a longer coevolutionary interaction with the large hawk‐cuckoo than the Chinese babax, which is a recent host acquired through a host switch by the hawk‐cuckoo. We discuss the possible outcome of the interaction between the large hawk‐cuckoo and these two host species, and emphasize that host switch behaviour in brood parasites is more likely an adaptation to expand the range of host species rather than a change in host species favoring an increase in reproductive output. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, ●● , ●●–●●.     

Integrating phylogenetic and ecological distances reveals new insights into parasite host specificity

The range of hosts a pathogen infects (host specificity) is a key element of disease risk that may be influenced by both shared phylogenetic history and shared ecological attributes of prospective hosts. Phylospecificity indices quantify host specificity in terms of host relatedness, but can fail to capture ecological attributes that increase susceptibility. For instance, similarity in habitat niche may expose phylogenetically unrelated host species to similar pathogen assemblages. Using a recently proposed method that integrates multiple distances, we assess the relative contributions of host phylogenetic and functional distances to pathogen host specificity (functional–phylogenetic host specificity). We apply this index to a data set of avian malaria parasite (Plasmodium and Haemoproteus spp.) infections from Melanesian birds to show that multihost parasites generally use hosts that are closely related, not hosts with similar habitat niches. We also show that host community phylogenetic ß‐diversity (Pßd) predicts parasite Pßd and that individual host species carry phylogenetically clustered Haemoproteus parasite assemblages. Our findings were robust to phylogenetic uncertainty, and suggest that phylogenetic ancestry of both hosts and parasites plays important roles in driving avian malaria host specificity and community assembly. However, restricting host specificity analyses to either recent or historical timescales identified notable exceptions, including a ‘habitat specialist’ parasite that infects a diversity of unrelated host species with similar habitat niches. This work highlights that integrating ecological and phylogenetic distances provides a powerful approach to better understand drivers of pathogen host specificity and community assembly.     

Attractive blue‐green egg coloration and cuckoo−host coevolution

Recent evidence suggests that blue‐green coloration of bird eggshells may be related to female and/or egg phenotypic quality, and that such colour may affect parental effort and therefore the nutritional environment of developing nestlings. Here we suggest that these relationships and the signal function of eggshell coloration would affect the outcome of coevolution between avian brood parasites and their hosts in at least three different non‐exclusive evolutionary pathways. First, by laying blue‐green coloured eggs, cuckoo females may exploit possible sensory biases of their hosts, constraining the evolution of parasitic egg recognition, and thus avoid rejection. Second, because of the relatively high costs of laying blue eggs, cuckoo females may be limited in their ability to mimic costly blue‐green eggs of their hosts because cuckoo females lay many more eggs than their hosts. Furthermore, costs associated with foreign egg recognition errors would be relatively higher for hosts laying blue eggs. Third, cuckoos may use coloration of host eggs for selecting individuals or specific hosts of appropriate phenotypic quality (i.e. parental abilities). We here explored some predictions emerging from the above scenarios and found partial support for two of them by studying egg coloration of European cuckoos (Cuculus canorus) and that of their 25 main hosts, as well as parasitism and rejection rate of hosts. Cuckoo hosts parasitized with more blue, green, and ultraviolet cuckoo eggs, or those laying more blue‐green eggs, were more prone to accept experimental parasitism with artificial cuckoo eggs. In addition, coloration of cuckoo eggs is more variable when parasitizing hosts laying bluer‐greener eggs, even after controlling for the effect of host egg coloration (i.e. degree of egg matching). Globally, our results are consistent with the proposed hypothesis that host egg traits that are related to phenotypic quality of hosts, such as egg coloration, may have important implications for the coevolutionary interaction between hosts and brood parasites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 154–168.     

Do common cuckoos (Cuculus canorus) possess an optimal laying behaviour to match their own egg phenotype to that of their Oriental reed warbler (Acrocephalus orientalis) hosts?

Optimality theory suggests that parasitic cuckoos should evolve an optimal laying behaviour aiming to positively select host nests in which the eggs match the phenotype of their own eggs, thus minimizing the rejection risk from hosts and, in turn, maximizing the cuckoos' fitness. We tested this hypothesis by investigating cuckoo‐egg matching between parasitized and nonparasitized nests in a common cuckoo (Cuculus canorus) host, the Oriental reed warbler (Acrocephalus orientalis), by use of Vorobyev–Osorio and Nature‐Pattern‐Match models to quantify the matching of egg colour and pattern from avian vision, respectively. The results of our study indicated that cuckoo‐egg matching in parasitized nests was no better than that in nonparasitized nests, and thus we found no support for the optimal laying hypothesis in cuckoos. The mixed conclusions from all previous studies, including the present study, may be explained by (1) the parallel coevolution in different cuckoo–host systems; (2) the inappropriate methodology; and (3) the deficiency of the assumption itself. We suggest that a better methodology should be developed to solve the puzzle of whether cuckoos choose to lay eggs matching those of the host.     

Egg Discrimination in an Open Nesting Passerine Under Dim Light Conditions

Many hosts of avian brood parasites such as the common cuckoo (Cuculus canorus) show refined egg discrimination behaviour. Egg recognition in most open‐nesting hosts seems to be based entirely on differences in colour. However, hole‐ and dome‐nesting hosts may rely largely on luminance contrasts. Here, we studied egg rejection behaviour in nightingales (Luscinia megarhynchos), an open‐nesting species that nests in deeply shadowed positions and lays very specific dark olive‐green eggs. Although being theoretically suitable as hosts of the cuckoo, nightingales are very rarely parasitized and no cuckoo egg morph mimicking nightingale eggs is known. Thus, we predicted high rejection rate of foreign eggs, but because of the dim nesting environments, luminance contrasts would be an important cue in egg rejection decisions, similar to cavity‐ or dome‐nesting species. We experimentally parasitized nightingale nests with two groups of model egg types: ‘bright eggs’ and ‘dark eggs’. Within each group, one of the egg types was an effective match while the other type was a poor colour match (whitish vs. pale blue and olive‐green vs. black).We used a discrimination visual model to quantify host‐model egg similarity and compared egg rejection predicted by the model with the observed rejection pattern. Consistent with a scenario of largely luminance‐based egg recognition, blue and white eggs, which had larger achromatic mismatching, were rejected at a higher relative rate than the better achromatic matching black and green eggs. Nightingales showed strong aggression to a cuckoo dummy, suggesting that they were involved in coevolutionary interactions with the cuckoo in the past. However, because of the highly distinct appearance of nightingale eggs relative to the other sympatrically breeding passerines, and the largely luminance‐based egg recognition, this arms race was likely terminated at an early stage.     

How can distinct egg polymorphism be maintained in the rufescent prinia (Prinia rufescens)–plaintive cuckoo (Cacomantis merulinus) interaction—a modeling approach

In avian brood parasitism, both the host and the parasite are expected to develop various conflicting adaptations; hosts develop a defense against parasitism, such as an ability to recognize and reject parasitic eggs that look unlike their own, while parasites evolve egg mimicry to counter this host defense. Hosts may further evolve to generate various egg phenotypes that are not mimicked by parasites. Difference in egg phenotype critically affects the successful reproduction of hosts and parasites. Recent studies have shown that clear polymorphism in egg phenotype is observed in several host–parasite interactions, which suggests that egg polymorphism may be a more universal phenomenon than previously thought. We examined the mechanism for maintaining egg polymorphism in the rufescent prinia (Prinia rufescens) that is parasitized by the plaintive cuckoo (Cacomantis merulinus) from a theoretical viewpoint based on a mathematical model. The prinia has four distinct egg phenotypes: immaculate white, immaculate blue, white with spots, and blue with spots. Only two egg phenotypes, white with spots and blue with spots, are found in the cuckoo population. We show that the observed prinia and cuckoo phenotypes cannot be at an equilibrium and that egg polymorphism can be maintained either at stationary equilibrium or with dynamic, frequency oscillations, depending on the mutation rates of the background color and spottiness. Long‐term monitoring of the prinia–cuckoo interaction over a wide geographic range is needed to test the results of the model analyses.     

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.