Reed warbler hosts fine-tune their defenses to track three decades of cuckoo decline

Oriental reed warblers retain strong egg recognition abilities during the nestling stage

Egg recognition and rejection are the most common and effective anti-parasitic strategies against avian brood parasitism in terms of maintaining stability over time and plasticity in response to environmental cues. Conversely, parasites have evolved multiple counter-adaptations to the anti-parasitic defenses of hosts. Among them, the crypsis hypothesis suggests that eggs that appear more cryptic in color and are closely matched to the environment are helping to counter the egg recognition strategy of the host. In this study, we investigated whether the egg recognition ability of Oriental reed warblers (Acrocephalus orientalis), a common host of common cuckoos (Cuculus canorus), changed during different reproductive stages by using model egg experiments. The effect of the crypsis hypothesis on the egg recognition ability of the hosts was also investigated by controlling the color contrast between the inside of the experimental nests and the model eggs. The results showed that the Oriental reed warbler retained strong egg recognition abilities, which were similar to the incubation stage (GLMMs: F 1,27 = 0.424, p = .521), even after entering the nestling stage and preferentially rejected model eggs with distinct contrasting colors (binomial test: Fisher's exact, p = .016). These results are consistent with the crypsis hypothesis. The present study suggests that the host retains a strong egg recognition ability even during the nestling stage and that cryptic-colored eggs that are closely matched with the breeding nest environment help counter the host's egg recognition abilities and increase the chances of successful parasitism by cuckoos. However, the effectiveness of cryptic egg may be weaker than mimic egg in countering egg recognition and rejection by hosts with open-cup nests.

Defense behavior of two closely related but geographically distant host species against cuckoo parasitism: A next test for the parallel coevolution

Interactions between avian brood parasites, such as common cuckoos (Cuculus canorus), and their hosts are one of the best-studied examples of the coevolutionary arms race. Different stages of this arms race can be seen in different races of common cuckoos and their hosts across their range. However, little is known whether selected populations of two closely related but geographically distant species with probably different coevolutionary histories with the common cuckoo are also at different stages of the arms race. In this study, we tested this prediction experimentally using the same non-mimetic model eggs and three-dimensional (3D) printed models of the gray adult common cuckoo (Cuculus canorus). We examined egg recognition and egg rejection and aggression against the common cuckoo in the great reed warbler (Acrocephalus arundinaceus) and Oriental reed warbler (Acrocephalus orientalis), in Slovakia of Europe and northeast China of Asia. The results showed that the great reed warbler exhibited stronger responses to experimental model eggs and 3D models of the common cuckoo than the Oriental reed warbler. We conclude that both the great reed warbler and Oriental reed warbler have well-developed antiparasitic behaviors against common cuckoos in the studied populations, but with different levels of defense intensity, which may be due to local differences in parasitic pressure and the risk of parasitism. This provides an opportunity to study coevolutionary processes between the brood parasite and its hosts together in both species at large geographical scales.

The mere presence of cuckoos in breeding area alters egg-ejection decisions in Daurian redstarts

Brood parasitic cuckoos and their hosts serve as model systems for studying host–parasite coevolution. Egg-rejection behavior constitutes an effective defense against brood parasitism, but some host species show phenotypic plasticity in egg-rejection behavior. Direct exposure to a cuckoo near the nest can increase egg-rejection likelihood, and long-term studies have shown that increased the egg-rejection rates generally correlate with higher parasite prevalence. However, it remains unclear whether such increases result from interactions between parasites and hosts, as these can be surprisingly common, or whether the mere presence of cuckoos in the breeding area is sufficient. Daurian redstarts Phoenicurus auroreus are a common host of the common cuckoo Cuculus canorus that defend against cuckoo parasitism mainly by ejecting the parasitic egg from the nest. This species is unique, as its first breeding attempt of the year takes place before the arrival of cuckoos, excluding the possibility for direct interactions. We simulated the ambient presence of cuckoos or hoopoes Upupa epops (control) in sub-populations of redstarts during their first egg-laying period by presenting taxidermic models and playing back vocalizations. Redstarts in cuckoo-treated plots showed significantly higher egg-ejection rates than individuals in control plots, even though females in both groups were equally likely to recognize the parasitic egg. Among females that did recognize the parasitic egg, those exposed to the cuckoo treatment were more likely to eject it than those exposed to the control treatment. Our results demonstrate unequivocally that the mere presence of cuckoos in the environment is sufficient to provoke egg-ejection behavior.

Female Cuckoo Calls Deceive Their Hosts by Evoking Nest-Leaving Behavior: Variation under Different Levels of Parasitism

Simple Summary

Obligate brood parasite birds such as the common cuckoo (Cuculus canorus) can trick their hosts via a variety of means. Cuckoos are threats to the host nests but not to the host adults themselves. To successfully parasitize the host nests, female cuckoos have been hypothesized to distract the hosts’ attention from their nests by mimicking the calls of sparrowhawks (Accipiter nisus), a predator of the hosts. We performed playback experiments in two populations of the host oriental reed warbler (Acrocephalus orientalis) that have experienced different levels of parasitic pressure. We found that female cuckoo calls evoked both populations of the hosts to leave their nests more frequently than did the calls of male cuckoos or doves that do not pose threats to the hosts. This indicated that the call of the female cuckoo functions to deceive the host and thus favors host brood parasitism. However, we propose that such a deceiving effect of the female cuckoo call is due to the rapid cadence of the call rather than sparrowhawk mimicry.

Abstract

The common cuckoo (Cuculus canorus) is an obligate brood parasite that has evolved a series of strategies to trick its hosts. The female cuckoo has been hypothesized to mimic the appearance and sounds of several raptors to deceive the hosts into exhibiting anti-predator behavior. Such behavior would relax the protection of the host nest and thus allow the female cuckoo to approach the host nest unopposed. Many anti-parasite strategies have been found to vary among geographical populations due to different parasitic pressures from cuckoos. However, the effect of female cuckoo calls related to different levels of parasitic pressure has not been examined. Here, we studied the effect of female cuckoo calls on the oriental reed warbler (Acrocephalus orientalis), one of the major hosts of the common cuckoo, in two geographical populations experiencing different levels of parasite pressure. Four kinds of sounds were played back to the hosts: the calls from female common cuckoos, male common cuckoos, sparrowhawks (Accipiter nisus), and oriental turtle doves (Streptopelia orientalis). The results showed that the female cuckoo calls induced the hosts to leave their nests more frequently than the male cuckoo or dove calls in both populations, and two populations of the hosts reacted similarly to the female cuckoo calls, implying that the function of female cuckoo calls would not be affected by the difference in parasitism rate. This study indicates that female cuckoo calls function to distract the hosts’ attention from protecting their nests. However, we propose that such a deception by the female cuckoo call may not be due to the mimicry of sparrowhawk calls, but rather that the rapid cadence of the call that causes a sense of anxiety in the hosts.

The Evolution of Predator Resemblance in Avian Brood Parasites

Predators have profound effects on prey behavior and some adult brood parasites use predator resemblance to exploit the antipredator defenses of their hosts. Clarifying host perception of such stimuli is important for understanding the adaptive significance of adult brood parasite characteristics, and the mechanisms by which they misdirect hosts. Here I review the literature to explore the adaptive basis of predator resemblance in avian brood parasites, and natural variation in host responses to these stimuli. I also provide a framework for the information ecology of predator resemblance, which is based on the principles of signal detection theory and draws from empirical evidence from the common cuckoo, Cuculus canorus, as the most widely studied system. In this species, visual and acoustic hawk-like stimuli are effective in manipulating host defenses. Overall, contrasts across host responses suggest that different modalities of information can have independent effects on hosts, and that predator resemblance takes advantage of multiple sensory and cognitive processes. Host perception of these stimuli and the degree to which they are processed in an integrated manner, and the physiological processes underlying regulation of the responses, present new avenues for brood parasitism research.

Persistent fine-tuning of egg rejection based on parasitic timing in a cuckoo host even after relaxation of parasitism pressure

Egg rejection is one of the most effective defenses to avoid avian brood parasites by hosts, and reflects avian cognition during parasite-host coevolution. However, egg rejection varies with different populations and species, or under different contexts. Here we studied the egg recognition behavior in a non-parasitized population of the red-whiskered bulbul (Pycnonotus jocosus), which has been recorded as a host of the banded bay cuckoo (Cacomantis sonneratii). By comparing its egg rejection frequency and latency between egg-laying and egg-incubation stages with a variety of nest ages, which represent different costs of cuckoo parasitism, the results show that the hosts rejected parasite eggs more frequently in the egg-laying stage than in the egg-incubation stage without a difference in latency, and accepted parasite eggs with an increase of nest age. These findings indicated that the hosts are able to adjust the intensity of their anti-parasitic responses according to the parasitic timing. A relaxation of parasitism risk and the tradeoff between parasitism risk and egg rejection cost may contribute to explaining our results.

A Chromosome-Level Genome Assembly of the Reed Warbler (Acrocephalus scirpaceus)

The reed warbler (Acrocephalus scirpaceus) is a long-distance migrant passerine with a wide distribution across Eurasia. This species has fascinated researchers for decades, especially its role as host of a brood parasite, and its capacity for rapid phenotypic change in the face of climate change. Currently, it is expanding its range northwards in Europe, and is altering its migratory behavior in certain areas. Thus, there is great potential to discover signs of recent evolution and its impact on the genomic composition of the reed warbler. Here, we present a high-quality reference genome for the reed warbler, based on PacBio, 10×, and Hi-C sequencing. The genome has an assembly size of 1,075,083,815 bp with a scaffold N50 of 74,438,198 bp and a contig N50 of 12,742,779 bp. BUSCO analysis using aves_odb10 as a model showed that 95.7% of BUSCO genes were complete. We found unequivocal evidence of two separate macrochromosomal fusions in the reed warbler genome, in addition to the previously identified fusion between chromosome Z and a part of chromosome 4A in the Sylvioidea superfamily. We annotated 14,645 protein-coding genes, and a BUSCO analysis of the protein sequences indicated 97.5% completeness. This reference genome will serve as an important resource, and will provide new insights into the genomic effects of evolutionary drivers such as coevolution, range expansion, and adaptations to climate change, as well as chromosomal rearrangements in birds.

Reed Warbler Hosts Do Not Fine-Tune Mobbing Defenses During the Breeding Season, Even When Cuckoos Are Rare

Hosts of brood parasitic cuckoos often employ mobbing attacks to defend their nests and, when mobbing is costly, hosts are predicted to adjust their mobbing to match parasitism risk. While evidence exists for fine-tuned plasticity, it remains unclear why mobbing does not track larger seasonal changes in parasitism risk. Here we test a possible explanation from parental investment theory: parents should defend their current brood more intensively as the opportunity to replace it declines (re-nesting potential), and therefore “counteract” any apparent seasonal decline to match parasitism risk. We take advantage of mobbing experiments conducted at two sites where reed warblers (Acrocephalus scirpaceus) experience (in Italy), or do not experience (in Finland), brood parasitism. We predicted that mobbing of cuckoos should be higher overall in Italy, but remain constant over the season as in other parasitised sites, whereas in Finland where cuckoos do not pose a local threat, we predicted that mobbing should be low at the beginning of the season but increase as re-nesting potential declined. However, while cuckoos were more likely to be mobbed in Italy, we found little evidence that mobbing changed over the season at either the parasitized or non-parasitized sites. This suggests that re-nesting potential has either little influence on mobbing behavior, or that its effects are obscured by other seasonal differences in ecology or experience of hosts.

Environmentally driven escalation of host egg rejection decimates success of an avian brood parasite

The black-headed duck (Heteronetta atricapilla) of South America is the only known avian obligate brood parasite with precocial offspring. In Argentina, it relies on two species of coots as primary hosts, which typically reject 35–65% of duck eggs. We show that environmentally driven increases in host egg rejection behavior lead to substantial reductions in the reproductive success of the brood parasite. Episodes of flooding and vegetation loss caused dramatic shifts in host egg-rejection behavior, resulting in rejection (85–95%) of almost all duck eggs. Coots respond to fluctuating water levels by building up their nest, raising their own eggs but leaving duck eggs behind. Coots can apparently recognize parasitic duck eggs, but large-scale rejection is triggered only when hosts must actively make a choice. We use a simple population model to illustrate the unique demographic challenges that black-headed ducks face with their parasitic lifestyle and to explore the potential impact of environmentally induced escalation of egg rejection. Using the best available estimates for key vital rates, we show that obligate parasitism may provide a demographically precarious existence for black-headed ducks, even under benign environmental conditions. Environmentally mediated increases in egg rejection rates by hosts could impact significantly the viability of this enigmatic species of brood parasitic duck. Our results demonstrate that egg rejection rates are not fixed properties of host populations or individuals but are strongly influenced by social and ecological factors. Shifts in these environmental drivers could have important and unforeseen demographic consequences for brood parasites.

Signal detection and optimal acceptance thresholds in avian brood parasite-host systems: implications for egg rejection

Signal detection theory addresses the challenge of successfully identifying informative signals in noisy information contexts, allowing optimal behavioural decisions in diverse ecological contexts. The optimal acceptance threshold hypothesis proposed by Reeve (Reeve 1989 Am. Nat. 133, 407-435. (doi:10.1086/284926)) is an elegant theoretical model to predict the flexibility of acceptance thresholds for conspecific discrimination. This model has provided a robust framework used to explore recognition systems in a broad range of contexts such as animal communication, nest-mate discrimination or anti-parasitic host responses. In this review, we discuss key concepts related to the optimal acceptance threshold hypothesis applied to egg rejection decisions in avian brood parasite-host interactions. We explore those factors determining signal detectability in parasitized nests and how hosts adjust their rejection decisions to both the risk of parasitism and the potential costs associated with egg rejection. In addition, we discuss recent results that challenge some traditional assumptions of the optimal acceptance threshold hypothesis and provide a novel perspective to explore rejection decisions, such as the existence of single-threshold decision rules or acceptance decisions. An integrative view combining current evidence with traditional theory is needed to further advance the comprehension of optimal acceptance thresholds. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Age-Related Brood Parasitism and Egg Rejection in Magpie Hosts

When the strength or nature of a host-parasite interaction changes over the host life cycle, the consequences of parasitism can depend on host population age structure. Avian brood parasites reduce hosts' breeding success, and host age may play a role in this interaction if younger hosts are more likely parasitized and/or less able to defend themselves. We analyzed whether the age of female magpie (Pica pica) hosts is associated with parasite attack or their ability to reject foreign eggs. We recorded parasitism and model egg rejection of known-age individuals over their lifetime and established whether the likelihood of parasitism or egg rejection changed with age or longevity. Parasitism probability did not change with female age, and there was a trend toward longer-lived females being less likely to be parasitized. However, model egg rejection probability increased with age for each individual female, and longer-lived females were more prone to reject model eggs. Most females in the population were young, and the majority of them accepted model eggs, suggesting that brood parasites exploiting younger host individuals are benefitting from a lower defense level of their hosts. Our results stress that the intensity of selection by brood parasites may be mediated by the age structure of host populations, a to-date neglected aspect in brood parasite-host research.

Probing the Limits of Egg Recognition Using Egg Rejection Experiments Along Phenotypic Gradients

Brood parasites lay their eggs in other females' nests, leaving the host parents to hatch and rear their young. Studying how brood parasites manipulate hosts into raising their young and how hosts detect parasitism provide important insights in the field of coevolutionary biology. Brood parasites, such as cuckoos and cowbirds, gain an evolutionary advantage because they do not have to pay the costs of rearing their own young. However, these costs select for host defenses against all developmental stages of parasites, including eggs, their young, and adults. Egg rejection experiments are the most common method used to study host defenses. During these experiments, a researcher places an experimental egg in a host nest and monitors how hosts respond. Color is often manipulated, and the expectation is that the likelihood of egg discrimination and the degree of dissimilarity between the host and experimental egg are positively related. This paper serves as a guide for conducting egg rejection experiments from describing methods for creating consistent egg colors to analyzing the findings of such experiments. Special attention is given to a new method involving uniquely colored eggs along color gradients that has the potential to explore color biases in host recognition. Without standardization, it is not possible to compare findings between studies in a meaningful way; a standard protocol within this field will allow for increasingly accurate and comparable results for further experiments.

Mating system and extra-pair paternity in the Fan-tailed Gerygone Gerygone flavolateralis in relation to parasitism by the Shining Bronze-cuckoo Chalcites lucidus

Extra-pair copulation can increase genetic diversity and offspring fitness. However, it may also increase intra-nest variability in avian hosts of brood parasites, which can decrease the discrimination ability of host parents towards the parasite. In New Caledonia, the Fan-tailed Gerygone (Gerygone flavolateralis), which is parasitized by the Shining Bronze-cuckoo (Chalcites lucidus), has two nestling morphs, dark and bright, that can occur in monomorphic and polymorphic broods. Gerygone parents recognize and eject parasite nestlings from their nest, but the presence of polymorphic broods may increase the chances of recognition errors. Using 17 microsatellite markers, we investigated the mating system of the Fan-tailed Gerygone to understand the mechanisms underlying nestling polymorphism. We hypothesised that extra-pair copulations would lead to a higher proportion of polymorphic broods caused by higher genetic variability, thus creating a trade-off between genetic benefits and host defence reliability. Extra-pair paternity occurred in 6 of 36 broods, which resulted in 6 of 69 offspring sired by extra-pair males. Broods with and without mixed paternity were comparably often parasitized. Extra-pair paternity did not influence the proportions of bright, dark and polymorphic broods. Compared to bright siblings in polymorphic broods, dark nestlings tended to have lower heterozygosity, particularly in loci associated with skin coloration. The results also suggested that there is no obstacle for genetic exchange between individuals from forest and savannah, possibly due to dispersal of offspring. We conclude that the Fan-tailed Gerygone is a socially monogamous species with a low rate of extra-pair paternity compared to closely related species. Extra-pair paternity increased offspring genetic variability without measurable associated costs by brood parasitism. The results highlight the importance of studying host mating systems to assess the trade-offs between host defence and offspring fitness in co-evolutionary arms races.

An experimental test of host's life history traits modulation in response to cuckoo parasitism risk

Hosts can counteract parasites through defences based on resistance and/or tolerance. The mechanistic basis of tolerance, which involve defensive mechanisms minimizing parasite damage after a successful parasitic attack, remains poorly explored in the study of cuckoo-host interactions. Here, we experimentally explore the possibility that the risk of great spotted cuckoo Clamator glandarius parasitism may induce tolerance defences in magpie Pica pica hosts through plasticity in life-history traits. We predict that magpies exposed to auditory cues indicating high parasitism risk will more likely exhibit resistance and/or modify their life-history traits to minimize parasitism costs (i.e. tolerance) compared to magpies under low parasitism risk. We found that manipulating the perceived parasitism risk did not affect host resistance (i.e. rejection of parasitic eggs) nor host life-history traits. Unexpectedly, host’s egg volume increased over the season in nests exposed to auditory cues of control non-harmful hoopoes Upupa epops. Our results do not provide support for inducible defences (either based on resistance or tolerance) in response to risk of parasitism in magpie hosts. Even so, we encourage studying plastic expression of breeding strategies in response to risk of cuckoo parasitism to achieve a better understanding of the mechanistic basis of tolerance defences.

A generalist brood parasite modifies use of a host in response to reproductive success

Avian obligate brood parasites, which rely solely on hosts to raise their young, should choose the highest quality hosts to maximize reproductive output. Brown-headed cowbirds (Molothrus ater) are extreme host generalists, yet female cowbirds could use information based on past reproductive outcomes to make egg-laying decisions thus minimizing fitness costs associated with parasitizing low-quality hosts. We use a long-term (21 years) nest-box study of a single host, the prothonotary warbler (Protonotaria citrea), to show that local cowbird reproductive success, but not host reproductive success, was positively correlated with the probability of parasitism the following year. Experimental manipulations of cowbird success corroborated that female cowbirds make future decisions about which hosts to use based on information pertaining to past cowbird success, both within and between years. The within-year pattern, in particular, points to local cowbird females selecting hosts based on past reproductive outcomes. This, coupled with high site fidelity of female cowbirds between years, points to information use, rather than cowbird natal returns alone, increasing parasitism rates on highly productive sites between years.

Combining personal with social information facilitates host defences and explains why cuckoos should be secretive

Individuals often vary defences in response to local predation or parasitism risk. But how should they assess threat levels when it pays their enemies to hide? For common cuckoo hosts, assessing parasitism risk is challenging: cuckoo eggs are mimetic and adult cuckoos are secretive and resemble hawks. Here, we show that egg rejection by reed warblers depends on combining personal and social information of local risk. We presented model cuckoos or controls at a pair's own nest (personal information of an intruder) and/or on a neighbouring territory, to which they were attracted by broadcasts of alarm calls (social information). Rejection of an experimental egg was stimulated only when hosts were alerted by both social and personal information of cuckoos. However, pairs that rejected eggs were not more likely to mob a cuckoo. Therefore, while hosts can assess risk from the sight of a cuckoo, a cuckoo cannot gauge if her egg will be accepted from host mobbing. Our results reveal how hosts respond rapidly to local variation in parasitism, and why it pays cuckoos to be secretive, both to avoid alerting their targets and to limit the spread of social information in the local host neighbourhood.

Collective defence portfolios of ant hosts shift with social parasite pressure

Host defences become increasingly costly as parasites breach successive lines of defence. Because selection favours hosts that successfully resist parasitism at the lowest possible cost, escalating coevolutionary arms races are likely to drive host defence portfolios towards ever more expensive strategies. We investigated the interplay between host defence portfolios and social parasite pressure by comparing 17 populations of two Temnothorax ant species. When successful, collective aggression not only prevents parasitation but also spares host colonies the cost of searching for and moving to a new nest site. However, once parasites breach the host's nest defence, host colonies should resort to flight as the more beneficial resistance strategy. We show that under low parasite pressure, host colonies more likely responded to an intruding Protomognathus americanus slavemaker with collective aggression, which prevented the slavemaker from escaping and potentially recruiting nest-mates. However, as parasite pressure increased, ant colonies of both host species became more likely to flee rather than to fight. We conclude that host defence portfolios shift consistently with social parasite pressure, which is in accordance with the degeneration of frontline defences and the evolution of subsequent anti-parasite strategies often invoked in hosts of 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.

Oriental reed warbler (Acrocephalus orientalis) nest defence behaviour towards brood parasites and nest predators

Brood parasites and predators pose different threats to passerines that may favour the evolution of enemy-specific defence strategies. Furthermore, potential sex-specific variation in parental investment may be manifested in differences between male and female nest defence behaviour. We investigated these hypotheses in Oriental reed warblers (Acrocephalus orientalis), by recording sex- and stage-specific (nests with eggs or nestlings) responses to stuffed dummies placed at their nests. Warblers showed the highest level of aggression to the co-occurring parasite, the common cuckoo (Cuculus canorus), colour morph (grey), but showed reluctance to mob or attack the co-occurring nest predator, the magpie (Pica pica). There was a sex difference in rate of body attacks towards rufous morph common cuckoo, sparrowhawk (Accipiter nisus) (locally absent parasite and predator, respectively) and the spotted dove (Streptopelia chinensis) (locally present, harmless species), with females showing better ability to distinguish between these species than males.

The role of egg-nest contrast in the rejection of brood parasitic eggs

Hosts of avian brood parasites can avoid the reproductive costs of raising genetically unrelated offspring by rejecting parasitic eggs. The perceptual cues and controls mediating parasitic egg discrimination and ejection are well studied: hosts are thought to use differences in egg color, brightness, maculation, size, and shape to discriminate between own and foreign eggs. Most models of brood parasitism implicitly assume that the primary criteria to which hosts attend when discriminating eggs are differences between the eggs themselves. However, this assumption is confounded by the degree to which chromatic and achromatic characteristics of the nest lining co-vary with egg coloration, in that egg-nest contrast per se might be the recognition cue driving parasitic egg detection. Here we systematically tested whether and how egg-nest contrast itself contributes to foreign egg discrimination. In an artificial parasitism experiment, we independently manipulated egg color and nest lining color of the egg-ejector American robin (Turdus migratorius), a host of the obligate brood parasitic brown-headed cowbird (Molothrus ater). We hypothesized that the degree of contrast between foreign eggs and the nest background would affect host egg rejection behavior. We predicted that experimentally decreasing egg-nest chromatic and achromatic contrast (i.e. rendering parasitic eggs more cryptic against the nest lining) would decrease rejection rates, while increasing egg-nest contrast would increase rejection rates. In contrast to our predictions, egg-nest contrast was not a significant predictor of egg ejection patterns. Instead, egg color significantly predicted responses to parasitism. We conclude that egg-egg differences are the primary drivers of egg rejection in this system. Future studies should test for the effects of egg-nest contrast per se in predicting parasitic egg recognition in other host-parasite systems, including those hosts building enclosed nests and parasites laying cryptic eggs, as an alternative to hypothesized effects of egg-egg contrasts.

Cuckoo hosts shift from accepting to rejecting parasitic eggs across their lifetime

One of the best-known outcomes of coevolution between species is the rejection of mimetic parasite eggs by avian hosts, which has evolved to reduce costly cuckoo parasitism. How this behavioral adaptation varies along the life of individual hosts remains poorly understood. Here, we identify for the first time, lifetime patterns of egg rejection in a parasitized long-lived bird, the magpie Pica pica and show that, during the years they were studied, some females accept, others reject, and some others modify their response to model eggs, in all cases switching from acceptance to rejection. Females tested in their first breeding attempt always accepted the model egg, even those individuals whose mothers were egg rejecters. A longitudinal analysis showed that the probability of egg rejection increased with the relative age of the female, but was not related to the risk of parasitism in the population. We conclude that ontogeny plays a fundamental role in the process leading to egg rejection in magpies.

Evolution: predator versus parasite

Both predators and brood parasites can be major threats to the reproduction of many birds. A new study shows that some cuckoo chicks can help deter nest predators, potentially improving host reproductive success when predation risks are high.

Host responses to interspecific brood parasitism: a by-product of adaptations to conspecific parasitism?

Background

Why have birds evolved the ability to reject eggs? Typically, foreign egg discrimination is interpreted as evidence that interspecific brood parasitism (IP) has selected for the host’s ability to recognize and eliminate foreign eggs. Fewer studies explore the alternative hypothesis that rejection of interspecific eggs is a by-product of host defenses, evolved against conspecific parasitism (CP). We performed a large scale study with replication across taxa (two congeneric Turdus thrushes), space (populations), time (breeding seasons), and treatments (three types of experimental eggs), using a consistent design of egg rejection experiments (n = 1057 nests; including controls), in areas with potential IP either present (Europe; native populations) or absent (New Zealand; introduced populations). These comparisons benefited from the known length of allopatry (one and a half centuries), with no gene flow between native and introduced populations, which is rarely available in host-parasite systems.

Results

Hosts rejected CP at unusually high rates for passerines (up to 60%). CP rejection rates were higher in populations with higher conspecific breeding densities and no risks of IP, supporting the CP hypothesis. IP rejection rates did not covary geographically with IP risk, contradicting the IP hypothesis. High egg rejection rates were maintained in the relatively long-term isolation from IP despite non-trivial rejection costs and errors.

Conclusions

These egg rejection patterns, combined with recent findings that these thrushes are currently unsuitable hosts of the obligate parasitic common cuckoo (Cuculus canorus), are in agreement with the hypothesis that the rejection of IP is a by-product of fine-tuned egg discrimination evolved due to CP. Our study highlights the importance of considering both IP and CP simultaneously as potential drivers in the evolution of egg discrimination, and illustrates how populations introduced to novel ecological contexts can provide critical insights into brood parasite-host coevolution.