Foreign Eggs Research Articles

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.

Now you see it, now you don't: flushing hosts prior to experimentation can predict their responses to brood parasitism.

Brood parasitic birds lay their eggs in other birds' nests, leaving hosts to raise their offspring. To understand parasite-host coevolutionary arms races, many studies have examined host responses to experimentally introduced eggs. However, attending parents often need to be flushed from their nests to add experimental eggs. If these birds witness parasitism events, they may recognize and reject foreign eggs more readily than parents who did not. We found that, after being flushed, female blackbirds, Turdus merula, remained close to their nests. Flushed females were more likely to eject foreign eggs and did so more quickly than females that were not flushed during experimentation. In contrast, flushing did not predict responses and latency to responses to parasitism by song thrush, Turdus philomelos, which flew farther from their nests and likely did not witness experimental parasitism. When statistically considering flushing, previously published conclusions regarding both species' response to experimental parasitism did not change. Nevertheless, we recommend that researchers record and statistically control for whether hosts were flushed prior to experimental parasitism. Our results have broad implications because more vigilant and/or bolder parents can gain more information about parasitism events and therefore have better chances of successfully defending against brood parasitism.

Absence of egg rejection in an Asian population of house sparrow (Passer domesticus), a conspecific brood parasite in Europe

The house sparrow (Passer domesticus) is well known for conspecific brood parasitism in Europe, and there it is capable of recognizing and rejecting non-mimetic foreign eggs and even mimetic conspecific eggs. Here, we tested the egg rejection of an Asian population of house sparrows by using model eggs, sparrow eggs painted red and sparrow eggs with added brown spots, of which their mimicry to sparrow eggs were quantified by granularity analysis based on avian vision. Surprisingly, house sparrows in Asia did not show any egg rejection. We suggest that the evolutionary equilibrium theory, which was first proposed to explain the absence of egg rejection in some hosts of interspecific brood parasites, may explain the deficiency of egg rejection in house sparrows, as a result of trade-offs between the high rejection costs and the costs of raising conspecific foreign chicks.

The Value of Artificial Stimuli in Behavioral Research: Making the Case for Egg Rejection Studies in Avian Brood Parasitism

AbstractExperimentation is at the heart of classical and modern behavioral ecology research. The manipulation of natural cues allows us to establish causation between aspects of the environment, both internal and external to organisms, and their effects on animals' behaviors. In recognition systems research, including the quest to understand the coevolution of sensory cues and decision rules underlying the rejection of foreign eggs by hosts of avian brood parasites, artificial stimuli have been used extensively, but not without controversy. In response to repeated criticism about the value of artificial stimuli, we describe four potential benefits of using them in egg recognition research, two each at the proximate and ultimate levels of analysis: (1) the standardization of stimuli for developmental studies and (2) the disassociation of correlated traits of egg phenotypes used for sensory discrimination, as well as (3) the estimation of the strength of selection on parasitic egg mimicry and (4) the establishment of the evolved limits of sensory and cognitive plasticity. We also highlight constraints of the artificial stimulus approach and provide a specific test of whether responses to artificial cues can accurately predict responses to natural cues. Artificial stimuli have a general value in ethological research beyond research in brood parasitism and may be especially critical in field studies involving the manipulation of a single parameter, where other, confounding variables are difficult or impossible to control experimentally or statistically.

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 their own and foreign eggs. Most theories 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, so 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 those parasites laying cryptic eggs, as an alternative to hypothesized effects of egg-egg contrast.

Ambient light in domed nests and discrimination of foreign egg colors

In cuckoo-host coevolution, rejection of parasite eggs based on visual discrimination is the key host-defensive mechanism reducing the costs of parasitism. Although host discriminatory tasks often occur in variable environmental conditions, the influence of nest light variation on the perceptual processes involved in egg discrimination has been seldom considered. Here, we combine visual modeling, experimental manipulation of nest ambient light, and egg recognition experiments with model eggs differing in background color (cream vs. blue) to explore the possibility that variation in ambient light in magpie (Pica pica) domed nests may affect the perceptual process involved in discrimination of foreign eggs. We found that the architecture of magpie nests affects the quality of ambient light for egg recognition and that changes in luminosity did not differently affect rejection of blue and cream models. However, we found that ejection of model eggs declined throughout the season in nests in which luminosity remained unmodified, but that magpies rejected eggs at a similar rate over the season in nests in which luminosity was increased. These results therefore suggest that variation in ambient light at the nests might potentially influence the perceptual processes involved in visual detection of parasite eggs by cuckoo hosts.

The evolutionary causes of egg rejection in European thrushes (Turdusspp.): a reply to M. Soler

Correspondence A response to: Soler: No evidence of conspecific brood parasitism provoking egg rejection in thrushes. Frontiers in Zoology 2014, 11:68. Several species of European thrushes (genus Turdus) reject foreign eggs, yet the evolutionary causes of this behavior remained unclear. Recently, we concluded that conspecific (CBP), rather than interspecific brood parasitism (IBP) is responsible for the patterns of foreign egg rejection in these taxa [1]. In his comment M. Soler does not agree with this conclusion. Here we address each of his four main points to demonstrate the principles that (i) a theory is never out of date but is either supported or discredited, and (ii) it is not the use of references, but instead the collection and analysis of new data, which advances scientific progress. First, Soler argues that the costs of CPB are too low to select for the evolution of host defenses and that the foreign eggs are too similar to host eggs for effective discrimination. That costs of CPB seem to be generally low across various species does not necessarily mean that such costs are always low (see also p. 51 in [2]). For example, IBP is generally highly costly to hosts but in several hosts of the brown-headed cowbird (Molothrus ater), there is little or no detectable cost of IBP [3]. Regarding egg similarity, inter-clutch variation in both Turdus species that we studied is sufficiently high to allow for foreign conspecific egg discrimination both in theory [4] and in reality [1]. Second, Soler takes issue with the maintenance versus decay of egg rejection behaviors as an antiparasitic host defense in the absence of ongoing brood parasitism. Crucially, the empirical studies referenced by Soler reported that the rejection costs associated with the ejection

Effect of parasite-to-host egg ratio on egg rejection by a Brown-headed Cowbird host

ABSTRACT American Robins (Turdus migratorius) typically eject parasitic Brown-headed Cowbird (Molothrus ater) eggs from their nests. In order to successfully remove parasitic eggs, robins must first differentiate between their eggs and foreign eggs, and then remove the foreign egg(s). Our primary objectives were to determine (1) whether the robins reject cowbird eggs because they are in the minority (“discordancy hypothesis”) or because robins have learned the appearance of their own eggs regardless of whether they form the majority of eggs in the nest (“true egg recognition hypothesis”); and (2) whether the robin's ability to recognize its eggs and reject parasitic eggs was affected by the parasite-to-host egg ratio. We added artificial cowbird eggs to robin nests to create 3 treatments: (1) a majority of robin eggs, (2) an equal number of robin and cowbird eggs, and (3) a majority of cowbird eggs. Parasite-to-host egg ratios were between 1:3 and 3:1. Robins ejected all cowbird eggs at 88% of nests (51 o...

Rejection of parasitic eggs in passerine hosts: Size matters more for a non-ejecter

The evolution of egg mimicry by parasites and the recognition of foreign eggs by hosts are important components of the coevolutionary arms race between brood parasites and their hosts, which is one of the most behaviorally complex of all host–parasite interactions. To examine cues used by the Yellow Warbler (Setophaga petechia), a non-ejecter that buries or deserts eggs laid by parasitic Brown-headed Cowbirds (Molothrus ater), we added model eggs of different sizes and colors to nests. We also reviewed the literature to investigate the effects of model egg surface color (background and maculation) and size on the response of hosts that eject. We predicted that size would be more important for Yellow Warblers as they likely use tactile cues to bury or desert parasitized clutches and color cannot be assessed tactilely. In Yellow Warblers, rejection frequency increased as size and color diverged more from real warbler eggs. Egg size was not generally used as a criterion for egg rejection, however, across different species that eject parasitic eggs. Color was the only model egg parameter, out of color and size, that significantly affected rejection in these ejecter hosts. Tactile cues are therefore not used by ejecters but are more important in a host that uses methods of rejection that do not require egg discrimination, such as burial and desertion. Of metrics that took into account the ultraviolet range, achromatic Just Noticeable Differences (brightness) in Yellow Warblers better predicted rejection of model eggs based on color than chromatic Just Noticeable Differences (hue). The high costs of burial and desertion may have led to multiple (size and color) egg discrimination abilities in the Yellow Warbler.

Nest sanitation behavior in hirundines as a pre-adaptation to egg rejection to counter brood parasitism.

Previous studies suggested that nest sanitation behavior may have been a pre-adaptation from which egg rejection of brood parasite eggs evolved. We tested this hypothesis in two swallow species, the red-rumped swallow (Cecropis daurica) and the barn swallow (Hirundo rustica). Our results indicated that the red-rumped swallow, which is an accepter of foreign eggs, rejected a low percentage of non-egg-shaped objects and did so less often than the barn swallow, which is an intermediate rejecter of foreign eggs. Furthermore, the egg rejection rates of the barn swallow increased with the increase in rejection rates of non-egg-shaped objects among different populations. These results showed that nest cleaning behavior could have evolved into a means of reducing the costs of brood parasitism, suggesting that egg recognition ability has evolved from recognition of non-egg-shaped objects. This finding advances our understanding of the evolution of egg recognition behavior in birds.

Does nest luminosity play a role in recognition of parasitic eggs in domed nests? A case study of the red bishop.

Certain light environments may hinder egg discrimination by hosts of foreign eggs, which could in some circumstances lead to the acceptance of non-mimetic eggs by hosts. We measured light parameters at red bishop (Euplectes orix) nests and used a model of avian visual processing to quantify the detectability of eggs in the light environment in which they are perceived. We found that the overall amount of light was very variable between red bishop nests and always sufficient for colour discrimination. A model of avian visual processing revealed that nest luminosity had no influence on host responses towards eggs which were painted dark brown. Dark eggs do not appear to be cryptic in red bishop nests and can be distinguished with ease, whereas natural red bishop eggs are usually accepted, despite the domed structure of the nest. We found little variation in both chromatic and achromatic contrasts between host and artificial eggs, indicating that there was very little variation in the light quality inside nests. We suggest that nest luminosity is likely to play a role in egg recognition in situations when light reaches threshold values for colour discrimination, i.e. in scotopic as opposed to photopic vision. Rejection rates for dark eggs were higher than for bright (conspecific) foreign eggs. More investigation of domed nest-building species is required, as this type of nest appears to have a highly variable light environment, dependent on both nest structure and habitat.

Who moved my eggs? An experimental test of the egg arrangement hypothesis for the rejection of brood parasitic eggs.

Avian brood parasitism is an exceptional reproductive strategy whereby parasites reduce their own costs associated with parental care and impose them on the host parents. Consequently, host species have evolved multiple defensive mechanisms to combat parasitism. The vast majority of research attention to date has examined host defenses to recognize and reject parasitic eggs. The recently proposed “egg arrangement hypothesis” suggests that hosts may not focus solely on individual eggs’ features, but instead the overall arrangement of the clutch may also provide a cue that parasitism has occurred. Correlative data revealed that host females maintaining a consistent egg arrangement across the incubation period were more likely to reject foreign egg models than females that did not keep a consistent egg arrangement. Here, we provide the first experimental test of this hypothesis in the European blackbird (Turdus merula). We experimentally parasitized nests such that the egg arrangement was either disrupted or not disrupted. We found no evidence that altered egg arrangement was used as a cue for egg rejection by host females. Therefore, we suggest that females that keep consistent egg arrangement are more likely to eject foreign eggs for other correlated reasons. Thus, egg arrangement does not serve as an independent cue to trigger egg rejection responses to parasitism in this host species.Electronic supplementary materialThe online version of this article (doi:10.1007/s10071-014-0800-x) contains supplementary material, which is available to authorized users.

Testing for correlations between behaviours in a cuckoo host: why do host defences not covary?

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.

Introduction: ‘flour power’ and feminism between the waves

Did feminist militancy cease when the Pankhursts called a halt to the activities of the WSPU in 1914? Was the suffragette spirit quashed with the advent of the First World War, and due to the achievement of women’s partial (1918) and then equal (1928) suffrage thereafter, having to wait to be reclaimed by the Women’s Liberation Movement in the atmosphere of ‘flower power’ in the late 1960s? Indeed, there is some compelling evidence to suggest that during the 1930s this legacy was a living one, even among young women who were coming to terms with their political identities and resisting through direct action the keenly felt backlash against sexual equality. A rather unusual incident in Ilfracombe, an otherwise sleepy seaside town in North Devon, illustrates well the persistence of feminist consciousness and even of militancy. On 19 February 1937, an organised gang of local ‘spinsters’ in Ilfracombe ‘took war into the enemy camp’ and stormed a smoking concert being held at the Bachelors Club at the Royal Clarence Hotel. Wearing masks and armed with bags of flour and water pistols, they led the raid on the hotel at 9.30 in the evening, taking entirely off guard members who were seated comfortably smoking, drinking, and enjoying the performance of an artiste. The women, one of whom was reported to be carrying a banner, ‘flung their bags of flour right and left’ until many of the bachelors were covered head to foot, and then from atop tables and chairs they fired their water pistols at the flour-covered bachelors. Scuffles ensued when some of thebachelors caught hold of some of the spinsters but the men failed to maintain their grip, their hands pasty with the flour and water mixture. One of the women assailants eventually requested to be photographed with the men, and refused to leave until this was granted. The ring leader was identified as Miss Dinah Hewett, described in the local paper as an ‘Ilfracombe blonde’ (together with her photo portrait showing a young, attractive woman, fashionably attired with pearl earrings and a neck kerchief). She had organised it all with a friend, because ‘We were determined to force an entry, although the men said we should not. We showed them that we women can be really tough when put to the test.’1 Their actions to close down the Bachelors Club were relatively spontaneous but they also intended to solicit the support of Miss Marjorie Graves, ex-MP, to champion their cause once she was adopted as the Conservative candidate for the NorthWest Devon Division. There was an immediate ripple effect to the actions of the Ilfracombe spinsters,and both men and women armed for a copy-cat confrontation in near-by Plymouth. Clearly the local papers that reported these incidents did so tongue in cheek, the parodying narrative tone consistent with that evident in so much of the press coverage of feminist political mobilisation earlier in the century. There was an escalation in violence with the Plymouth incident when Miss Jessie Lugg mustered a force of spinsters, described as ‘25 Amazons’, to wreak the first meeting of the Plymouth and District Bachelors Club, formed by Stocker P. Moss of the Royal Naval Barracks. Lugg said:The Ilfracombe girls doused their beer with flour but we are prepared to use more drastic measures than that. Arrangements have been made for a stock of foreign eggs and over-ripe tomatoes. That’s the kind of artillery to turn loose on those fellows. We may use flour also, but if we do it will be liberally mixed with pepper.2

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

BackgroundWhy 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.ResultsHosts 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.ConclusionsThese 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.

Recognizing odd smells and ejection of brood parasitic eggs. An experimental test in magpies of a novel defensive trait against brood parasitism.

One of the most important defensive host traits against brood parasitism is the detection and ejection of parasitic eggs from their nests. Here, we explore the possible role of olfaction in this defensive behaviour. We performed egg-recognition tests in magpie Pica pica nests with model eggs resembling those of parasitic great spotted cuckoos Clamator glandarius. In one of the experiment, experimental model eggs were exposed to strong or moderate smell of tobacco smoke, whereas those of a third group (control) were cleaned with disinfecting wipes and kept in boxes containing odourless cotton. Results showed that model eggs with strong tobacco scent were more frequently ejected compared with control ones. In another experiment, models were smeared with scents from cloacal wash from magpies (control), cloacal wash or uropygial secretions from cuckoos, or human scents. This experiment resulted in a statistically significant effect of treatment in unparasitized magpie nests in which control model eggs handled by humans were more often rejected. These results provide the first evidence that hosts of brood parasites use their olfactory ability to detect and eject foreign eggs from their nests. These findings may have important consequences for handling procedures of experimental eggs used in egg-recognition tests, in addition to our understanding of interactions between brood parasites and their hosts.

How to Spot a Stranger's Egg? A Mimicry‐Specific Discordancy Effect in the Recognition of Parasitic Eggs

AbstractEgg discrimination by hosts is an antiparasitic defence to reject foreign eggs from the nest. Even when mimetic, the presence of brood parasitic egg(s) typically alters the overall similarity of all eggs in a clutch, producing a discordant clutch compared to more homogenous clutches of composed only of hosts’ own eggs. In multiple parasitism, the more foreign eggs are laid in the nest, the more heterogeneous the overall clutch appears. Perceptual filters and recognition templates cannot explain the known pattern of lower rejection rates of foreign eggs in multiple vs. single parasitism. We therefore assessed the role of clutch homogeneity and manipulated the colour of one or more eggs in the clutches of great reed warbler (Acrocephalus arundinaceus) hosts of common cuckoos (Cuculus canorus). Varying the colours of both the majority and the minority eggs caused predictable shifts in the rejection of the focal egg(s), and ejection rates of the minority egg colour consistently increased but only when it belonged to a more mimetic egg colour, relative to the less mimetic colour of majority eggs. The results imply that in addition to sensory filters, and template‐based cognitive decision rules, discordancy‐based rejection is affected by the overall clutch appearance and interacts with specific colours varying in the extent of mimicry, to contribute to the recognition decisions of hosts to reject parasitic eggs.

A recoverable cost of brood parasitism during the nestling stage of the American robin (Turdus migratorius): implications for the evolution of egg rejection behaviors in a host of the brown-headed cowbird (Molothrus ater)

Hosts of brood parasitic birds face reduced reproductive success as a direct consequence of rearing parasitic young. The most commonly evolved host behavior to combat costly parasitism is the rejection of foreign eggs. Despite consistent patterns of reduced nesting success in broods parasitized by brown-headed cowbirds (Molothrus ater), most of its host species do not reject foreign eggs. Paradoxically, where it is present, egg rejection is more common among cowbird hosts with larger body sizes, yet cowbird chicks are less likely to outcompete larger host nest mates. This raises a fundamental question whether egg rejection, even in the minority of cowbird hosts, has specifically evolved in response to costly brood parasitism. Here we tested predictions associated with this hypothesis in an egg-rejecter host, the American robin (Turdus migratorius), by assessing whether experimental cowbird parasitism causes reduced nesting success. We cross-fostered cowbird and control, host chicks into robin nests, testing for experimental effects of chick species, brood size, and hatching asynchrony; unmanipulated nests served as additional controls. Rearing a foreign chick in the brood reduced the reproductive output of host robins, however, we detected no effect of parasitizing host nests with a cowbird versus robin chick. Cowbird chicks were significantly less likely to fledge than cross-fostered robin chick controls. These experiments reveal that parasitism itself can exert a cost on robin hosts during the nestling stage, representing a recoverable cost of cowbird parasitism that can be avoided by the host through rejecting parasite eggs prior to hatching. These results support the assumption that foreign egg rejection is an evolved host response to brood parasitism in this system, despite the poor survival rate of cowbird chicks in robin broods.

Mafia or Farmer? Coevolutionary consequences of retaliation and farming as predatory strategies upon host nests by avian brood parasites

Many host species of avian brood parasites have evolved to recognize and reject foreign eggs and chicks in the nest. Yet, other hosts accept and care for parasitic young, despite the fitness losses associated with raising non-kin. It has been suggested that nest predation upon host nests by brood parasites could select for coevolved acceptance by hosts, even when their cognitive and motor traits allow for the successful rejection of brood parasitism. Using a modeling approach, I analyzed the conditions that favor the evolution of two predatory strategies by parasites and the acceptance of parasitism in the presence of predatory parasites. The Mafia strategy represents retaliatory parasites that punish rejecter hosts by depredating their nests. In contrast, the Farmer strategy represents farming parasites which depredate advanced stage host nests. Both predatory strategies benefit when hosts become available for future parasitism by renesting. The modeling showed that higher rates of parasitism and rejecti...

Naïve hosts of avian brood parasites accept foreign eggs, whereas older hosts fine-tune foreign egg discrimination during laying.

BackgroundMany potential hosts of social parasites recognize and reject foreign intruders, and reduce or altogether escape the negative impacts of parasitism. The ontogenetic basis of whether and how avian hosts recognize their own and the brood parasitic eggs remains unclear. By repeatedly parasitizing the same hosts with a consistent parasitic egg type, and contrasting the responses of naïve and older breeders, we studied ontogenetic plasticity in the rejection of foreign eggs by the great reed warbler (Acrocephalus arundinaceus), a host species of the common cuckoo (Cuculus canorus).ResultsIn response to experimental parasitism before the onset of laying, first time breeding hosts showed almost no egg ejection, compared to higher rates of ejection in older breeders. Young birds continued to accept foreign eggs when they were subjected to repeated parasitism, whereas older birds showed even higher ejection rates later in the same laying cycle.ConclusionsOur results are consistent with the hypotheses that (i) naïve hosts need to see and learn the appearance of their own eggs to discriminate and reject foreign eggs, whereas (ii) experienced breeders possess a recognition template of their own eggs and reject parasitic eggs even without having to see their own eggs. However, we cannot exclude the possibility that other external cues and internal processes, accumulated simply with increasing age, may also modify age-specific patterns in egg rejection (e.g. more sightings of the cuckoo by older breeders). Future research should specifically track the potential role of learning in responses of individual hosts between first and subsequent breeding attempts by testing whether imprinting on a parasitized clutch reduces the rates of rejecting foreign eggs in subsequent parasitized clutches.