Animal populations, with a known history of introduction events, provide opportunities to study the dynamics of how rapid shifts in ecological context affect behavioral (e.g., responses to brood parasitism) and life-history (e.g., clutch and egg parameters) traits. We studied the European Greenfinch (Carduelis chloris) introduced to New Zealand, regarding foreign-egg rejection behaviors and also compared their clutch characteristics with data from the source populations in the United Kingdom. Although previously this species had been considered an unsuitable host for the Common Cuckoo (Cuculus canorus), and not impacted by selection pressure associated with brood parasitism, we found that Greenfinches in our study population were able to eject experimental eggs at low frequencies. In contrast, nest desertion rates were similar in experimentally parasitized and control unmanipulated nests, implying that nest desertion is not an antiparasite adaptation in this species. Contrary to previous studies, we did not find significant differences in clutch and egg sizes between introduced and source populations. This study emphasizes (1) the importance of using control treatments in studies of host responses to experimental parasitism, (2) including apparently unsuitable hosts of brood parasites, and (3) meta-replicating prior studies to further the process of gaining and validating scientific knowledge.

Common Cuckoos (Cuculus canorus) parasitize nests of small passerines. The Cuckoo chicks cause the death of their nest-mates when evicting eggs or nestlings from the nests; consequently, hosts suffer from a high loss of reproduction. Host adaptations against parasitism, e.g., by egg discrimination behavior, and cuckoo counter-adaptations to hosts, e.g., by mimetic eggs, are often regarded as a result of the arms race between the two interacting species. In Hungary Great Reed Warblers (Acrocephalus arundinaceus) are the main hosts of cuckoos, suffering from heavy parasitism (ca. 40-65%). The Oriental Reed Warbler (A. orientalis), formerly a subspecies of the Great Reed Warbler (A. a. orientalis), is also a highly parasitized host in Japan (25-40%). We compared main characteristics of Cuckoo parasitism in these two distant areas from the Western and Eastern Palearctic by comparing cuckoo egg mimicry. We measured color characteristics of host and parasitic eggs by spectrophotometer. Visual modeling revealed lower chromatic distances between Cuckoo and host eggs in Hungary than in Japan, but high variation both in host and Cuckoo eggs may cause matching problems in Hungary. Achromatic (brightness) difference between host and Cuckoo eggs were lower in Japan than in Hungary, and it proved to be the most important factor affecting egg rejection. Hosts rejected Cuckoo eggs at similar frequencies (37% and 35% in Hungary and Japan, respectively). Host adaptation, i.e., egg rejection behavior, seems to be preceding Cuckoo counter-adaptations to hosts in Japan. We suggest that the Cuckoo-Great/Oriental Reed Warbler relationships developed in alternative ways in Japan and Hungary, and they represent different stages of their arms race.

Understanding the occurrence of multiple distinct phenotypes in a population of a species, i.e., polymorphism, is one of the challenges encountered in evolutionary biology. Egg color polymorphism in birds is one example of morphological polymorphism and disruptive selection has been proposed as a hypothetical mechanism to explain its occurrence. We studied how polymorphic egg colors (immaculate blue and white) occur in Korean populations of the Vinous-throated Parrotbill (Paradoxornis webbianus). Egg color ratios (the proportion of nests with blue eggs in a population) were monitored over a large spatial scale and egg colors were quantified using a spectrophotometer. We found egg color ratios to vary spatially among populations. Interestingly, there was a latitudinal morph-ratio cline in egg color ratios. The proportion of nests with blue eggs increased considerably with the latitude declined towards the southern part of the Korean peninsula. There were some quantitative variations in egg colors among populations. However, the pattern of variations was not consistent with those of the population egg color ratios. Based on these results, we discuss a potential scenario for the evolution of egg color polymorphism in the Vinous-throated Parrotbill.

The coevolutionary arms race between cuckoos and their hosts predicts that low variation in egg features within a host clutch would facilitate discrimination of mimetic parasite eggs. Here, we experimentally examine this prediction by using artificial and natural parasite eggs showing contrasting level of matching with host eggs. We quantified, based on human assessment, intra-clutch variation in egg appearance and egg discrimination in the Iberian Azure-winged Magpie (Cyanopica cyanus), a presumed former host of the Great Spotted Cuckoo (Clamator glandarius). Azure-winged Magpies rejected parasitic eggs in relation to their degree of dissimilarity with own eggs: Great Spotted Cuckoo model eggs were relatively more often rejected (73.7%) than real Great Spotted Cuckoo eggs (44.4%) and the less contrasting conspecific eggs (35.5%). Contrary to our predictions, we found that, irrespective of mimicry level of parasitic eggs, intra-clutch variation in host egg appearance did not differ significantly between rejecters and acceptors. We found, however, that individuals with higher variation in egg-size were almost significantly more prone to be rejecters than individuals showing lower variation in egg size. Our results do not support the hypothesis that the extent of intraclutch variation in egg discrimination varied with parasite egg mimicry in this particular system, and add to previous findings suggesting that perhaps an increase in intra-clutch variation in egg appearance, rather than a decrease, might be advantageous when discriminating against non-mimetic Great Spotted Cuckoo eggs.

Different lineages of birds show varying sensitivity to light in the ultraviolet (UV) wavelengths. In several avian brood parasite-host systems, UV-reflectance of the parasite eggs is important in discriminating own from foreign eggs by the hosts. In turn, for parasitic females it may be beneficial to lay eggs into host clutches where eggs more closely match the parasite's own eggs. While the visual sensitivities of numerous cuckoo- and cowbird-host species have been described, less is known about those of their respective parasites. Such sensory characterization is important for understanding the mechanisms underlying potential perceptual coevolutionary processes between hosts and parasites, as well as for better understanding each species' respective visual sensory ecology. We sequenced the short wavelength-sensitive type 1 (SWS1) opsin gene to predict the degree of UVsensitivity in both of New Zealand's obligate parasitic cuckoo species, the Shining Cuckoo (Chalcites[Chrysococcyx] lucidus) and the Long-tailed Cuckoo (Urodynamis[Eudynamis] taitensis). We show that both species are predicted to possess SWS1 opsins with maximal sensitivity in the human-visible violet portion of the short-wavelength light spectrum, and not in the UV. Future studies should focus on the (mis)matching in host-parasite visual sensitivities with respect to host-parasite egg similarity as perceived by the avian visual system and the behavioral outcomes of foreign egg rejection.

With the knowledge that cuckoos and cowbirds lay their eggs parasitically, and that some hosts eject parasitic eggs, ornithologists began to ponder the question of how host females discriminate between a foreign egg and their own eggs, wondering how hosts "know" which egg to remove. Results of one of the first uncontrolled experiments were inappropriately interpreted to imply ejection was based on discordancy, with hosts simply ejecting the egg in the minority, or the "odd-looking" egg. Controlled experiments eventually revealed that hosts first learn the appearance of own their eggs and discriminate between them and any odd egg in their nest, regardless of which egg type is in the minority. Recent work has shown that discordancy may play a role in discrimination by males mated successively with females that lay polymorphic eggs. We examine the details of the early experiments, in light of recent advances in studies of egg recognition. An ability to recognize eggs also has been extended, implicitly, to include obligate brood parasites, as it underlies several hypotheses in explanation of the behavior of parasites toward their hosts. Egg recognition in parasites, however, has not been experimentally confirmed, nor has a mechanism been identified by which parasites could discriminate between their own eggs and the other eggs in a nest. We review hypotheses (parasite competition, egg removal and multiple parasitism, mafia, farming) that require the ability of obligate brood parasites to discriminate eggs at different levels and the potential mechanisms used by parasites to recognize their own eggs and suggest experiments to test for egg discrimination. An assessment of the egg recognition ability of parasites is germane to our understanding of how parasites counteract defenses of hosts.

A central tenet of coevolutionary theory, including theory of the coevolutionary relationship between brood parasites and their hosts, is that temporal and spatial patterns may reveal important information about ecological and evolutionary dynamics. For instance, level of genetic structure of populations provides important information about the role of genetics and gene flow in determining local patterns of selection on hosts due to parasitism (i.e., egg rejection) and on parasites due to selection by hosts (i.e., egg mimicry). Furthermore, abiotic (i.e., climatic conditions) and biotic (phenotypic characteristics of animals) factors that also vary spatially may directly or indirectly affect populations of hosts and brood parasites and, therefore, their interaction. By reviewing the literature, we found considerable evidence for an effect of the spatially and temporally structured abiotic environment on the phenotype of both parasite and host eggs and the degree of mimicry. Moreover, we found examples suggesting that specific life history characteristics of hosts that vary geographically and/or temporally may affect the probability of initial colonization of a new host species and the direction and the speed of coevolution. We provide an exhaustive review of studies investigating temporal and spatial patterns of the interaction between brood parasites and their hosts. Such temporal and spatial trends in parasite and host traits are, together with genetic information on rejection and significant effects of gene flow, consistent with coevolutionary dynamics. However, gene flow and changes in the temporal and spatial patterns of abundance of both parasites and hosts may result in frequent cases of counter-intuitive relationships between the phenotype of the parasite and that of the host (i.e., poor or no mimicry), which may suggest limits to the degree of adaptation. We provide a list of scientific questions in need of further investigation, concluding that studies of brood parasites and their hosts may play a central role in testing the geographic theory of coevolution and several alternative hypotheses.

Brood parasitic birds constitute a model system for the study of coevolution. Such parasites are unique by having evolved unusually thick eggshells for their body size. Thick eggshells have been hypothesized to evolve as 1) a means of preventing damage to parasite eggs when the brood parasite lays its egg at a distance from the host clutch (the laying damage hypothesis); 2) a consequence of host puncture ejection (the puncture resistance hypothesis); 3) a means for the brood parasite to allocate calcium to development of a disproportionately large skeleto-muscular system in evicting parasite chicks (the chick vigour hypothesis); or 4) a means of protecting the cuckoo embryo from microorganisms in the nest of the host (the anti-bacterial protection hypothesis). Here we review the literature studying the evolutionary mechanisms promoting thick eggshells in avian brood parasites,and provide proposals for future studies to test their validity. Available data are insufficient to rigorously test exclusive predictions and assumptions of these not necessarily exclusive hypotheses,although the laying damage and the puncture resistance hypotheses seem to currently be the most well supported alternatives. We discuss how quantification of rejection modes (grasp ejection,puncture ejection and desertion) may disclose the validity of the puncture resistance hypothesis,and finally we provide perspectives for future research on testing this specific hypothesis.