Choosing suitable hosts: Common cuckoos Cuculus canorus parasitize great reed warblers Acrocephalus arundinaceus of high quality

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Egg rejection behavior and clutch characteristics of the European Greenfinch introduced to New Zealand

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.Peter Samaš, Lenka Polačiková, Mark E. Hauber, Phillip Cassey, Tomáš Gri

A sum of its individual parts? Relative contributions of different eggshell regions to intraclutch variation in birds

The rejection of eggs of brood parasites in several species of hosts is based on cues only at the blunt pole (BP) and not at the sharp pole (SP) of the foreign and own eggshell. We investigated whether intraclutch variation is confined to a specific egg pole in species where the extent of intraclutch variation in the overall egg appearance is known to positively covary with either egg rejection rate or the probability of being parasitized. For the two poles separately, we analysed intraclutch variation of eggshell brightness and blue chroma. We quantified intraclutch variation as the standard deviations of these colour metrics, instead of their coefficients of variation which would represent a statistically flawed approach. Pooling measurements of brightness across the whole egg surface led to statistically non-significant results and masked positive correlations of BP brightness with egg rejection or parasitism risk, respectively. In contrast, patterns of blue chroma were important across the whole egg. Thus, the traditional whole egg ‘averaging’ approach may mask biologically important effects of intraclutch variation when the variation and potential signalling functions of egg appearance are confined to a specific egg part (brightness). However, analyses based on only BP and SP eggshell region specific data may also lack the power to detect effects of phenotypic traits that do not vary between egg poles (blue chroma). We advocate the use of a combination of region-specific and whole-eggshell based colour metrics and manipulations in cognitive, perceptual, and ecological studies of foreign egg rejection.Lenka Polačková, Mark E. Hauber, Petr Procházka, Phillip Cassey, Marcel Honza and Tomáš Gri

Parents, predators, parasites, and the evolution of eggshell colour in open nesting birds

The colourful surface of birds’ eggshells varies dramatically between species, but the selective pressures driving this variation remain poorly understood. We used a large comparative dataset to test several hypotheses proposed to explain the evolution of eggshell colouration. We tested the hypothesis that predation pressure might select for cryptic eggshells by examining the relationship between predation rate and egg colouration. We found that predation rates were positively related to eggshell brightness. The blackmail hypothesis suggests that females lay colourful eggshells to coerce males into providing additional care during incubation to keep colourful eggs covered. According to this hypothesis, conspicuous eggs should be found in situations with high risk of visual detection from predators or brood parasites. In support of this hypothesis, proportional blue-green chroma was positively related to parasitism risk, and eggs with higher proportional blue-green chroma or higher ultraviolet chroma received higher combined parental nest attendance during the incubation period. The sexual signalling hypothesis states that blue-green colour indicates female quality; however, we did not find that blue-green eggshell colour was greater in species where males participate in any form of parental care, and relative male provisioning was unrelated to blue-green eggshell chroma. We found some support for the hypothesis that brood parasitism may select for high inter-clutch variation in eggshell colour to facilitate egg recognition. In our dataset, parasitism risk was negatively related to inter-clutch repeatability of blue-green chroma. Our study highlights the diversity of selection pressures acting on the evolution of eggshell colour in birds and provides suggestions for novel areas of future key research direction.Daniel Hanley, Phillip Cassey, Stéphanie M. Douce