Biogeographic history, egg colouration, and habitat selection in Turdus thrushes (Aves: Turdidae).

Despite broad interest and recent experimentation, there is no single ecological model accounting for the adaptive significance of the diversity of avian eggshell colouration. The often blue-green eggs of Turdus thrushes are a charismatic example of this, having long captured cultural and scientific attention. Although the biology and evolutionary history of "true" thrushes is well understood, little is known about correlated evolution between shifts in habitat and eggshell pigmentation, and how these shifts map with Turdus biogeography. We applied phylogenetic comparative methods to assess the evolutionary timing of divergence and variation of life history traits and eggshell colouration and maculation presence in the genus. We found that eggshell colour diversified independently on several occasions in the past 11 million years, with much of the variation occurring within the last 4 million years. The majority of Turdus species lay blue-green eggs and also tend to be sedentary and forest-dwelling. Diet generalist species and species which have transitioned to a forest habitat are more likely to lay white eggs (10% of studied species). In turn, lineages in any habitat were more likely to transition to blue-green eggs. We found that variation in egg colour is increased in some clades, of which two lineages radiated in South America and the East Palearctic, in the past 2-4 million years. These findings provide support for the hypothesis that white eggs are more conspicuous to predators in open environments and that multiple, non-mutually exclusive constraints operate on the adaptive function of avian eggshell colour.

Coloniality and development impact intraclutch consistency of avian eggs: a comparative analysis of the individual repeatability of eggshell size and shape metrics.

In oviparous animals, egg morphology is considered an aspect of the extended phenotype of the laying mother and, thus, can be directly assessed for consistency both within and between individual females. Despite a recently renewed interest in the evolution and mechanics of avian eggshell morphology, we still lack a large-scale, comparative understanding of which egg traits are individually plastic and whether individual consistency is shaped by ecological and life history traits at the species level. Here, we aimed to understand whether intraclutch repeatability per se of different eggshell metrics is an evolving trait that responds to selection pressures from socio-ecological contexts across a diverse group of avian species for which clutch-level eggshell morphology data were available to us. Coloniality, ontogeny, and incubation period had significant impacts on the comparative patterns of relative individual repeatability among two egg metrics (i.e., size and shape), whereas other life history traits (including adult size, clutch size, nest type, migration, breeding latitude, host status of brood parasitism) did not have statistical impacts. Our results also demonstrate that individual consistency has a more widespread phylogenetic distribution than expected by evolutionary contingency across avian diversity. Future analyses should also incorporate the effects of intra- and interspecific covariation in other morphological and physiological traits on the evolution of individual consistency, especially those relevant to egg recognition, including eggshell color and maculation.

A review of the cues used for rejecting foreign eggs from the nest by the Eurasian blackbird (Turdus merula).

Avian brood parasitism is reproductively costly for hosts and selects for cognitive features enabling anti-parasitic resistance at multiple stages of the host's breeding cycle. The true thrushes (genus Turdus) represent a nearly worldwide clade of potential hosts of brood parasitism by Cuculus cuckoos in Eurasia and Africa and Molothrus cowbirds in the Americas. The Eurasian blackbird (Turdus merula) builds an open-cup nest and is common within much of the common cuckoo's (C. canorus) breeding range. While this thrush is known to be parasitized at most only at low rates by this cuckoo, the species is also a strong rejector of nonmimetic foreign eggs in the nest. Given their open-cup nesting habits, we predict that Eurasian blackbirds primarily use visual cues in making a distinction between own and parasitically or experimentally inserted foreign eggs in the nest. We then provide a comprehensive and quantitative review of the literature on blackbird egg rejection studies. This review corroborates that vision is the primary sensory modality used by blackbirds in assessing eggs, but also brings attention to some other, less commonly studied cues which appear to influence rejection, including predator exposure, individual experience, stage of clutch completion, and maternal hormonal state. Blackbirds are also able to recognize and eject even highly mimetic eggs (including those of conspecifics) at a moderate rate, apparently relying on many of the same sensory cues. Although the cues involved in foreign egg recognition by Eurasian blackbirds do not appear specialized to nonmimetic cuckoo parasitism, we cannot differentiate between the possibility of egg rejection being selected by mostly conspecific parasitism or by the evolutionary ghost of a now-extinct, mimetic cuckoo host-race.