Eggshell conspicuousness in ground nesting birds: do conspicuous eggshells signal nest location to conspecifics?

Abstract

Predators can adversely impact nesting success and therefore the presence of seemingly conspicuous eggshells would appear maladaptive. The ratites, especially the tinamous, exhibit a diverse array of eggshell colours that appear to differ strikingly from their nesting substrate, while most ground-nesting species that do not build a nest lay camouflaged eggs. Surprisingly, there is little research on how these nest contents appear to ecologically-relevant viewers, including conspecifics and predators. Here we use visual modelling to compare eggshell conspicuousness in two distinct avian lineages differing in eggshell colour and breeding biology: ratites and galliformes. Ratites generally lay vibrant, unspeckled eggs directly on the ground, while galliformes tend to lay subtler, speckled eggs on nests built on the ground. We test the hypothesis that eggshell colours in ratites are more conspicuous, from the perspective of an avian conspecific, than those of galliformes. We found that the uniform colour covering the surface of the eggshell colour (hereafter, eggshell background colour) differs noticeably from the nesting substrate in both ratites and galliformes. However, the speckling pattern of galliform eggs often masks their conspicuous eggshell background colour, which contributes to a less conspicuous appearance overall. We tested the hypothesis that eggshell conspicuousness in ratites serves an intraspecific signalling function to advertise nest location to females in communally nesting species. We found no support for this hypothesis, suggesting that selection pressure for communal laying did not result in the diversity of conspicuousness found in avian eggs. Overall, we argue that future investigations of egg coloration should consider egg appearance (eggshell background colour and speckling) in the context of the natural nest substrate, all from the perspective of the relevant visual receiver.