Kittiwake eggs viewed by conspecifics and predators: Implications for colour signal evolution

Abstract

Colour traits evolve under the antagonistic selective pressures of increasing detectability to intended receivers (e.g. conspecifics) while minimizing detectability to unintended receivers (e.g. predators). Primarily selected for camouflage, avian egg coloration has also been suggested to have intraspecific signalling functions. A first step in assessing the relative importance of camouflage/communication requires quantifying detectability for different visual systems, which is rarely done. Here, we compared brightness and colour contrasts of black-legged kittiwake (Rissa tridactyla) speckled and soiled eggs as seen from a distance by conspecifics and their main avian predator, the common raven (Corvus corax). We also examined how egg speckling and soiling deposition on incubated eggs affected egg detectability and egg coloration variability. We found that eggs were detectable by both species, but were more conspicuous to conspecifics than to predators. Egg speckling reduced brightness contrast and increased colour contrast, contributing to reducing detectability by predators while maintaining potential for intraspecific communication, respectively. Soiling reduced egg detectability for both perceivers, but soiled eggs remained more conspicuous to conspecifics than predators. Moreover, soiling did not homogenize the clutch signature of egg colour and speckling, thus allowing intraspecific exploitation of information on female quality that egg coloration may convey or individual clutch recognition. We suggest that further studies of egg colour adaptive functions and evolution should consider all perceivers as well as soiling deposition, an overlooked mechanism that may be favoured by selection against predation.