Imperfectly camouflaged avian eggs: Artefact or adaptation?

For many birds, the fight for survival begins at the egg stage: avoiding predation is paramount. At a broad phylogenetic level, selection by predators for egg camouflage appears to be the primary driver of variation in egg colouration and patterning. Despite this, experiments at fine-scale taxonomic levels have largely failed to find support for adaptively camouflaged egg appearance. How are we to resolve this baffling contradiction? Here we present and evaluate five explanations which are not mutually exclusive and which may explain why eggs appear imperfectly camouflaged at the genus or species level. First, imperfect camouflage may be an artefact of imperfect measurements. In studies of egg camouflage, researchers have consistently neglected to account for predator vision, and only rarely have egg appearance and camouflage been quantified carefully and objectively. Recalibrating our assessment of egg camouflage may answer many questions, but it is unlikely to wholly explain why many eggs do not appear seamlessly cryptic. Instead, imperfect camouflage may stem from mechanistic constraints on pigment production in some avian lineages or may be a consequence of selection for anti-microbial protection. Or perhaps the manifold demands of the egg and selection for functions other than protective concealment have tugged the egg phenotype away from an ideally cryptic appearance. Finally, imperfect egg camouflage may occur if other forms of defence are simply more effective, or if a diverse brigade of predators imposes different selective pressures on egg appearance. A combination of these factors is likely involved. Avian eggs provide an excellent model system for investigating animal camouflage, and recent advances in numerous fields make this area particularly ripe for future research.