Why is a robin’s egg blue? Exploring the evolution of egg colour in birds
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A recent hypothesis for explaining blue-green eggs in birds suggests that they may be a sexually selected signal of female (and thus nestling) quality that males use to make parental investment decisions. Although this hypothesis has found mixed correlational support in a few species, well-controlled experimental support has been lacking in non-cavity-nesting species with vivid blue eggs. In this study, we isolate the influence of egg colour on male behaviour by replacing natural American robin Turdus migratorius clutches with four artificial eggs (all of the same colour) representing extremes in natural colour intensity. After incubation, three unrelated nestlings were fostered into each experimental nest immediately after the normal incubation period and parental behaviour was monitored when nestlings were 3, 6, and 9 days old. For the youngest nestlings, male provisioning rate significantly increased in the dark egg treatment, but the effect of egg colour disappeared at the older nestling stages. This result remains significant when controlling for variation in female behaviour. Male feeding rate at unmanipulated nests was also significantly related to natural egg colour. These results suggest that blue-green egg colour acts as post-mating signal of female quality in at least some cup-nesting species, but does not exclude the possibility that the blue-green pigment also serves other adaptive purposes.