Recognition and rejection of foreign eggs of different colors in Barn Swallows

Abstract

Brood parasitic birds lay eggs in the nests of other birds, and the parasitized hosts can reduce the cost of raising unrelated offspring through the recognition of parasitic eggs. Hosts can adopt vision-based cognitive mechanisms to recognize foreign eggs by comparing the colors of foreign and host eggs. However, there is currently no uniform conclusion as to whether this comparison involves the single or multiple threshold decision rules. In this study, we tested both hypotheses by adding model eggs of different colors to the nests of Barn Swallows (Hirundo rustica) of two geographical populations breeding in Hainan and Heilongjiang Provinces in China. Results showed that Barn Swallows rejected more white model eggs (moderate mimetic to their own eggs) and blue model eggs (highly non-mimetic eggs with shorter reflectance spectrum) than red model eggs (highly non-mimetic eggs with longer reflectance spectrum). There was no difference in the rejection rate of model eggs between the two populations of Barn Swallows, and clutch size was not a factor affecting egg recognition. Our results are consistent with the single rejection threshold model. This study provides strong experimental evidence that the color of model eggs can has an important effect on egg recognition in Barn Swallows, opening up new avenues to uncover the evolution of cuckoo egg mimicry and explore the cognitive mechanisms underlying the visual recognition of foreign eggs by hosts.