AMERICAN ROBINS AND European blackbirds reject brown eggs and accept blue-green eggs regardless of the colour differences between their own eggs and parasitic eggs, a new study has found.
Hosts of avian brood parasites have long been assumed to reject foreign eggs from their nests based on the total degree of dissimilarity in colour to their own eggs. But new research published in the journal Proceedings of the Royal Society B show that two studied hosts pay attention to some colour differences more than others: bluer eggs were accepted while equally dissimilar browner eggs were rejected.
Egg discrimination along a gradient of natural variation in eggshell coloration is the collaboration of a global team of researchers, including Dr Daniel Hanley of Long Island University Post, and Dr Mark Hauber of Hunter College and the Graduate Center of City University of New York (CUNY).
The team tested hosts’ responses to gradients of natural (blue-green to brown) and artificial (green to purple) egg colours by painting foreign eggs. Colours were varied along the natural and artificial colour gradients. The foreign eggs were then added to the nests of blackbirds and American robins (Turdus migratorius) and were observed to see whether the hosts accepted or rejected the foreign eggs from their nests.
After each egg introduction, the team monitored the nest daily for six consecutive days, and hosts were considered “rejecters” when the foreign egg or one of their own eggs disappeared from their nests during this study period. To ensure rejection responses were consistent, foreign eggs were uniform in size and shape. The only difference was in their colour.
Results showed that hosts based rejection decisions on both the direction and degree of colour dissimilarity along the natural, but not artificial, gradient of egg colours, which demonstrates that egg recognition is specifically tuned to the natural gradient of avian eggshell colour.
Co-author Dr Hanley said: “By using a simple experiment we show that two hosts are much more likely to remove parasitic eggs from their nests if they are browner than their own, but not if they are more blue-green. This result is surprising because the prevailing assumption of previous research had been that greater perceived differences between host and parasitic eggs would result in a greater likelihood of rejection.
“The findings highlight an unexplored cognitive mechanism in egg recognition and show both sensory reception and cognitive processes are critical in host perception.”
● Watch an American robin reject a cowbird egg at: www.youtube.com/watch?v=GBTML1zcqQA