The American coot is a bird famous for playing favorites with its children, and it is especially superficial. It has been observed previously that American coot parents will give food to those with the brightest red colors on their feathers and beak. It was previously theorized that this parental preference was related to the bird’s “brood parasitism”, the act of placing eggs in the nests of unknowing birds of the same species. This behavior lets the birds have more offspring without needing to spend the energy to feed them. It was hypothesized that coots capitalize on the innate preference for red color by making their parasitic offspring bright red in order to take the biggest share of the other nest’s resources, but a recent study challenges this idea.
Bruce Lyon and Daizaburo Shizuka tracked a large number of American coots in the wild and determined a quantitative measure of the “redness” of the chicks. From these measurements, they were able to show that parasitic chicks tend to be less red than are host chicks (chicks raised by their biological parents). This is directly counter to the idea that red coloration serves to help parasites. However, they did notice that redness is strongly linked to the order in which chicks are born, with redness increasing with each sequential chick. This led to a new theory that redness serves as a way for parents to even the odds of survival among their offspring. Finding enough food is difficult, so American coots will usually lose about 50% of each brood to starvation. It then follows that they favor the later born chicks because they have not yet received as much food as the older ones and will be less strong and capable of competing for their fair share.
In the end, these results suggest that parental favoritism of red coloration is unrelated to brood parasitism. The authors point out that when coots do lay a parasitic egg, it does not appear to be planned very far ahead of time. They wouldn’t have time to influence coloration. Parasitic eggs also tend to be laid very early before the mother has established her own nest, which explains why parasitic chicks tend to be less red overall. Although this study is a substantial contribution to our understanding of red coloration in these birds, there is more to be discovered. The authors here used one statistic to summarize overall redness of the birds, but the beak, the feathers on their neck, the top of the head, among other features, can all be colored red individually. It may be that these different components of the “redness” score communicate more information than is appreciated.
Managing correspondent: Julian Segert
Press article: The mysterious case of the ornamented coot chicks has a surprising explanation Phys.org
Image credit: Mike Baird/ Flickr