The American redstart is a small songbird that nests in North America and winters in the tropics. The adult male is black with bright orange-red patches in its tail and on its flanks.
So what function might these bright red patches serve? They are certainly not useful for camouflaging the bird from predators.
Scientists studying this bird found that each patch serves a different function.
The birds with the brightest red tail patches were more likely to have two territories (each with a female and a nest). They frequently fan their tails to show off the bright patch to other males to keep them away.
You see, redstarts, like most migratory birds, are not particularly faithful. It is not unusual at all for a nest to contain young that are sired by several males.
However, birds with bright red flank patches are more likely to be the fathers of the young in their own nest. Females are less likely to stray from a male with a bright flank patch.
So these bright red patches are serving as signals, one for males and one for females. The significance of the red color is likely due to the fact that the birds cannot make red, they must get the red coloration from their diet, from the food that they eat. It is "expensive" for the male redstarts to make the bright red patches so having them means that the birds are healthy and is a sign of fitness.
This article summarizes the information in this publication:
Reudink, M. W., Marra, Peter P., Boag, P. T. and Ratcliffe, L. M. 2009. Plumage colouration predicts paternity and polygyny in the American redstart. Animal Behaviour, 77: 495-501.
Many animals display multiple signals that can be used by conspecifics to gather information about the condition or quality of potential mates or competitors. Different signals can indicate different aspects of individual quality or function in spatially or temporally separated periods. However, for long-distance migratory birds, it is unclear if signals, such as plumage traits, function in different phases of the annual cycle. We investigated the potential role of carotenoid-based tail and flank plumage, and bib size, in relation to extrapair paternity and polygyny in the American redstart, Setophaga ruticilla. This work complements our previous research suggesting tail feather brightness acts as a status signal, mediating territory acquisition during the nonbreeding season in Jamaica. Here, we show that tail feather brightness also serves as an important signal during the breeding season. Specifically, our results indicate that polygyny, a behaviour highly dependent on obtaining and defending multiple territories, is significantly predicted by tail brightness. Interestingly, flank redness best predicted whether individuals secured paternity at their nest and the proportion of within-pair offspring sired. We suggest that by expanding the study of plumage function in long-distance migrants to events occurring throughout the annual cycle, we gain a critical perspective on the function and evolution of ornamental traits.
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