Why Are Redstarts Red?
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 scientific paper:
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.
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