Redstart Tail Color Influenced by Molting Location

The American redstart is a small songbird that nests across southern Canada and south through the eastern United States to the Gulf coast. It has bright red to yellow patches in its tail.

The bright tail colors come from carotenoids, chemicals the bird gets from its diet (mostly insects). The carotenoids are also useful in the immune and endocrine systems.

Redstarts grow a new tail once a year, in late summer on the breeding grounds, and before the migration to the tropical wintering grounds in Central America and the Caribbean islands.

Some male redstarts have bright red patches in their tail, others orange, and still others have yellow patches. Scientists hypothesized 2 explanations for the color difference.

1. One possibility is that birds in poor health use the carotenoids first for health reasons (immune response) and would not have enough to make a bright red tail. Healthy birds would have plenty of carotenoids and red tails. A yellow tail would be sign of a bird that was in poor health when it molted.
2. Another possibility is that the tail color varies by geography. Birds from a particular location might have redder or yellower tails because the food available in the molting area was either rich or poor in carotenoids.

To test the hypotheses, scientists sampled redstart tail feathers on their breeding grounds. They found no relationship between how fast the tail feathers grew (faster growth means a healthier bird) and the color. Hypothesis one seemed unlikely.

To test the second hypothesis, redstart tail feathers were collected from across the wintering grounds. They were analyzed for stable isotopes. Stable isotopes are variations in common elements (such as hydrogen) and are known to vary geographically. So, the broad geographic region where the tail feather grew could be pinpointed.

The stable isotopes differentiated birds that molted in the northwest, midwest, northeast, central east, or the southeast. And the birds from the northern regions had redder tails.

So redstarts in the northern part of their range are getting more carotenoids in their diet than birds in the southern part of their range and this makes their tails redder.

This article summarizes the information in this scientific paper:

Norris, D. R., Marra, Peter P., Kyser, T.K., Sherry, T. W., Ratcliffe, L. M., and Montgomerie, Robert. 2007. Continent-wide variation in feather colour of a migratory songbird in relation to body condition and moulting locality. Biology Letters, 3(1): 16-19.

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