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Why different birds have different numbers of tail feathers

A Swallow-tailed Kite spreads the feathers of its wings and tail.
Swallow-tailed Kite in Ohio, September 2016, by Randy Smith.

In the column “Since You Asked” in every issue of BirdWatching, Contributing Editor Julie Craves answers readers’ questions about birds and bird behavior. Here is a question about feathers from our October 2016 issue:

How many tail feathers do most birds have, 10 or 12? Why are there different numbers? – James Hooey, Kelowna, British Columbia

Most species have 12 tail feathers, which are known as rectrices. Ten is also a common number, but totals range from as few as 6 to as many as 32. Related birds — that is, those in the same genus — typically have the same number, but some species within a genus occasionally have different numbers. The number of wing and tail feathers is typically fixed within a species. The reason for the variable number of rectrices in related species is something of a mystery. Habitat and behavior have often not provided solid evidence. The variation has been attributed to random genetic drift.

There are also instances of individual birds within a species having the “wrong” number of rectrices — usually extra feathers. This has been reported across a wide range of families.

A study of extra rectrices in five raptor species found that the bonus feathers usually occurred on the right side and, most frequently, on the outer part of the tail. Since detecting extra feathers usually requires examining birds in the hand — either alive or as museum specimens — the phenomenon may be more widespread than currently documented. While slightly less common, extra wing feathers have also been noted. – Julie Craves

About Julie Craves

Julie-Craves-120Julie is supervisor of avian research at the Rouge River Bird Observatory at the University of Michigan Dearborn and a research associate at the university’s Environmental Interpretive Center. She writes about her research on the blog Net Results, and she maintains the website Coffee & Conservation, a thorough resource on where coffee comes from and its impact on wild birds.

Read other questions that Julie has answered in “Since You Asked.”

If you have a question about birds for Julie, send it to [email protected] or visit our Contact page.

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