Shedding feathers early may enhance sex appeal, new songbird study shows

Birds that migrate early in the season may have a distinct advantage when it comes to attracting the opposite sex, say researchers from Queen’s University and the Smithsonian Institution.

And it’s all about the feathers.

Researchers were surprised to discover that the timing of a male songbird’s reproduction cycle affects the colour of his feathers and may have important implications for his success in attracting mates. When migratory songbirds raise their young extremely late in the summer, many don’t have time to molt (shed their feathers and replace with new growth) before heading south, the new study shows.

“This means they must molt at stopover sites on their journey to tropical winter habitats,” explains Ryan Norris, who conducted the research as part of his PhD in biology at Queen’s, supervised by Professors Laurene Ratcliffe (Queen’s Biology) and Peter Marra (Smithsonian Environmental Research Center).

“Their replacement feathers, grown en route, are less colourful than those of birds that had time to molt before migration, which may put them at a disadvantage in attracting females the following breeding season,” says Dr. Norris. “Both findings – that molting in some songbirds occurs after migration has begun, and that their new feathers are duller in colour – were surprising.”

The study will be published Dec. 24 in the journal Science.

Until now scientists have assumed that most species of migratory birds molt before they migrate. The team discovered that in fact some begin their migration, molt at a “stopover” site, then continue to their winter habitat. Forty per cent of the male American Redstarts in the study molted in their tail feathers at areas up to 2000 kilometers south of their breeding grounds.

By measuring stable hydrogen isotopes in the newly grown feathers when birds returned the following spring to breed at the Queen’s University Biology Station north of Kingston, the researchers were able to determine the approximate region where molting had occurred. And when the feathers were analyzed with a spectrometer measuring how much light of different wavelengths is reflected, significant differences in colour were also detected.

A key indicator of the songbirds’ health and quality is the concentration of carotenoid in the feathers, which causes orange-red light to be reflected in their feathers. Physiological stress during molting can reduce carotenoid deposits in the feathers.

“Studies of other bird species have shown that females prefer males with higher concentrations of carotenoids, and thus brighter, more intense colours,” says Queen’s biologist Bob Montgomerie, who did the colour analysis of feathers for this project. “What we didn’t know until now is that birds’ colours in any given year may be affected by what happened to them in the previous breeding season.

“That’s exciting because ’cost of reproduction’ is a general, organism-wide problem of many species, not just birds.”

The other member of the research team from Queen’s is geology professor Kurt Kyser, director of the university’s Facility for Isotope Research, where the stable isotope measurements were conducted.