The songbird who migrates early may get the mate because he has an attractive advantage, Canadian researchers say.

Biologists discovered that when migratory songbirds raise their young late in the summer, many of the future fledglings don't have time to moult their feathers before heading south.

Female American redstart feeds her young (Courtesy: T. Sherry, T. Werner)

• FROM DEC. 11, 2003: Tropical habitat key to migrating birds' success

Instead of sporting brightly coloured feathers, the latecomers end up replacing their feathers at stopover sites during the winter migration, which can result in less colourful plumage.

Scientists say the concentration of orange and red pigments in the feathers signals the health of a bird. Birds that experience physiological stress during moulting often have fewer deposits of pigment in their feathers.

As part of his doctoral research, Ryan Norris of Queen's University in Kingston tracked forms or isotopes of hydrogen that naturally accumulate in the birds' feathers during long migration flights.

By measuring the isotopes in the feathers after the birds returned north, Norris and his colleagues were able to estimate where the moulting occurred.

They found 40 per cent of the male American redstarts in the study moulted some of their tail feathers up to 2,000 kilometres south of their breeding grounds.

The researchers also quantified colour differences by measuring how much light the feathers reflected.

Previous bird research shows females prefer males with brighter plumage.

The new study show males pay a cost for not producing colourful feathers in the previous breeding season, said Queen's biology Prof. Bob Montgomerie, who co-authored the study in the Dec. 24 issue of the journal Science.

The research was funded by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, Ontario Innovation Trust, and several U.S. institutes.