As more tropical forests are destroyed, Canadian songbirds could face serious breeding problems, biologists say.

The declining quality of the birds' winter home hurts their ability to reproduce when they fly north in the spring, according to researchers at Queen's University in Kingston, Ont.

Biology Prof. Laurene Ratcliffe and doctoral student Ryan Norris analysed blood from a small, migratory warbler called the American redstart.

Female American redstart feeding young Courtesy: T. Sherry, T. Werner

The birds weigh about eight grams. Males are black with orange flanks and tails, while the females are grey with yellow splotches.

When the redstarts returned to their temperate breeding ground in Kingston in 2000 and 2001, scientists took blood samples from the birds.

The researchers looked for a tracer in the blood – a "food chain signature" of the birds' diet down in the Caribbean and Central America. That way biologists could tell if the songbirds came from a high- or low-quality wintering ground.

Until now, scientists weren't able to track individual small birds along their migration route. Although the birds are banded, no one could go into the tropics to find them.

Wetter habitat better

The Queen's team and their colleagues found birds coming from high-quality winter habitat such as mangroves or low, wet areas did better.

"Better" in this case means reproductive success – the birds fledged more young in their northern breeding ground.

"Most of the birds you hear singing in the spring are migrating from the tropics," Norris told CBC News Online.

Norris said the study shows coastal mangroves are a key habitat for migrating birds, but the land is threatened by development.

The research was sponsored by the Natural Sciences and Engineering Research Council, Canada Foundation for Innovation and National Science Foundation. The study appears in this week's online issue of Proceedings of the Royal Society: Biological Sciences.