"Snowbirds" versus real birds

Southern vacation resorts encroaching on key winter habitats crucial to success of migratory birds: Queen’s researchers

(Kingston, ON) – The destruction of tropical forests to create vacation resorts for human “snowbirds” who fly south from Canada and the northern U.S. every winter is creating serious breeding problems for real migratory birds, say Queen’s University biologists.

A new study, headed by Ph.D. student Ryan Norris and his advisor, Professor Laurene Ratcliffe, shows for the first time that declining winter habitats of migratory songbirds significantly affect their ability to reproduce when they return north in the spring – and the evidence is found in tiny drops of their blood.

The study, highlighted recently in the journal Science, and co-authored by Dr. Peter Marra of the Smithsonian Institute, appears on-line in the current Proceedings of the Royal Society: Biological Sciences, and will be published in an upcoming print edition of Proceedings.

“Our findings help explain why many species of long-distance migratory songbirds have declined over the last few decades,” says Mr. Norris, noting that a relatively small geographic band – across the Caribbean, Greater Antilles, and central America – is the annual destination of an estimated five billion migratory birds flying south each year from Canada. The species in this study is a small, migratory warbler called the American redstart.

Until now, understanding how and why these migratory populations are declining has been a problem, since it is difficult to track individuals throughout their yearly travels. A new technique for detecting “biological signatures” provides the solution.

Using equipment and expertise of the Queen’s Facility for Isotope Research (QFIR) headed by geology professor Kurt Kyser, the team measures stable carbon isotopes found in the warblers’ blood. These samples are taken immediately after the birds arrive at the university’s biological field station north of Kingston, Ontario in the spring.

Since the turnover time of the blood cells is from six to eight weeks, they provide a good indicator of the quality of the birds’ previous habitat on the wintering ground. The carbon signature of each redstart has been deposited by insects the bird ate, and the insects in turn fed on vegetation growing in the winter habitat. “It’s basically a ‘food chain signature’,” says Mr. Norris.

The researchers first determined what were high and low quality habitats for redstarts, which winter in the Caribbean and central America, and breed in deciduous forest throughout Canada and the U.S. They found that what makes a better winter habitat is primarily the degree of wetness, particularly at the end of the season (late winter/early spring) when low quality habitats tend to dry out.

The second step was to develop isotope “markers” that identify the quality of each type of habitat. Next, blood samples were taken from warblers in their breeding grounds, and finally the birds’ reproductive success was measured by counting the number of “fledged” offspring to leave their nests.

The study, funded by the Natural Sciences and Engineering Research Council (NSERC), Canada Foundation for Innovation (CFI), and National Science Foundation (NSF), shows that redstarts wintering in high quality habitats, such as mangroves and lowland tropical forests, arrive earlier on the breeding grounds, nest earlier, and are more successful in producing young.

“Our work shows that destroying these high quality habitats has a disproportionate effect on the redstart populations: they lose the areas most capable of supporting them,” says Dr. Ratcliffe.

“As biologists, to see such a significant effect on the birds’ reproductive success is absolutely fascinating,” the biology professor continues. “Now we’ll need to go much deeper to discover all the complexities, and to determine if there are applications to other organisms as well.”