Tracking Migratory Birds Using Isotopes
Most birds in North America fly south for the winter. Some species like robbins may migrate relatively few miles, other species like swallows may migrate thousands of miles each year. Migratory birds need to have suitable habitat in their summer breeding grounds, their winter residences, and along the flyways in between.
It is difficult to know if populations of migratory birds are stable or declining because their numbers fluctuate from year to year, they move north and south every year, and they also may change the places they use as the landscape changes.
How do habitat changes affect migratory birds? If they are forced to use degraded habitat do they have fewer young or do they lose weight? Scientists have had a difficult time answering these most basic questions because there has been no reliable way to follow birds from their winter to summer ranges. Peter Marra used the same isotope technique described in the preceeding monarch butterfly article to study migratory birds in their winter range. Good winter habitat for American redstarts is dominated by plants with one isotopic signature while less desireable habitat has a different signature. He found that birds that had to live in marginal habitats lost weight over the winter and had higher levels of stress hormone. Good winter habitat is limiting for at least some neotropical migratory birds. Meanwhile natural areas throughout the region are under continuous and increasing pressure from human development.
We spoke with Peter about his work and its implications for migratory birds.