Tracking Bird Migration Using the Sun
The spectacle of migration has long been of interest to biologists and bird watchers alike and while we know that 5 billion birds migrate thousands of miles annually our understanding of this amazing phenomenon is surprisingly poor. Over the past 10 years the Smithsonian Migratory Bird Center has focused much of its research on non-breeding season ecology.
One essential component of this research is an understanding of migratory connectivity or the linkages between breeding and non-breeding populations of a given bird species. For example knowing where birds breeding in North America spend their winters (primarily Central America) is key to understanding a species' ecology and evolution. Moreover, knowledge of migratory connectivity will ultimately have important implications for the conservation and management of migratory bird species.
To date, our understanding of migration and the linkages between breeding and non-breeding populations of migratory birds has been limited by our ability to track birds throughout the year. As such, when the robins, sparrows, orioles and warblers leave for the winter, where they go, has been largely left up to our imagination. Historically we have used banding (birds captured in breeding locations and recaptured in non-breeding locations) as the primary tool for understanding the connections between these disparate times in the annual cycle (Fig. 1). Recently, however, new technology has begun revolutionizing our ability to track bird migration and determine non-breeding locations.
Figure 1. Estimated patterns of migratory connectivity for gray catbirds breeding in the Mid-Atlantic and Midwest using mark-recapture data and geolocators (inset). Solid circles indicate breeding regions and dashed circles indicate non-breeding regions. The inset shows non-breeding locations as estimated using light levels collected from 6 geolocators place on birds in Washington D.C. Both data sources show that birds breeding in the Washington D.C. region spend their winters in Florida and Cuba.
Geolocators are tiny light-level data loggers that collect daily data on light intensity. These tags can be fitted to the backs of migratory birds to collect light data as the bird moves southward, over-winters and makes its return migration the following spring (Fig. 2). Specifically, light data provide researchers with daily sunrise and sunset times that can be used to estimate latitude and longitude locations given the sun elevation angle. As a result researchers can track the large-scale movements of birds and identify where specific populations spend the non-breeding period (Fig. 1).
Figure 2. A wood thrush fitted with a light level geolocator that will be used to track its movements during migration and determine where it spends the winter.
The Migratory Bird Center is one of the leaders in the applications of these tools having deployed geolocators on warblers, sparrows, blackbirds, thrushes, and shorebirds. In addition to using this new technology on a diversity of species we have tagged birds in a number of geographic locations including Maryland, Vermont, North Carolina, New Jersey, Michigan, Indiana, and Belize.
These tools have provided new information about migratory pathways, timing and the linkages or lack thereof between breeding and wintering populations of migratory birds. With the right resources and leadership, these tools will advance our understanding of migratory bird movements and their life cycles. Ultimately, as we advance our understanding of the complex life cycles of migratory birds we can create proactive management and policies that promote conservation for both breeding and non-breeding locations alike.