Until recently, scientists knew little about songbird migration. They speculated and hypothesized based on the rare recoveries of banded birds on the wintering grounds. Although these discoveries were exciting, they did little to reveal the mysteries or details of the actual migration.
In 2007, however, a giant leap toward understanding songbird migration occurred when Dr. Bridget Stutchbury of York University, in a ground breaking study (published inScience, February 2009), tracked the first songbird migration using tiny geolocators harnessed on the backs of purple martins, Progne subis.
The first generation of geolocators work by measuring and recording light levels every few minutes. Upon return from migration, the geolocators must be retrieved from the bird and the data downloaded. Special software is used to convert the light measurement readings into the calculation of latitude and longitude allowing analysis of where the bird was when. They are accurate to approximately 200 miles.
Newer technology Pinpoint GPS geolocators record locations with GPS accuracy on preprogrammed dates.
Unfortunately, they are not true GPS technology must be removed from the returning bird to retrieve the data.
The biology of purple martins makes them ideal candidates for this type of study. (Please read more about them and about the plight of our aerial insectivores in the About Purple Martins page.) They are neo-tropical migrants (wintering in South America) with a relatively long distance migration. Because they are dependent on man-provided housing and have strong site loyalty, they can be relatively easily recaptured. Although on average, one can only expect a 50%-60% migration survival rate, approximately 92% of the surviving birds return to the colony from which they started, making the retrieval of geolocators possible.
In one of our publications (See the 'Publications" page), we showed that the declining numbers of our northern aerial insectivores vs. our stable southern populations (using purple martins as a model species) is not due to different environmental factors on the wintering grounds as previously hypothesized. Using geolocators, we were able to show that both the northern and southern populations of martins winter in the same geographical areas. This study is a world-first, using the largest sample size yet for tracking songbirds and shows why geolocators are so important for unraveling the causes of breeding declines in migratory birds.