Every fall, thousands of raptors migrate through Montana on their annual journey from breeding to wintering grounds. But figuring out where that eagle, northern goshawk or other raptor calls home is a more complicated question. By utilizing innovative sampling techniques, RVRI has been able to more accurately pinpoint a raptor’s “place of birth.”
Specifically, an isotope of hydrogen, called deuterium, was selected due to the ratios of deuterium changing consistently with latitude. With this technique we only need to take a “thumb-sized” feather sample, which then can be analyzed to determine the ratio of deuterium. By sampling only juvenile birds, whose feathers are grown in the nest, we can estimate the individual bird’s natal origin.
We have been sampling fall migrant juvenile golden eagles and northern goshawks since 2004 and to date have collected 58 and 35 samples, respectively. We are now in the process of analyzing our 2007 samples and will be writing up our results this winter for publication. Nonetheless, we will continue collecting samples and adding to our data sets, as this is an innovative, cost-effective way to learn more about avian migratory ecology.
Golden Eagle C-21 was banded as a first-year female October 10, 2006. The stable isotope analyses indicated the Mackenzie Mountains located in the Northwest Territories as her natal origin. She was later found in Chihuahua, Mexico in the spring of 2006,after migrating nearly 3000 miles. She was discovered by rancher Benito Sanchez, who noticed the blue wing tags on C-21 as she lay dead in a dried up creek bed. The wing tags prompted Bennito to dismount from his horse and take a closer look. It was then that he saw RVRI’s contact information and phoned us. We don’t know how she died, as she showed no signs of obvious trauma. According to Bennito, there is an old saying in Mexico, “old eagle, young snake, the eagle wins…old snake, young eagle, the snake wins…”
This is a prime example of how isotope analysis incorporated with wing tags can be used to bring us new insight and understanding about Golden Eagle migratory ecology in North America.