Home > Predatory Birds > Bald Eagles > Research > Migration
Bald Eagle Migrations
Small satellite telemetry transmitters can provide more information useful to understanding bald eagle ecology and life history than has been gathered in decades of banding and conventional telemetry studies. As the bald eagle continues its recovery in the face of human expansion in California, this exciting new tool can effectively produce a wealth of information to help plan for the continuance of healthy local and wintering populations of this beautiful and important species in California, and illuminate the international importance of conservation measures.
Until recently, banding was essentially the only way to study bird migration. However, because of the low probability of bands being reported, large numbers of banded birds were required to gain a small amount of scattered information. The use of conventional telemetry transmitters offered an improvement, but required monitoring either from the ground or by airplane, both of which present logistical difficulties, especially in the case of long-range migrants. Advances in satellite-based telemetry have only recently resulted in transmitters small and lightweight enough to be carried by birds. The new transmitters are neither heavier nor bulkier than the conventional ones that SCPBRG staff has been using on eagles for many years without any indication of harm or discomfort.
The implications are very promising. Where a band can, at best, reveal a single point along a migratory continuum, a satellite transmitter can show the entire schedule of annual movement, complete with information on stopover and wintering areas.
Migration Update
We are not currently receiving signals from any bald eagles due to battery wear of the transmitters or other factors. Pending funding, we plan to continue the program in the future. Meanwhile, we have been summarizing results for reports to the funding agencies of these studies. Here are some summary maps of migration routes of wintering bald eagles tagged over several years in cooperation with the California Department of Parks and Recreation at Millerton Lake near Fresno, CA. They include the north- and southward migration routes of adults tagged at Millerton Lake, and all movements of all birds tagged there.
When we began these studies, we could never have imagined the consistency of results we would find, with virtually all tagged wintering eagles migrating to a relatively consistent area within Canada for the summer. Some of the adults appeared to be breeders there, while others were "floaters", non-territorial adults who roam the countryside seeking a place within a breeding territory. Note how the eagles tend to cross the Rocky Mountains in a relatively small area in southern Alberta, then fan out again. This area is a known migration corridor for both bald, but especially golden, eagles (e.g., Yates et. al. 2001 Journal of Raptor Research).
In looking at these maps, one might think that all bald eagles from that Canadian region migrate to California for the winter. However, it must be borne in mind that by definition, all of the eagles we followed migrate here, or we would not have encountered them. That does not mean that eagles migrating south do not spread out over the southern United states, a phenomenon which we would not see in this study.
Sometimes an eagle's transmitter is active during a day on which the eagle migrates, allowing a look at just how far and how fast they can move. This occurred on 15 March 2001, when Millerton Subadult Male #3 traveled approximately 100 miles in 6+ hours before the transmitter switched off. A weather front was moving through that day that probably assisted the eagle in crossing the Sierras and traveling up their eastern slope.
Click image above to enlarge map.
We hope to bring you more bald eagle migration sagas in the near future. With all the suprises we have seen, there is clearly more to learn. Not only has it been a privilege to be involved in these studies, but our ability to share them via the internet has brought a whole other dimension to the program.
Related Articles:
Linthicum, J., Jackman, R. E., Latta, B. C., Koshear, J. and M. Smith. 2006 in review. Annual migrations of Californian and Canadian bald eagles as revealed by satellite telemetry.
