Bird migrations, especially those that are long-distance and/or across rugged terrain and oceans, have long fascinated us flight-less humans. One such amazing cross-continental journey is taken by the Swainson’s Hawk, which travels from its breeding grounds in North America to its overwintering site in Argentina.
As with the California Condor, our founder Dr. Pete Bloom was at the forefront of critical research efforts with the California population of Swainson’s Hawks. He completed extensive statewide surveys of these hawks for the California Department of Fish & Game in the late 1970’s, which culminated in a determination that over 90% of the California population had disappeared during the past three decades. In 1980, only about 400 individuals were known to exist (in California), and those individuals were limited to only a few locations, like the Butte Valley. It was known at the time that Swainson’s Hawks migrated to Argentina for the winter, but very little was understood about their intense journey, until U.S. Forest Service biologist Brian Woodbridge put a small transmitter on one and used satellite telemetry to track its southward path. This provided very useful information, although there were still questions about how and why the population was declining so rapidly. Then, in the 1990’s, the struggling population was dealt another blow; an extremely toxic organophosphate compound was being used to control a heavy grasshopper infestation in the Argentinian pampas. In Argentina, the Swainson’s Hawk is known locally as the “grasshopper hawk”, as these nuisance insects are a favorite food. Not surprisingly, the pesticide-laden fields were killing the hawks by the tens of thousands. But fortunately, once the Argentinian government realized the ecological disaster that was occurring in the pampas, they immediately curtailed use of these toxins. Both American and Argentinian researchers got right back to work – trapping, banding, and placing satellite tags on the surviving hawks, and in the past few decades we have seen their numbers slowly but steadily increasing.
A Swainson’s Hawk fledgling, banded by Pete Bloom in July 2019.
Last summer, a reunion of sorts happened in the Butte Valley, and author Scott Weidensaul wrote a great article about it for the Cornell University’s Living Bird magazine (https://www.allaboutbirds.org/a-swainsons-hawk-reunion-celebrates-a-conservation-success/). Four generations of researchers, including Woodbridge and Bloom, all got together to work on banding efforts and collecting genetic samples from Swainson’s Hawk chicks. When the research first began, adults were banded with both a federal leg band and a color leg band, while chicks only received the federal band. If the offspring returned the following year, they were recaptured and color banded as well. But then, starting in 2008, every chick received both a color and a federal band, and blood samples were taken to map the genetics of the growing population. Close to 1,100 chicks have been banded as such, and the California Swainson’s Hawk population is now one of the most marked populations of raptors across the globe.
Dr. Pete Bloom with three Swainson’s Hawks chicks banded in the Butte Valley - July 2019. (Photo by Geoff Bedrosian)
Dr. Bloom is currently in Northern California, banding the next generation of Swainson’s Hawks there, and was excited to find a nest containing two chicks belonging to a previously banded female. Swainson’s Hawk chicks are highly philopatric, meaning they will most likely return to the same area they were born to breed and raise their own young. As adults, they also show strong site fidelity, as they have a tendency to keep the same mate and territory for the rest of their lives. There are many factors that have helped contribute to the increase in Swainson’s Hawk populations, but new threats are always taking shape. We hope that the long-term monitoring efforts by Dr. Bloom and other researchers will continue to show the persistent tenacity of these raptors in California’s ever-changing landscape. Another reunion is happening in Butte Valley again this year.
Kerry Ross
Other critical bird habitats include the desert ecosystems of southern California & Mexico. Human development is an issue here, uniquely including wind and solar farms that are expanding across these remote regions. There is a lot of missing data regarding birds who migrate through the harsh conditions of these deserts, mostly because these areas are difficult for humans to traverse. There are limited resources, very few roads, and very little shelter from extreme temperatures. Most people would choose to observe birds in areas with more favorable settings, so Point Blue Conservation Science collaborated with the Sonoran Joint Venture and eBird to create a citizen science project that encouraged people to visit under-represented areas of the Colorado and Mojave Deserts in southern California. Kerry Ross, our full-time biologist and an avid birder, participated in this “Desert Avicaching” project, submitting 148 checklists and recording 95 species in 2018, ultimately leading to him being awarded the grand prize! The data submitted by all 72 participating volunteers will be crucial to helping researchers learn more about the movement and abundance of migrating bird species in areas of the desert that are already developed, or may have plans to be developed in the near future, and can hopefully be utilized in land management decisions.
For more information on the Desert Avicaching project, please visit: https://sonoranjv.org/avicaching/
To learn more about our staff, go to http://bloom.bio/staff-2
Written By: Nicollet Overby