Raptor use of the riparian woodland (Bosque) and adjacent habitat in Middle Rio Grande Valley, New Mexico

The Middle Rio Grande valley provides valuable summer and winter habitat for many species of raptors, including Cooper’s Hawk, Great Horned Owl, Swainson’s Hawk, Red-tailed Hawk, Western Screech-Owl, American Kestrel, and Mississippi Kite. Some raptors, such as Cooper’s Hawk, Great Horned Owl, Western Screech-Owl, and American Kestrel are resident in the Bosque year-round. During the winter months, the Bosque and particularly the adjacent agricultural lands are home to other raptors, including Bald and Golden Eagle, Red-tailed Hawk, Ferruginous Hawk, Northern Harrier, and Prairie and Peregrine Falcons.

Red-tailed Hawk, Juvenile - photo by Doug Brown

Hawks Aloft has conducted driving surveys along the levee to document raptor use of the riparian woodland and the adjacent lands. Surveys were conducted during the winter and breeding seasons over a two-year period, from December 2002 through June 2004. These surveys have been completed, and the data are being analyzed. The data will be compared to numbers detected in the Middle Rio Grande Biological Study, conducted in 1981-1982. Additional comparisons of our current study will include the number of raptors detected in urban versus rural areas, effects of disturbance, and detection rates between different survey methods.

American Kestrel female - photo by Keith Bauer

Preliminary results indicate that our highest abundance of raptors occurred on transects adjacent to agricultural lands. Therefore, we consider that the presence of agriculture adjacent to bosque habitat attracts a greater number of raptors than urban areas. Differences in habitat within the riparian corridor, e.g., native versus exotic vegetation, also could influence raptor numbers, particularly species such as Cooper’s Hawk and Great Horned Owl. Our study has been designed to examine multiple factors influencing raptor abundance in the Middle Rio Grande Valley. In addition to the raptor survey method detailed here, we also locate and monitor active raptor nests. We hope that continuing studies and further analyses will help us unlock some of the mysteries of how raptors use these habitats, including the true value of agricultural land and large stands of native vegetation.