The riparian cottonwood forest, known as Bosque, that borders the Middle Rio Grande provides important habitat for wildlife, including many species of raptors that use the woodlands for nesting. Diurnal raptor species known to breed in the Rio Grande Bosque and adjacent lands include Cooper’s Hawk, Swainson’s Hawk, Mississippi Kite, and American Kestrel. Owls that nest within this riparian corridor include Great Horned Owl, Long-eared Owl, Western Screech-Owl, and Barn Owl. The dominant stick nest builder, and the most numerous nesting raptor in the Bosque is the Cooper’s Hawk. Great Horned Owls, Swainson’s Hawks, and Mississippi Kites nest in smaller numbers in the Bosque. The American Kestrel, a cavity nesting falcon, nests in high concentrations in areas with standing dead trees, particularly where fire has occurred in the previous five years.
Western Screech-Owl in tree - photo by Doug Brown
A dedicated team of 6-8 Hawks Aloft staff comb the Bosque from Bernalillo, south to Belen, searching each tree for evidence of stick nests, likely documenting more than 90% of the nests that are present. To date, we have mapped more than 800 stick and cavity nests along 50+ river miles. Each nest is visited three times during the nesting season to determine occupancy, nest improvement, and construction of new nests. Of these nests, 100-150 become active annually (12%-18%). Active nests are monitored throughout the nesting season to determine success and productivity. Each nest is visited at least once every ten days until the young fledge, or the nest fails. The intensity of this search effort allows us to document nest densities for many species.
The data collected through this study document raptor nesting preferences and productivity relative to vegetation, and might provide additional evidence that agriculture adjacent to the bosque supports higher numbers of raptors. For instance, the nesting study has shown that one area of the bosque, in the village of Corrales, has nearly twice the number of active Cooper’s Hawk nests than other areas. In fact, the nesting density of this reach, might be higher than that reported anywhere else in North America. The adjacent land use there is a combination of agriculture and homes located on large properties, many of which contain horses or other livestock.
Several management issues, ranging from development and fire protection to water conservation and removal of exotic vegetation, impact the bosque and the adjacent lands. A variety of habitat restoration initiatives also may have short or long-term effects on the wildlife dependent upon this habitat. Because of the potential conflict between raptor conservation and management issues, it is important to monitor raptor use of the bosque and the urban and rural land use adjacent to the riparian woodland that affect raptor populations. Our long-term monitoring could provide insight on how land use and management affects nesting New Mexico raptors.
