Between the years of 2004 and 2013, Hawks Aloft monitored 70 miles of bosque habitat, from Rio Rancho on the north, to Belen on the south. During these surveys, we monitored an average of 521 stick nests per year. Because raptors often re-use the same nests in multiple years, we visited all previously documented nests each year and added any new nests that were found, resulting in a cumulative total of 4009 nests documented during the 10 year period. We also opportunistically documented nest cavities.
Among these nests, a total of 668 (16.6%) became active, representing 10 different species. All active nests were monitored until the young fledged or the nest failed. Cooper’s Hawk was the dominant nest-building raptor in the bosque, with a total of 488 active nests, 73% of all active nests documented. A total of 122 (18%) Great Horned Owl nests were documented. A combined six other species represented the remaining 9% of active nests: American Kestrel (n=22), Swainson’s Hawk (n=12), American Crow (n=11), Common Raven (n=8), Common Black-Hawk (n=3), and Long-eared Owl (n=2).
Cooper’s Hawk is by far the most abundant nesting raptor in the middle Rio Grande bosque. Of the 488 active nests we monitored during the first 10 years of this study, 400 (82%) were successful, 71 failed and the outcome was undetermined at 17. A total of 1,059 chicks were reared to fledging, for an average of 2.19 young fledged per nest. The average fledging date differed only slightly among the ten years, ranging from July 1 to July 10, with young in some nests fledging as early as June 23 and as late as August 8.
Great Horned Owl, the earliest nesting bosque raptor, typically does not construct its own nest, but chooses an existing nest and refurbishes it by adding nest lining material. We monitored 122 (18.3%) active Great Horned Owl nests during the study. Of these, 84 nests were successful producing 166 fledged young, 18 failed and the outcome was undetermined at 20 nests. Great Horned Owl fledglings generally leave the nest in early May and are exceptionally difficult to detect once they learn to fly, a factor that contributed to the relatively high number of nests without a known outcome.
American Kestrel, an obligate cavity nesting species, occurred in low numbers throughout the study. A total of 22 active nest sites were documented, primarily in formerly burned areas with standing snags, or open areas. Due to their secretive nesting habits, nests of this species and the number of fledglings are particularly difficult to detect. For more information on this species, please see the lead article in this issue of Aloft.
Swainson’s Hawk also nested in low numbers in the bosque, with a total of 12 nests. Of these, 9 nests were successful producing 15 fledged young, 1 failed and the outcome was undetermined at 2 nests. They nest primarily in areas with a tall forest canopy and adjacent open areas for hunting. Because this species nests in the tops of the highest cottonwoods, and are particularly secretive near their nests, they are challenging to locate and monitor.
Common Raven and American Crow, both corvid species, also were monitored as part of this study. Although they are not raptors, both build stick nests and also repair and use existing nests built by raptors. Thus, the presence of a nesting corvid could create a suitable nesting substrate that might be utilized by a raptor species in future years. Both corvid species nested in low numbers in the bosque throughout the study. We monitored at total of 11 American Crow nests that produced 14 young, and 8 Common Raven nests that produced 11 young.
Common Black-Hawk, a riparian obligate buteo, was documented only in recent years, with three active and successful nests, one each in 2011, 2012, and 2013, producing a total of 4 fledged young. All nests were located in the southern reaches of the study area, in densely forested bosque managed by the Middle Rio Grande Conservancy District.
Long-eared Owl is rare in the bosque and generally nests only in areas with a densely vegetated understory. In 2005, a Long-eared Owl nest was monitored near the far north end of Corrales, prior to any understory clearing of that reach, and successfully fledged four young. In 2007, an active nest that failed was monitored on the east side of the Rio Grande, south of Montano Blvd., following vegetation management. Since then, although Long-eared Owls have been occasionally seen early during the nesting season, no nests have been located.
When this study began, we expected that Red-tailed Hawk would be a nesting raptor. However, although Red-tailed Hawks were routinely observed in the bosque during spring, fall, and winter, no bosque nests were documented.
Turkey Vulture is another species that was regularly observed in (or flying above) the bosque. But, no nests have been documented. During spring and fall migration periods, vultures sometimes congregate in historic roost sites, with as many as 80 individuals roosting in a single tree.
Barn Owls were occasionally documented during raptor surveys and also during songbird surveys for the Middle Rio Grande Songbird Study, but no nests were located. Barn Owls are secretive, cavity nesting owls, making nest location extremely difficult.
In 2002-2004, Hawks Aloft conducted nighttime surveys for owls in the Middle Rio Grande bosque, using tape playback of calls of four owl species. Although these surveys documented approximately one active Western Screech-Owl territory for every 0.2 miles of bosque, no active nest cavities were located during daytime follow-up surveys. Because screech-owls are nocturnal, small, and well-camouflaged, confirming active nesting is nearly impossible without actually searching inside potential nest cavities.
An analysis of the data from the first 10 years of this study provides a snapshot of the raptor species that nest in the bosque and their reproductive success. But, it does not adequately measure the educational benefit to the public or the passion of our surveyors for their survey areas and the birds they monitor. With surveyors regularly present throughout the middle Rio Grande bosque during the breeding season, our team has interacted with many human visitors to the riparian woodland, listened to their hawk or owl stories, educated them about our work, and built relationships beneficial to bosque wildlife and vegetation. About twice each season we encounter illegal clearing during the nesting season that is quickly reported to the appropriate agency. Sometimes, we also encounter thoughtless humans engaging in activities such as vegetation destruction, littering, use of campfires, smoking, shooting, and letting dogs run uncontrolled. Team members share a rich culture of stories, and each team member cares passionately about the hawks, falcons and owls that nest within their patches of the woods. It has been an extraordinarily rewarding project with far greater value than the numbers evident in the data reported.
The Bosque Nesting Raptors Study has been funded almost entirely by the US Army Corps of Engineers, with occasional support from the US Fish and Wildlife Service. Hawks Aloft also contributed a significant match via the services of trained volunteers in addition to staff to monitor some areas. All volunteers were trained in the use of Global Positioning Units, search methods, identification of common bosque nesting raptors, nest preferences, and in aging nestling Cooper’s Hawks.
We thank Ondrea Hummel and William DeRagon, US Army Corps of Engineers for their financial and logistical support, Matt Schmader, City of Albuquerque Open Space Division, Yasmeen Najmi, Middle Rio Grande Conservancy District, Lara Aho, Corrales Bosque Advisory Commission, Anthony Martinez, Corrales Fire Department Chief, and Jay Hart, City of Rio Rancho Parks and Recreation Department for their logistic support. We also thank our dedicated team of volunteer bosque surveyors: John Acklen, Vicki Dern, Bob Kipp, Maurice Mackey, Arlette Miller, Mariah Oeser, David Parsons, Chellye Porter, Jeff Porter, Larry Rimer, Stephen Saletta, Allison Schacht, Mary Walsh, and Christie Wilcox.