A recently published Wildlife Monographs article, Long-Term Demography of the Northern Goshawk in a Variable Environment, by Richard Reynolds, a Research Wildlife Biologist with the Rocky Mountain Research Station (RMRS) and other RMRS scientists and university collaborators reports on a 20-year mark-recapture study of northern goshawks (Accipiter gentilis). Northern goshawks are forest-dwelling raptors and sensitive species in all of the Forest Service’s Regions except the Southeastern region. The 425,000-acre study area encompasses spatially isolated coniferous forests of Arizona’s high elevation Kaibab Plateau and includes the Kaibab National Forest and the North Rim of the Grand Canyon National Park. Monitoring goshawk populations is difficult because of their relatively low density and elusive behavior and, as a result, most goshawk studies have been restricted in scope.
Research on the Kaibab has shown that goshawks, predators of birds and small mammals, are strongly food-limited; whether the hawks lay eggs or not depended on prey abundance, which fluctuated over 3–4 year periods of El Niño Southern Oscillation (ENSO) wet and dry years. The amount of ENSO precipitation affected the primary productivity of forest under- and overstory vegetation, and primary productivity controlled the availability of food resources to ground- and canopy-dwelling prey and ultimately their abundance. Extensiveannual variation in breeding was observed; in drought periods, only 8% of breeders laid eggs, whereas in exceptional wet periods, up to 87% of pairs laid eggs. The extent to which the goshawk population was shown to be food-limited indicated that the combined abundance of individuals within the suite of prey species (from ground and tree squirrels, to rabbits, jays, woodpeckers, and grouse) was important for sustaining the goshawk population through time. The presence of all species in the local suite of prey depends on the amount, quality, and intermixture of the particular habitats needed by each prey. The habitat mixture included a fine-scale (10 acres or less) mix of small groups of mature trees with interlocking crowns for tree squirrels, woodpeckers, grouse, and other birds, and scattered small grass-forb openings for rabbits, ground squirrels, grouse, and other prey. Interestingly, this habitat mix characterized the natural conditions of the ponderosa pine and mixed conifer forests on the Kaibab Plateau before initiation of fire suppression and organized tree harvests. The mix of habitats had been maintained historically by frequent surface fire with small patches of stand-replacing or high-severity fire, which left much of the forest canopy intact, but regularly reduced the constant buildup of fuels and kept trees from invading into small grass-forb openings. The increased density of today’s forests, the consequence of fire suppression perhaps enhanced by climate warming, has contributed to increased numbers and sizes of stand-replacing wildfires, which has resulted in long-term displacement of goshawks from affected Kaibab territories. During the study period, the goshawk population was slightly declining to stable. However, further habitat loss to crown fire, prolonged drought periods, and other stressors could prompt a decline in Kaibab Plateau population.
Reynolds, Richard T.; Lambert, Jeffrey S.; Flather, Curtis H.; White, Gary C.; Bird, Benjamin J.; Baggett, L. Scott; Lambert, Carrie; Bayard De Volo, Shelley. 2017. Long-term demography of the Northern Goshawk in a variable environment. Wildlife Monographs. 197: 1–40.