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Long-term demography of the Northern Goshawk in a variable environmentAuthor(s): Richard T. Reynolds; Jeffrey S. Lambert; Curtis H. Flather; Gary C. White; Benjamin J. Bird; L. Scott Baggett; Carrie Lambert; Shelley Bayard De Bolo
Source: Wildlife Monographs. 197: 1-40.
Publication Series: Scientific Journal (JRNL)
Station: Rocky Mountain Research Station
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DescriptionThe Nearctic northern goshawk (Accipiter gentilis atricapillis) is a resident of conifer, broadleaf, and mixed forests from the boreal to the southwestern montane regions of North America. We report on a 20-year mark-recapture investigation (1991-2010) of the distribution and density of breeders, temporal and spatial variability in breeding, nestling sex ratios, local versus immigrant recruitment of breeders, breeding age structure, age-specific survival rates, and rate of population change (λ) of this species on the Kaibab Plateau, a forested sky island in northern Arizona, USA. We used an information-theoretic approach to rank models representing alternative hypotheses about the influence of annual fluctuations in precipitation on the annual frequency of goshawk breeding and fledgling production. We studied 125 goshawk breeding territories, representing approximately 87% of an estimated 144 total territories based on a mean distance of 3.8 km between territory centers in a 1,728-km2 study area. The salient demographic feature of the population was extensive annual variation in breeding, which manifested as large inter-annual variation in proportions of pairs laying eggs, brood sizes, nest failure rates, and fledgling production. The percent of territories known in a prior year in which eggs were laid in a current year ranged from 8% to 86% (math formula = 37%, SE = 4.51), annual mean nest failure rate (active nests that failed) ranged from 12% to 48% (overall math formula = 23%, SE = 2.48), and mean annual brood size of successful nests (fledged ≥1 fledgling) ranged from 1.5 young to 2.5 young (overall math formula = 2.0 young, SE = 0.03). Inter-annual variation in reproduction closely tracked inter-annual variation in precipitation, which we hypothesize influenced primary forest productivity and bird and mammal prey abundance. The best breeding years (1992-1993, 77-87% of pairs laid eggs) were coincident with a record-long El Niño-Southern Oscillation (ENSO) wet period and the worst breeding year (2003; 8% of pairs laid eggs) was the last of a 3-year record drought. Overall breeding success was 83% with most failures occurring during incubation; once eggs hatched, goshawks tended to fledge young.
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CitationReynolds, Richard T.; Lambert, Jeffrey S.; Flather, Curtis H.; White, Gary C.; Bird, Benjamin J.; Baggett, L. Scott; Lambert, Carrie; Bayard De Bolo, Shelley. 2017. Long-term demography of the Northern Goshawk in a variable environment. Wildlife Monographs. 197: 1-40.
KeywordsAccipiter gentilis, age structure, Arizona, brood sex ratio, demography, immigration, Kaibab Plateau, lambda, precipitation, recruitment, reproduction, survival
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