Northern Goshawks on the Kaibab Plateau: A 20-year Investigation Into Factors Affecting Their Demography
The northern goshawk is designated as a “sensitive species” in all Forest Service regions. This designation is a consequence of a contention that forest management practices can cause population declines. The hawk has been proposed for listing under the Endangered Species Act and its response to forest management continues to be the subject of frequent litigation. Goshawks are long-lived, large-area predators that, because of their elusiveness, are difficult to find and monitor. Researching the effects of forest management and natural disturbances on goshawk viability requires large study areas, large samples of goshawks, and long-term studies to distinguish any population responses to environmental disturbances (e.g., weather, climate) from disturbances caused by forest management, each of which may take years to manifest. To determine their population density and vital rates, Forest Service scientists at the agency’s Rocky Mountain Research Station surveyed for, trapped, banded, and monitored breeding goshawks and nestlings on 125 territories for 20 years. The study area included forests on the Kaibab National Forest and Grand Canyon-North Rim and allowed for comparison of goshawk demography in areas with (National Forest) and without (National Park) tree harvests. To determine the population's status (increasing, decreasing, stable), they focused research on births, deaths, and immigration. They used capture-mark-resight methods of banded individuals to determine lifespan, breeding lifespans, and to estimate age-specific survival and rates of population change. Re-sightings of individuals at nests also provided estimates of turnovers and recruitment rates of new breeders. Because young were banded on nearly all territories on the forested portions of the Kaibab Plateau, they determined rates of recruitment of locally (in situ) banded young, which in turn allowed us to estimate immigration rates of (unbanded) recruits from distant forests. Intensive monitoring at nests allowed them to determine age at first breeding, how production of young changed with breeder age, and causes of mortalities of breeders and nestlings. Because several large crown fire burned all or parts of multiple goshawk territories during our study, we were also able to monitor the effects of habitat loss on goshawk breeding. The scientists’ key findings are: A salient feature of the population was extensive annual variation in breeding with as few as 8 percent of territories to as many 87 percent of territories with egg laying pairs in a year. Variable reproduction tracked variation in El Niño-La Niña precipitation, which influenced primary forest productivity (vegetation growth, flower and seed production), which in turn affected bird and mammal (goshawk prey) abundance and goshawk reproduction. Mean lifespan of was 6.9 years with a maximum age of 15-years for both sexes. Mean annual survival of adults was 0.77 for both sexes; there was slight increase in survival early in life but after age 9-years survival slowly declined to 0 at 20-years. Immigration rate of new breeders was 0.63 percent. Population change (lambda) estimates showed a slight decline in the population of breeders over the study, likely due to continued drought through the last 15 years of the study and associated low prey populations. Climate change-related drought effects on prey abundance and habitat loss to catastrophic fire are existential threats to the population.
Forest Service Partners