M.F. Ingraldi | USGS Science

USGS2006

Diet, prey delivery rates, and prey biomass of Northern Goshawks in East-Central Arizona

Recent concern over persistence of Northern Goshawk (Accipiter gentilis) populations in Arizona has stemmed from two long-term demography studies that report substantial yearly fluctuations in productivity and evidence of a declining population. Although many factors could be involved in changes in productivity and population declines, availability of food is one such factor. As part of a demography study on the Sitgreaves portion of the Apache-Sitgreaves National Forest in Arizona, we used remote cameras to assess diets of goshawks. Northern Goshawks preyed upon 22 species during two nesting seasons. Adult pairs tended to specialize on particular species of prey. Prey delivery rates decreased throughout the nesting season with a corresponding increase in biomass in the latter stages of the nestling and fledgling periods. Adults appeared to take larger prey as nestlings increased in age.

Studies in Avian Biology↗

USGS2005

Quantifying Northern Goshawk diets using remote cameras and observations from blinds

Raptor diet is most commonly measured indirectly, by analyzing castings and prey remains, or directly, by observing prey deliveries from blinds. Indirect methods are not only time consuming, but there is evidence to suggest these methods may overestimate certain prey taxa within raptor diet. Remote video surveillance systems have been developed to aid in monitoring and data collection, but their use in field situations can be challenging and is often untested. To investigate diet and prey delivery rates of Northern Goshawks (Accipiter gentilis), we operated 10 remote camera systems at occupied nests during the breeding seasons of 1999 and 2000 in east-central Arizona. We collected 2458 hr of useable video and successfully identified 627 (93%) prey items at least to Class (Aves, Mammalia, or Reptilia). Of prey items identified to genus, we identified 344 (81%) mammals, 62 (15%) birds, and 16 (4%) reptiles. During camera operation, we also conducted observations from blinds at a subset of five nests to compare the relative efficiency and precision of both methods. Limited observations from blinds yielded fewer prey deliveries, and therefore, lower delivery rates (0.16 items/hr) than simultaneous video footage (0.28 items/hr). Observations from blinds resulted in fewer prey identified to the genus and species levels, when compared to data collected by remote cameras. Cameras provided a detailed and close view of nests, allowed for simultaneous recording at multiple nests, decreased observer bias and fatigue, and provided a permanent archive of data. ?? 2005 The Raptor Research Foundation, Inc.

Journal of Raptor Research↗