Forest Structure and Prey Abundance in Winter Habitat of Northern Goshawks

Abstract

The U.S. Forest Service manages most southwestern ponderosa pine (Pinus ponderosa) for forest structures designed to increase abundance of prey for northern goshawks (Accipiter gentilis). The rationale for this strategy is a hypothesis that goshawk populations are limited by prey abundance. However, Beier and Drennan (1997) found that during the breeding season goshawks selected foraging sites not for higher prey abundance but for higher canopy closure, greater tree density, and greater density of trees >40.6 cm diameter at breast height (dbh). This finding supports the argument that prey availability (as determined by forest structure suited to goshawk maneuverability and hunting behavior) is more important than prey abundance. During winter, goshawks are under greater thermal stress, most avian prey have migrated, and most mammalian prey are hibernating. Under these conditions, foraging habitats may differ from those described for the breeding season. We radiotracked 13 adult goshawks (6 F, 7 M) during 2 winters (1994-1995 and 1995-1996) to investigate seasonal movements, winter diet, and selection of foraging habitat. Most females continued to use their breeding-season home ranges in ponderosa pine forest during winter. Meanwhile, most males moved into lower-elevation pinyon-juniper forest. In contrast to the high prey diversity noted during breeding season, wintering goshawks specialized on only 2 species of large-bodied prey (cottontails [Sylvilagus spp.] and Abert squirrel [Sciurus aberti]). Goshawks minimized energy expense and thermal exposure of flight by caching and feeding behavior. Sites where goshawks foraged had more medium-sized trees (P= 0.06) and denser canopy closure (P = 0.06) than nearby reference plots that lacked evidence of goshawk use. However, indices of prey abundance were nearly equal at used and reference plots. Although our findings do not support the underlying premise of the Forest Service management strategy for goshawks, we have no evidence that goshawks will experience lower survival or fecundity under such management. Regardless of the impact of management on goshawk fitness, we question the policy of managing most ponderosa pine habitat in the Southwest United States based on the needs of this species alone.