ECOLOGICAL RELATIONSHIPS BETWEEN NESTING SWAINSONS AND RED-TAILED HAWKS IN SOUTHEASTERN IDAHO

We compared reproductive success, nest site characteristics, and food habits of nesting Swainson's hawks (Buteo swainsoni) and red-tailed ha wks (B. jamaicensis) along the Big Lost River and Birch Creek on the I daho National Engineering Laboratory, southeastern Idaho, from 1991-93 . Productivity was similar between specks. Twenty-four red-tailed hawk nests produced 34 fledglings (1.4/attempt) while 17 Swainson's hawk n ests produced 21 fledglings (1.2/attempt). Nest trees used by Swainson 's hawks were shorter, smaller, and more foliated than those used by r ed-tailed hawks (P < 0.01). Swainson's hawk nest trees were more folia ted than most trees along Birch Creek and the Big Lost River (P < 0.00 6). Red-tailed hawk nest trees were similar to available deciduous tre es (>25% dead), but were taller (P = 0.001). Prey remains and castings at nests (% frequency), indicated that Swainson's hawks preyed more c ommonly on birds than red-tailed hawks while the latter more commonly captured Lepus spp. and Sciuridae; Leporidae, including Lepus spp. and Sylvilagus spp., made up over 60% of the estimated prey biomass for b oth species. Riparian vegetation condition, notably the lack of narrow leaf cottonwood (Populus angustifolia) survival and regeneration, appe ared to be a major factor accounting for changes in hawk distribution on the study area.