Swainson's Hawk Nesting Ecology in North Dakota

Abstract

Swainson''s hawks (B. swainsoni) were studied at 270 occupied nest sites in south-central North Dakota on a 1259-km2 intensive study block and on a surrounding study area (16,519 km2) during 3 breeding seasons. On the intensive study block the number of occupied nests ranged from 46 in 1977 to 100 in 1979. Average nest densities were highest on ground moraine (0.119 nest/km2) and on eolian sand deposit (0.102 nest/km2) landforms. Pasture and haylands made up 75% of the land-use within 1.0 km of a sample of 27 nests. Some pairs nested successully in sites characterized by intensive agriculture and human activity; about 75% of all nests were in sites attributable to human activities. The most common nesting sites (43%) were in shelterbelts. Cottonwoods (Populus deltoides) were the most frequently used (44%) nest trees. At least 50% of the pairs constructed new nests each year. Mean nest success was 64% and mean number of young fledged per occupied nest was 1.5. Wind and hail caused over 30% of the nest failures each of the 3 yr. Northern pocket gophers (Thomomys talpoides) were the most frequent prey, accounting for 44% of all animal remains found at nests. Man-made changes in central North Dakota during the last century have provided many nesting sites in areas previously sparsely populated by Swainson''s hawks.