Prey and reproduction in a metapopulation decline among Swainson's Hawks, Buteo swainsoni

Abstract

Populations and productivity of Swainson's Hawks (Buteo swainsoni) have declined for a decade at two study areas (Hanna, Alberta, and Kindersley - Alsask, Saskatchewan) near the northern edge of their Great Plains breeding range, Near Hanna, reproduction in successful nests appeared stable through 1990 and then declined by 31%, followed by a hawk population drop in 1995 and 1996. In Saskatchewan, reproduction began to decline in 1988 and reached unprecedented low levels in 1993 and 1996, followed by a noticeable drop in hawk populations beginning in 1994. In both areas, Richardson's Ground Squirrels (Spermophilus richardsonii), the hawks' main prey, have declined drastically in numbers, forcing the hawks to prey more often on ducklings, songbirds and corvids. Forty Swainson's Hawks, marked as nestlings, bred on or near the Hanna study area, including five which moved from Saskatchewan. Recruitment of known-age breeders at Hanna was low at 4.4% of the breeding population, perhaps because the open prairie, which lacks geographic barriers, facilitates wide dispersal. The observed declines of this hawk and other species of grassland birds are of interest because they coincided with pervasive changes in the prairie ecosystem, especially an increasing use of biocides and fertilizers.