Effects of Habitat Fragmentation on Shrub-Steppe Birds in Southeastern Idaho

Rideout, Catherine

Some population declines of forest-dwelling songbirds in North America have been attributed to habitat fragmentation. However, few studies have addressed potential negative consequences of habitat fragmentation on passerines in shrub-steppe habitat of the West. My study examined potential effects of habitat fragmentation and patch size on three sagebrush-obligate species: Brewer's Sparrows (Spizella breweri), Sage Sparrows (Amphispiza belli), and Sage Thrashers ( Oreoscoptes mont anus). I surveyed birds and monitored reproductive success of sagebrush-obligate species in continuous (n = 9) and fragmented (n = 12) study plots located within and near the Idaho National Engineering and Environmental Laboratory (INEEL) in southeastern Idaho during 1999 and 2000. Avian species richness did not differ between continuous and fragmented habitat. However, relative abundance ofboth Sage Sparrows and Sage Thrashers was significantly higher in fragmented habitat. Nest success of Sage Sparrows was significantly lower in fragmented habitat. When all nests regardless of fate were considered, there was no difference in mean productivity between continuous and fragmented habitat. However, when only successful nests were considered, the mean number of fledglings per nest was lower in fragmented habitat for Brewer's Sparrows. There were 27 instances of brood parasitism by Brown-headed Cowbirds (Molothrus ater) for Brewer's (n = 14) and Sage Sparrows (n = 13), but events occurred equivalently in fragmented and continuous habitat. Furthermore, parasitized nests did not fail significantly more often than unparasitized nests. The most common nest predators were small rodents, but predators responsible for nest failure did not differ between continuous and fragmented habitat. Thus, effects of habitat fragmentation within shrub steppe in southeastern Idaho appear to be related to (1) increased abundance of Sage Sparrows and Sage Thrashers, (2) lower nest success for Sage Sparrows, and (3) lower productivity per successful nest for Brewer's Sparrows. In addition, within fragmented habitat, patch size was not correlated with the species richness, abundance or nesting success of the three sagebrush-obligate species.