BIRD-HABITAT RELATIONSHIPS IN INTERIOR COLUMBIA BASIN SHRUBSTEPPE

Vegetation structure is important in structuring avian communities. In the sagebrush biome, where continued habitat loss is thought to threaten shrusteppe-obligate birds, both remotely sensed and field-acquired measures of big sagebrush (Artemisia tridentata) cover have proven valuable in understanding avian abundance. Differences in structure between the exotic annual cheatgrass (Bromus tectorum) and native bunchgrasses are also expected to be important. We used abundance from 318 point-count locations, coupled with field vegetation measurements and a detailed vegetation map, to model abundance of four shrub- and four grassland-associated avian species in southeastern Washington. Specifically, we ask whether species' abundances in bunchgrass and cheatgrass differ and whether mapped categories of cover adequately explain species' abundances or whether fine-grained, field-measured differences in vegetation are also important in explaining abundance. We found that the abundance of shrub associates did not differ in sagebrush with a cheatgrass vs. bunchgrass understory, but grassland associates tended to use bunchgrass more than cheatgrass grasslands (Horned Lark, Eremophila alpestris; Grasshopper Sparrow, Ammodramus savannarum), or, in one case, cheatgrass more than bunchgrass (Long-billed Curlew, Numenius americanus). In the comparison of map- and field-based models, mapped cover types alone were sufficient for predicting abundance of five species studied, but models containing field-measured sagebrush cover outperformed models based on maps only for three species, the Sage Sparrow (Amphispiza belli)(+), Horned Lark (-), and Grasshopper Sparrow (-). We conclude that cover-type maps that consider understory composition and sagebrush density can predict avian distribution and abundance in the sagebrush biome efficiently.