DO SAGEBRUSH DENSITY AND VEGETATION CONDITION AFFECT DEMOGRAPHY OF BREWER\u27S SPARROWS NESTING IN THE BLACKFOOT VALLEY?

Vegetation condition can influence habitat quality by altering food abundance, predator abundance and species composition, and microclimate. In some habitats, livestock grazing alters vegetation condition in ways that affect the breeding success of passerines. Most sagebrush-dominated habitats have been heavily grazed by livestock, and the total area of sagebrush has declined substantially. Owing to this habitat loss and change, numbers of various sagebrush-obligate birds have declined, the Brewer\u27s Sparrow among them. I examined the effects of cattle grazing on Brewer\u27s Sparrow reproduction. Grazed plots, at the study-site scale, were characterized by higher potential nest-sagebrush density, and canopy cover, a higher number of exotic grass species, greater bare/rock ground cover, and lower grass/forb ground cover, compared with ungrazed plots. Trends in the characteristics of vegetation at the nest-patch scale were the same as those at the study-site scale. Reproductive performance was similar between grazed and ungrazed plots. Nestlings raised on ungrazed plots were larger than those on grazed plots in 2004, but not in other years. The numbers of breeding pairs were, however, higher in grazed plots than in ungrazed plots. Vegetation condition also influences nest-site selection of organisms. Many studies assume that this selection is adaptive; however, it does not necessarily increase the fecundity of organisms. Vegetation condition becomes one of the important cues to select nest sites because organisms use vegetation as nest hiding cover, thermal cover, foraging sites, and display sites. I examined two questions, using model selection analysis: (1) does vegetation condition affect nest survival of Brewer\u27s Sparrows?; and (2) does vegetation condition affect nestling size? There was no one best-approximating model to explain the correlation between vegetation features and nest survival, but all high-ranking models included nest-patch variables. In nestling-size models, nest-shrub size was positively correlated, and nest cover was negatively correlated, with nestling mass. Contrary to nest-survival models, nestling mass was strongly associated with nest-shrub variables rather than with nest-patch or understory-layer variables