Factors Affecting Songbird Nest Survival in Northern Mixed-Grass Prairie

Factors associated with the nest survival of mixed-grass prairie passerines are not well known, especially in the context of contemporary grassland management. We documented the nest survival of clay-colored sparrows (Spizella pallida), savannah sparrows (Passerculus sandwichensis), and bobolinks (Dolichonyx oryzivorus) in managed prairie in northwestern North Dakota, USA. We used logistic exposure models and an information-theoretic framework to estimate nest survival and evaluate support for mechanisms (grazing, temporal factors, nest parasitism, nest-site vegetation, and nest-patch factors) relevant to nest survival. Survival for the entire nesting interval (23–28 days) was low for clay-colored sparrow (18.2%), savannah sparrow (15.5%), and bobolink (3.5%). We found support for a cubic effect of nest age; survival of savannah and clay-colored sparrow nests was greatest during mid-incubation and least during the mid-nestling period. Parasitized clay-colored sparrow and bobolink nests had greater survival rates than nonparasitized nests. Nest survival of clay-colored sparrows increased with increasing vegetation height and density. For savannah sparrows, nest survival was lower when cattle were present than when cattle were absent. Characteristics of the nest patch did not have strong effects based on model coefficients and confidence intervals, though they appeared in many of the most supported models. Positive effects of vegetation height and density on nest survival of clay-colored sparrows and negative effects of cattle presence on nest survival of savannah sparrows suggest some detrimental effects of grazing. However, the need to restore and maintain intact prairies likely warrants the continuation of cattle grazing on conservation lands.