Quantifying multiple breeding vital rates in two declining grassland songbirds

Abstract

Many studies of reproductive success in North American songbirds have focused on nesting success, while relatively few have evaluated breeding-season adult survival and post-fledging survival. Grassland songbirds are among North America's most rapidly declining avian groups, and knowledge of factors that influence vital rates is needed to address declines, develop management strategies, and accurately model population limitation. We concurrently monitored nesting success, breeding-season adult survival, and post-fledging survival of two grassland obligates, Baird's Sparrow and Grasshopper sparrow, breeding in western North Dakota and northeastern Montana. Nesting success was monitored by locating and visiting nests at regular intervals while adult and post-fledging survival were assessed by daily telemetry tracking of radio-tagged birds. We analyzed the three variables using logistic exposure and modeled climate, temporal, and vegetative covariates to explain variation in rates. Cumulative nesting success, breeding-season adult survival, and post-fledging survival were 37%, 78%, and 25%, respectively, for Baird's Sparrow and 16%, 74%, and 55% for Grasshopper Sparrow. Both nesting success and post-fledging survival in Baird's Sparrow were responsive to environmental covariates including temporal effects and vertical vegetation structure. Conversely, vital rates of Grasshopper Sparrow were largely unresponsive to covariates we modeled, perhaps because of the species' broader habitat niche relative to Baird's Sparrow. Breeding season adult survival in both species showed little annual variation and was high relative to overwintering survival estimates for the same species, while post-fledging survival in Baird's Sparrow was low and may be a management concern. We suggest as a next step the formal comparison of vital rates across life-stages in an integrated population model capable of identifying sources of population limitation throughout the full annual cycle of the species.