Quantifying multiple breeding vital rates in two declining grassland songbirds

Abstract
Many studies on the reproductive success of North American passerines have looked at nesting success, but relatively few studies have evaluated adult survival during the nesting period and survival after fledging. Prairie passerines are among the fastest declining bird groups in North America and knowledge of the factors that influence vital rates is needed to reverse declines, develop management strategies, and accurately model what limits populations. We simultaneously tracked nesting success, adult survival during the breeding season, and post-flight survival in two prairie species, Baird’s Sparrow and Grasshopper Sparrow, breeding in western North Dakota and northeastern Montana. Nesting success was measured by locating and visiting nests at regular intervals, while adult survival and post-fledging survival were calculated using daily telemetric monitoring of birds with transmitters. We analyzed these three variables using logistic exposure and modelled climate, temporal and plant covariates to explain the rate change. Cumulative nesting success, adult survival in the breeding season, and post-flight survival were 37%, 78% and 25% for Baird’s Sparrow, and 16%, 74% and 55% for Grasshopper Sparrow, respectively. Nesting success and post-flight survival of Baird’s Sparrow were sensitive to environmental covariates, including temporal effects and vertical vegetation structure. Conversely, the vital rates of grasshopper sparrow did not vary depending on the covariates we modeled, perhaps due to the larger habitat niche of this species compared to Baird’s Sparrow. Adult survival in the breeding season in both species showed little annual variation and was high relative to their winter survival rate, while post-flight survival in Baird’s Sparrow was low and may need to be a priority. As a next step, we propose to formally compare vital rates across life stages in an integrated population model to identify the factors that limit populations throughout the full annual cycle of these two species.

PUBLICATION AVAILABLE AT: https://www.ace-eco.org/vol16/iss1/art19/