Habitat Targets for Imperiled Grassland Birds in Northern Mixed-Grass Prairie

Abstract

Grassland bird populations are declining faster than any other avian guild in North America, and promotion of favorable habitat conditions in rangeland breeding cores is important for their maintenance. There is much information on associations between breeding grassland songbirds and vegetation attributes. However, previous results have been difficult to translate into management practices due to mismatch between the scale and metrics used in biological sampling and those used in management. Here, we evaluate the response of imperiled grassland bird species to vegetation conditions using metrics and scales accessible to managers. We focus on four species that are experiencing particularly severe population declines: Baird's sparrow (Centronyx bairdii), chestnut-collared longspur (Calcarius ornatus), McCown's longspur (Rynchophanes mccownii), and Sprague's pipit (Anthus spragueii). In 2017 and 2018, we evaluated the abundances of these species within their core distributions in northern Montana. We used temporally replicated point-counts and hierarchical models to estimate abundance and associations with plot-level (9-ha) vegetation conditions while accounting for spatially and temporally variable detectability. Exotic grass encroachment and shrub cover had negative or neutral effects on all species. Birds responded strongly to biomass at this scale, with chestnut-collared longspurs and Sprague's pipit preferring a range of 1 100 kg ha−1 to 1 400 kg ha−1, and McCown's longspurs selecting for the lowest available. Residual grass and litter cover were important for Baird's sparrows. Variable results among species emphasize the need for heterogeneity in vegetation structure and composition at scales larger than the plot. Our results provide guidance for managers interested in improving habitat for these species.