Limited capacity of habitat features to buffer grassland bird declines

Grassland birds are among the most rapidly declining groups of species, largely due to historical habitat losses that have led to extinction debts. Beyond direct habitat manipulations (e.g., prescribed fire), conservation strategies for grassland birds often adopt a landscape-scale perspective, particularly when guiding decisions on reserve acquisition, prioritizing management across existing reserves, and restoring habitat. Additionally, climate is increasingly recognized as a critical consideration for grassland bird conservation, since temperature influences survival and nest success, while precipitation has an outsized effect on primary productivity and habitat structure in grassland ecosystems. Here, we used a 16-year breeding season avian point count dataset from some of the highest quality patches of remaining grassland habitat in Minnesota, USA, to (1) estimate occurrence trends for 33 species that use grasslands and (2) assess the influence of two traditional habitat variables (amount of grassland habitat, grassland edge density) and two climate factors (temperature and precipitation anomaly) on baseline occurrence and trends. Our model reveals declines across bird species that use grassland reserves; four out of eight grassland-obligate species declined (with ≥95% confidence; six out of eight declined with ≥68% confidence) whereas 6 out of 25 facultative grassland species declined (with ≥95% confidence). The traditional habitat variables were associated with baseline occurrence of relatively few species (e.g., proportion of grassland habitat showed a clear relationship with baseline occurrence of four species, including two obligates) and buffered declines for only a handful of species, none of them obligates (e.g., amount of grassland habitat showed a clear buffering effect for Red-winged Blackbird). In contrast, breeding-season climate anomalies showed strong relationships with occurrence of more species (temperature: 7 species; precipitation: 11 species). However, climate effects on trends were less common (e.g., wetter breeding seasons buffered declines of Savannah Sparrows). Considered holistically, our results suggest that grassland bird declines are apparent even in the highest quality remaining habitats of the upper midwestern United States. Conservation strategies focused exclusively on protection and management of high-quality prairie may have limited benefits for grassland bird populations, especially as temperature and precipitation anomalies exert increasingly strong impacts on population trajectories and climate conditions continue to change.