Grassland bird population declines at three Breeding Bird Survey spatial scales in contrast to a large native prairie

Grassland biomes in North America are threatened by agricultural intensification with implications for grassland-associated bird populations via habitat loss, alteration, pesticide use, and declining landscape heterogeneity. Despite decades of conservation concern, steep declines of grassland birds continue. Key to optimizing conservation effort is understanding how land-use practices, such as agriculture, across the annual cycle affect population status. Determining the relative influence of impacts on grassland bird declines is difficult given that the most robust estimate of trends, the North American Breeding Bird Survey (BBS), is conducted throughout agriculturally dominated regions. Our goal was to explore whether agriculture during the breeding season is a major driver of grassland bird declines, by evaluating population trends in regions with different amounts of agricultural development. We derived trends for 16 bird species spanning 23 years (1994–2016) at a large (459 km²), native prairie site, Suffield National Wildlife Area (SNWA) in Alberta, Canada. We compared those trends with the BBS across three spatial scales, a regional monitoring scheme with higher than average native grass cover (Grassland Bird Monitoring [GBM]), Bird Conservation Region 11 Canada (Canada), and all of Bird Conservation Region 11 (BCR 11). Trends measured as annual percent change and credible interval varied greatly among species and survey strata. Across all species, declines were greatest for Canada (−1.3%, CI: −2.8, 0.0) and BCR 11 (−1.9%, CI: −3.2, −0.6). This contrasts with positive trends for GBM routes (1.0%, CI: −0.4, 2.3) and the SNWA data (1.7%, CI: 0.3, 3.3). Six of 16 species at SNWA were increasing with one decreasing. Five species increased at GBM and four declined. Canada had 10 species declines and 3 increases and BCR 11 had 10 declines and no increases. None of the six grassland obligate species declined at SNWA, two declined at GBM, and all six declined over the two larger BBS strata. Our results showing fewer negative population trends at a large native grassland site compared with BBS at three spatial scales across the North American prairies support the prediction that agricultural intensification on breeding grounds may be driving population declines and protection of native grasslands is a key component of grassland bird conservation efforts.