Species Traits Predict Local Population Change Above and Beyond Regional Trend

Ivara Goulden , Casey Youngflesh , Allen H Hurlbert

Why do some populations persist in the face of environmental change while others decline? We tested the extent to which variation in local population change across species is attributable to three species traits: (i) habitat selectivity, (ii) temperature niche position, and (iii) degree of insectivory, with the null hypothesis that local trends are simply a reflection of broader regional trends. We modeled population change for 60 bird species on 806 route-years from 2000-2025 using the North Carolina Mini Breeding Bird Survey (NCMBBS), a long-term population survey of uniquely high spatial resolution, in a Bayesian hierarchical framework. Unlike previous studies, our studied species are all competing in the same landscape and experiencing the same changes in habitat and climate over time, allowing us to isolate the effects of species traits on ‘winners’ and ‘losers’ in a local landscape. We found that more species in the study area are in decline (56.7%) than are increasing (26.7%) or stable (16.7%). Local population change was positively related to regional trends, though local population change was generally more extreme. Above and beyond the influence of regional population trend, temperature niche position and habitat selectivity also predicted which species ‘win’ and ‘lose’, with species at the warm edge of their temperature niches and habitat specialists both more likely to decline. There was a weak positive effect of diet percent insectivory, but this likely reflects that grassland species, often not obligate insectivores, are in steeper decline rather than that insectivory provides a population benefit. We extend the frameworks set up in larger, but coarser scale, studies to show these species traits indeed shape avian populations and communities at the local level, above and beyond how they shape populations regionally.