Association Between June Very Warm Conditions and Annual Abundance of American Robin, Tree Swallow, Barn Swallow, and Mourning Dove, 1980-2019

Unusually warm conditions may influence breeding-season bird populations, but responses can vary among species. This study examined whether the percentage of the contiguous United States classified by the National Oceanic and Atmospheric Administration as "Very Warm" in June was associated with annual abundance indices for the American Robin, Tree Swallow, Barn Swallow, and Mourning Dove from 1980 to 2019. June temperature data were paired by year with model-based annual abundance indices from the North American Breeding Bird Survey. For each species, Pearson correlation and simple linear regression were used to evaluate the direction and strength of the association. The American Robin showed a weak positive correlation (r = 0.232, R² = 0.054, p = 0.150), whereas the Tree Swallow showed virtually no linear relationship (r = -0.051, R² = 0.003, p = 0.753). The Mourning Dove showed a weak negative, non-significant association (r = -0.281, R² = 0.079, p = 0.079). The Barn Swallow had the strongest negative relationship and was the only species with a statistically significant result (r = -0.356, R² = 0.127, p = 0.024). The findings indicate that broad-scale unusually warm June conditions were not associated with a uniform abundance response across the four species. The modest explanatory power of the models also suggests that habitat, precipitation, food availability, migration, and other ecological factors should be considered in future analyses.