Ecological Trap in the Nest: Human Hair Causes the Death of Breeding Barn Swallows ( <i>Hirundo rustica</i> )

) nests and their association with entanglement injury or mortality. We examined the nest material composition of 193 active barn swallow nests from two geographically distinct populations in China (Hainan in the south and Heilongjiang in the north), finding that 38.3% of nests contained anthropogenic materials, principally human hair (22.3%) and plastic (21.8%), with both occurring at significantly higher rates in the south Hainan population. We report, for the first time, seven cases (3.6%) of entanglement mortality in breeding adults and nestlings caused by human hair and plastic (fishing line) incorporated into nests. Although the number of individuals killed by entanglement in human hair and plastic was not large, our findings indicate that the ubiquitous presence of human hair and plastic in the environment may pose a direct threat to bird survival. A comprehensive, multidimensional understanding of the interactions between birds and anthropogenic materials will contribute to a broader assessment of the threats that environmental pollution poses to avian populations.