Preliminary Evidence of Anticoagulant Rodenticide Exposure in American Kestrels (Falco sparverius) in the Western United States

Although there is extensive evidence of declines in the American Kestrel (Falco sparverius) population across North America, the cause of such declines remains a mystery. One hypothesized driver of decline is anticoagulant rodenticide (AR) exposure, which could potentially cause mortality or reduced fitness. We investigated AR exposure in wild American Kestrels in Utah, USA. We collected and tested for AR residues in liver samples (n = 8) from kestrels opportunistically encountered dead and in blood samples (n = 71) from live wild kestrels, both nestlings and adults. We found high detection rates in both tissues. Adult kestrels were more likely to exhibit exposure than juveniles sampled in nests. Three-quarters (six of eight) of tested liver samples from adult kestrels exhibited evidence of AR exposure. Additionally, liver samples (n = 19) opportunistically collected from seven species of raptors within our study area had detectable levels of AR residues, with seven of eight raptor species evidencing exposure; across all raptors, five ARs were detected in liver samples, with brodifacoum the most prevalent, being found in over half (14 of 27) of samples. Over half (7 of 12) of the blood samples from adult kestrels had detectible levels of ARs, while only one of 59 juvenile nest samples tested positive. The difference in exposure rates between adults and juveniles could indicate differential exposure pathways by age class. Based on these findings, we recommend that ARs be further investigated as a potential cause of kestrel declines. Future research could focus on expanding sampling to provide sufficient sample sizes to test for potential nonlethal effects of AR exposure (e.g., fecundity, nesting success), identifying potential exposure pathways, and developing methods for passive sampling of ARs in excreta.

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