By Joe Eaton
American Kestrel (Photo: Dave Harper)
American kestrels—small colorful falcons—have experienced population declines in portions of their extensive range, including California and the northeastern US. Several possible causes have been suggested, including rodenticide poisoning, predation by Cooper’s hawks, and loss of nest sites, but so far no smoking gun has been found. A recent study supported a relationship between anticoagulant rodenticide (AR) exposure and negative population trends for the closely related and ecologically similar Eurasian kestrel in Great Britain. An article just published in the Journal of Raptor Research reports AR levels of concern in liver tissue of American kestrels in Utah, and the presence of ARs in the blood of kestrels using nest boxes there.
The paper, whose authors include Evan Buechley of The Peregrine Fund, Dave Olyear of the nonprofit HawkWatch International, and Barnett Rattner of the US Geological Survey, also noted apparent exposure differences between adult kestrels and fledglings, and recommended sampling excrement from nest boxes as a noninvasive method of detecting ARs. The Utah research was funded by Liphatech, the folks who brought us the first-generation AR (FGAR) chlorophacinone and the second-generation ARs (SGARs) bromadiolone and difethialone, and another co-author is affiliated with Liphatech.
Like most members of the falcon family, American kestrels don’t build their own nests. They usually lay their eggs in an unfurnished tree cavity but will accept artificial nest boxes. HawkWatch monitors 546 kestrel nest boxes in Utah’s Wasatch Front region, sandwiched between Great Salt Lake and the Wasatch Mountains. It’s a rapidly urbanizing area where sprawl has been devouring shrub-steppe wildland and dairy and other farmland. Even so, the kestrel population in the region is reportedly stable.
For the study, blood samples were taken from adult and fledging kestrels using boxes in wildland, agricultural, and urban habitat in 2019 and 2020. “I didn’t think we’d see too many adults with ARs,” Rattner recalled. “But seven of twelve had detectable levels.” An adult with the SGAR brodifacoum had the highest concentration; the SGARs bromadiolone and difethialone were also detected. ARs were found in samples from urban and agricultural habitats but not in those from wildlands.
Only one set of nestlings tested positive for an AR, brodifacoum. This represented pooled samples from all the young birds in the box. A difference in diet, with adults eating more rodents and nestlings being fed more insects, might account for the variance. The authors called for further research to clarify consumption patterns. Metabolic differences between adults and juveniles could also be a factor.
As an alternative to sampling blood, fecal samples have been used to detect insecticides and other contaminants in Eurasian skylarks, but apparently not for raptors and rodenticides. Rattner sees this as a promising noninvasive way to monitor exposure, along with analyzing pellets regurgitated by box-nesting raptors, pioneered by Ian Newton in a barn owl study in the 1990s. (This spring, RATS will be sampling the pellets of barn owls in the Central Valley.) Sampling feces and pellets would be useful in monitoring several raptor species besides American kestrels and barn owls. Eurasian kestrels, lesser kestrels, and the endangered Mauritius kestrel also use next boxes. So do a number of owls: barred owls and eastern and western screech-owls in North America; tawny, common scops, Ural, and little owls in Europe and Asia; and, at least on one occasion, New Zealand’s morepork.
A second data set came from analysis of the livers of eight kestrels and 17 other raptors of seven species that were either found dead in the study area or died after being taken to rehabilitation facilities. Three of the kestrels had summed AR concentrations exceeding 0.1 micrograms per gram, a level the authors characterized as “notable” and warranting further investigation as a potential threat to the species. Rattner clarified that this level is considered a threshold of concern: “This is an accepted value in the literature, used by many scientists. I’ve used it in monitoring studies of California condors and of birds in France.”
ARs were also found in the livers of six of seven great horned owls, two of three red-tailed hawks, two of three Cooper’s hawks, both western screech-owls, one of two merlins, and the single Swainson’s hawk. Most of the compounds detected were SGARs, with the FGAR diphacinone in one kestrel and one screech-owl. One great horned owl and both screech-owls had concentrations above the threshold of concern.
