by Joe Eaton
(Full article from RATS Tales May 2021)
In a recent article in Ecotoxicology, an international team of scientists presents what they describe as the first evidence that the use of second-generation anticoagulant rodenticides (SGARs) can limit the population of a raptor. Their research subject was the Eurasian or common kestrel, a rodent-specialist falcon. Lead author Staffan Roos—affiliated with the Royal Society for the Protection of Birds (RSPB) Centre for Conservation Science and the Swedish Species Information Centre—and his British co-authors used Breeding Bird Survey (BBS) data for 1997 through 2012 to indicate kestrel numbers in England and Scotland, and SGAR residues in the livers of kestrels found dead from 1997 through 2011 as a measure of rodenticide exposure. With an overall decline in population, the scientists found that annual kestrel abundance was lower in years when the concentration of the SGAR bromadiolone and of all SGARs combined was higher. No data on first-generation anticoagulant rodenticides (FGARs) was available.
The authors acknowledge that correlation doesn’t prove causation, and that other factors—changes in land use, decreasing numbers of small rodents, competition with barn owls and jackdaws for nest sites, predation by Northern goshawks—may have contributed to the kestrels’ decline. However, their findings suggest a rodenticide-based explanation for negative kestrel population trends in Britain and elsewhere in Europe. This has possible implications for other raptors like the American kestrel, a related species which has declined in Florida, New England, the Pacific Coast states, and elsewhere in its North American range.
Twenty years ago, the Eurasian kestrel was the most common raptor species in the United Kingdom with an estimated 60,000 pairs, having recovered from the effects of organochlorine pesticide use in the 1950s and 1960s. By 2016, though, the count was down to 31,000 pairs. BBS data from 1995 to 2018 indicated a 61 percent decline in Scotland and a 21 percent decline in England. Kestrel populations elsewhere in Europe fell by 24 percent between 1980 and 2016; significant decreases have been reported in France and Ireland. Given the predominance of voles and other small rodents in the birds’ diet and a spike in British agricultural and residential AR use beginning in the late 1990s, rodenticides had been suspected as a possible cause; but no one had documented a relationship between decreasing kestrel abundance and AR use.
Of the 241 kestrels tested, 66.8 percent had SGAR residues in their livers, with difenacoum in 120 birds, bromodialone in 116, and brodifacoum in 41; some had more than one SGAR. The researchers teased out exposure patterns by age, region, and habitat type. SGAR concentrations were higher in adult kestrels, consistent with bioaccumulation and extended sublethal exposure. The proportion of kestrels with detectable SGAR levels and the number of SGARs found was lower in Scotland than in England, although Scottish kestrels suffered a steeper population decline.
With extensive agricultural use of rodenticides, the positive association between SGAR exposure and the proportion of land covered by grain crops was not a surprise. (Under a recent stewardship plan, Scottish farmers have reduced their reliance on SGARs.) A similar association with broad-leaved woodland may have reflected SGAR use around holding pens and feeding stations for pheasants and partridges bred for English (more than Scottish) upland bird shooters.
None of the kestrels in the sample showed internal bleeding consistent with a lethal dose of SGARs, although nine had potentially life-threatening concentrations. Exactly how SGARs are impacting the kestrels remains to be determined. “Kestrels with non-lethal SGAR concentrations may have lower body condition, be less agile, less vigilant, or have a reduced immune system, making them more likely to be taken by predators, die in collisions with vehicles or die of disease,” Roos and his co-authors speculate. They note previous studies, including Laurel Serieys’ work with bobcats in California, relating exposure to compromised immune systems, weight loss, and poor body condition. Cumulative exposure may be limiting the recruitment of young kestrels into the breeding population. One clue comes from Spanish scientists led by Jesús Martinez-Padilla of the University of Oviedo, who found that nestling kestrels with traces of bromadiolone in their blood had lower body mass than AR-free nestlings. (Ironically, the kestrels were using nest boxes provided to attract rodent-eating raptors by vineyard operators, who at the same time deployed rodenticide.) Roos says immune system issues may be investigated in a new RSPB research project.
Small raptors like kestrels can be both predator and prey. In a second Spanish study, Irene Valverde of the University of Murcia and her co-authors reported that the persistence of bromadiolone in the carcasses of Eurasian kestrels posed a potential threat to predators and scavengers.
US Geological Survey biologist Barnett Rattner, whose experiments with American kestrels showed that exposure to the FGAR chlorophacinone after prior exposure to the SGAR brodifacoum exacerbated blood clotting problems, calls the British paper “a very good study,” with a sound statistical approach. “ARs are probably one of many factors that can affect populations,” he cautions. “There are more chemicals than SGARs in use in agricultural areas, with possible multiple exposures.” Further, Barnett points out that differences between the Eurasian and American species—American kestrels are smaller and often have a higher proportion of insects in their diet—should be considered in generalizing from the British study.
Future research may clarify whether population declines in several parts of the American kestrel’s range also track rodenticide use. Stay tuned.
