The current report is the eighth in a series of annual reports that describe the monitoring of second-generation anticoagulant rodenticide (SGAR) liver residues in barn owls Tyto alba in Britain. This work is an element of an overarching monitoring programme undertaken to track the outcomes of stewardship activities associated with the use of anticoagulant rodenticides. The barn owl is used for exposure monitoring as it is considered a sentinel for species that are generalist predators of small mammals in rural areas. The specific work reported here is the measurement of liver SGAR residues in 88 barn owls that died in 2022 at locations across Britain. The residue data are compared with those from 395 barn owls that died between 2006 and 2012 (hereafter termed baseline years), prior to changes in anticoagulant rodenticide (AR) authorisations and onset of stewardship in 2016.
As in the baseline years, the compounds detected most frequently in barn owls that died in 2022 were brodifacoum, bromadiolone, and difenacoum. Overall, 79.5% of the owls had detectable liver residues of one or more SGAR.
Numbers of barn owls containing detectable residues of flocoumafen and difethialone. There was no significant difference in the proportion of barn owls with detectable liver residues of flocoumafen between 2022 and the baseline years (3% vs 0%). In contrast, there was a significantly higher proportion of barn owls with detectable liver residues of difethialone in 2022 compared to baseline years (6.8% vs 0.3%), but this proportion was lower than in some of the intervening years (2016-2021).
The ratio of birds with “low” (100 ng/g wet wt.) concentrations for any single SGAR or for summed SGARs (ΣSGARs). There was a significantly higher proportion of birds with “high” concentrations of brodifacoum detected in their livers in 2022 than in the baseline years.
Average concentrations of brodifacoum, difenacoum, bromadiolone and ΣSGARs in the cohort of owls with “low” residues (100 ng/g wet wt.). There was no significant difference between barn owls from baseline years and from 2022 in the concentrations of “high” residues for all SGAR residues, including ΣSGARs. In contrast, “low” bromadiolone and difenacoum residues were significantly lower in birds from 2022 than in the baseline years, while “low” brodifacoum residues were significantly higher in birds from 2022 than in the baseline years.
Overall, there were significant differences in liver SGAR accumulation between barn owls that died in baseline years and in 2022: significant reductions of bromadiolone and difenacoum and an increase in brodifacoum residues from 2016. However, the lack of significant reductions in ΣSGAR residues in barn owls in 2022 suggests that full implementation of stewardship since 2018 has yet to result in a statistically significant reduction in exposure of barn owls to SGARs
