•Second-generation anticoagulant rodenticides (SGARs) can be toxic to all mammals and birds if consumed. Various studies have shown that, in Britain, there is widespread exposure to SGARs in a diverse range of predatory mammals and birds, including red kites (Milvus milvus) which scavenge dead rats, a target species for rodent control. The Wildlife Incident Investigation Scheme (WIIS) and the Predatory Bird Monitoring Scheme (PBMS) have shown that some mortalities result from this secondary exposure.
•In the present study, we analysed liver SGAR residues in 14 red kites that had been found dead in Britain in 2022. One bird collected in 2021 was also chemically analysed and added to the time trend analysis of this report. The carcasses were submitted to and necropsied by the Disease Risk Analysis and Health Surveillance (DRAHS) programme, the PBMS, and the WIIS for England & Wales. In 2022, there were no birds received from Scotland (i.e., no bird from the WIIS for Scotland and the Raptor Health Scotland study). All these organisations are partners in the WILDCOMS (Wildlife Disease & Contaminant Monitoring & Surveillance Network) network that promotes collaboration among surveillance schemes that monitor disease and contaminants in vertebrate wildlife in the UK.
•The UK Rodenticide Stewardship Regime (hereafter referred to as the stewardship scheme) began to come into force in mid-2016 as re-registration of products for use in the UK was approved by the HSE; full implementation of the scheme was in early 2018. The key aim of this stewardship initiative is to support competence among all users of professional SGAR products. A potential benefit of this may be the reduced exposure of non-target wildlife to anticoagulant rodenticides. However, the number and density of SGAR-contaminated rats may remain unchanged although diligent searching, removal, and safe disposal of poisoned rats, as promoted by the stewardship regime, might be expected to reduce the availability of poisoned dead rats to red kites (and other scavengers) and thereby reduce the proportion of birds that are exposed and/or the magnitude of exposure. Concomitant with the stewardship scheme was a relaxation of the indoor-use-only-restriction applied to brodifacoum, flocoumafen, and difethialone, the three most acutely toxic SGARs to use indoor and outdoor around buildings. Any consequent increase in outdoor use of these three SGARs could increase the risk of secondary exposure in red kites. We therefore compared the data in the current report with that collected in 2015 and 2016 to determine if there was any evidence of a change in pattern or magnitude of exposure in red kites that might be connected to stewardship and/or change in usage restriction.
•All of the 14 red kites from England & Wales in 2022 had detectable liver residues of at least one type of SGAR. Brodifacoum, difenacoum, and bromadiolone were each detected in 13, 13 and 6 red kites, respectively. Difethialone was found in two individuals while flocoumafen was detected in no bird.
•The proportion of analysed red kites exposed to SGARs in 2015 (91%), 2016 (90%), 2017 (96%), 2018 (100%), 2019 (91%), 2020 (88%), 2021 (98%), and 2022 (100%) was similar at circa 90% or more; the higher percentages in 2017 and 2018 were principally due to a greater proportion of birds from Scotland containing residues than observed in other years. However, all red kites from England and Wales were exposed to SGARs in 2022. Difenacoum, brodifacoum, and bromadiolone were the most prevalent compounds (detected in 87%, 87%, and 75% of red kites across the eight years for each compound, respectively). On average, there were detectable residues of two different SGARs in each red kite liver likely demonstrating multiple exposures.
•Sum liver SGAR concentrations in birds from 2022 ranged between 30 and 988 ng/g wet weight (arithmetic mean: 380 ng/g wet weight, median 257.9 ng/g wet weight). Necropsy examinations indicated that two red kites showed signs of being poisoned by SGARs (i.e., showing internal haemorrhaging that is not associated with detectable trauma and also having detectable liver SGAR concentrations). These samples accounted for 17% of the red kites of the year excluding uncertain poisoning cases. These two birds had sum SGAR liver concentrations of 544.2 and 661.6 ng/g wet weight. SGARs were considered a contributory cause of death resulting from unspecified use in these cases. SGARs were a contributory cause of death in 16% of the red kite cases examined across all eight years. Over the period 2015 to 2022, a reduction has been observed in the percentage of red kites examined that were diagnosed as birds in which SGARs were implicated as a contributory cause of death. However, given that the WIIS scheme specifically examines suspected poisoning incidents, it is likely that poisoned birds are over represented in this sample compared to the population as a whole in all eight years. Due to these reasons, caution should be used when interpreting evident changes in poisoned bird rates due to the opportunistic sampling methods used in this study that may lead to over reporting of poisoned birds. Those rates being subject to variations in relative contribution of the WIIS and PBMS to each year’s sample. It should be noted that sub-lethal poisoning due to SGAR exposure is not considered in this report.
•There were statistically significant differences between years in median summed SGAR residues for non-poisoned birds and in all red kites combined with poisoned and non-poisoned birds. The magnitude of accumulated summed SGAR residues, particularly sum of brodifacoum, flocoumafen, and difethialone concentrations, was significantly higher in 2022 than in 2019. Given low occurrence and low concentrations of flocoumafen and difethialone residues, it is demonstrated that the magnitude of brodifacoum residues has increased over recent years.
•Data on presence/absence of detectable brodifacoum, flocoumafen or difethialone residues were compared for 2015/2016 and 2017/18/19/20/21/22. The proportion of red kites with detectable residues of these three SGARs was not significantly different between 2015/2016 (82%) and 2017/18/19/20/21/22 (89%). Similarly, there was no significant difference in the proportion of red kites with detectable liver difenacoum or bromadiolone residues (90% in 2015/2016 vs. 94% in 2017/18/19/20/21/22). Since the implementation of the stewardship regime, no difference in exposure pattern relating to active ingredients has been detected with the exception of an increase in the concentrations of brodifacoum.
•Spatial analysis by county/region indicated that across the monitoring period highest exposure to SGARs in red kites appeared to be around the Berkshire/Hampshire and, to a lesser extent, North Yorkshire.
•Our findings do not indicate that there has been a consistent broad scale change in exposure in red kites to SGARs following implementation of stewardship in terms of either the proportion of the sample exposed or the magnitude of sum SGARs residues detected. However, there is evidence that the proportion of red kites in which SGARs were implicated as a contributory mortality factor has decreased in more recent years. Alternative approaches to monitoring SGARs in red kites could be considered that analyse a random but representative sample, and as part of such a programme there may also be value in monitoring SGARs in the blood of tracked individuals. Brodifacoum exposure has increased in recent years, but whether this change in exposure has been caused by the relaxation of usage restrictions on brodifacoum, difethialone and flocoumafen is still a question to be addressed
