Anticoagulant rodenticides in predatory birds 2010: a Predatory Bird Monitoring Scheme (PBMS) report

Abstract

The Predatory Bird Monitoring Scheme (PBMS; http://pbms.ceh.ac.uk/) is the umbrella project that encompasses the Centre for Ecology & Hydrology’s National Capability contaminant monitoring and surveillance work on avian predators. By monitoring sentinel vertebrate species, the PBMS aims to detect and quantify current and emerging chemical threats to the environment and in particular to vertebrate wildlife. Anticoagulant rodenticides, and in particular second generation anticoagulant rodenticides (SGARs), can be toxic to all mammals and birds. Predators that feed upon rodents are particularly likely to be exposed to these compounds. The PBMS, together with other studies, have shown that there is widespread exposure to SGARs of a diverse range of predators in Britain and that some mortalities occur as a result. This report summarises the PBMS monitoring for anticoagulant rodenticides in barn owls (Tyto alba), kestrel (Falco tinnunculus) and red kites (Milvus milvus) that were found dead in 2010 and presents long term trend analysis for barn owls. SGARs were detected in 91% of 53 barn owls analysed and the most prevalent compounds were difenacoum and bromadiolone. The majority of the residues were low and not diagnosed as directly causing mortality. The livers from 24 red kites analysed in 2010. Most (92%) had detectable liver SGAR concentrations, again mainly difenacoum and bromadiolone, although brodifacoum was also detected in over half the birds. Five of the red kites analysed showed signs of haemorrhaging thought possibly to be associated with rodenticide poisoning. However, only one of these birds had a sum SGAR liver concentration greater than 0.05 μg/g wet weight and the contribution of SGARs, if any, to the death of the other four birds is uncertain. SGARs were detected in 8 of 10 kestrels analysed. The most prevalent rodenticides detected in kestrel livers were those approved for outdoor use while the co-occurrence of multiple residues was also prevalent with 6 out of 10 kestrels having more than one SGAR present in their liver. Difethialone, a SGAR marketed since late 2011, was not detected in any of the three species tested in 2010. This represents a base-line against which future results can be compared. SGARs have been monitored in barn owls since 1983. Data on long-term trends have been adjusted to account for changes over time in sensitivity of analytical methods. This has meant that very low residues (<0.025 μg/g wet weight), which are now detectable, are not included in the time trend analysis. Overall, the proportion of barn owls with detectable liver concentrations of one or more SGAR has increased significantly over the course of monitoring. The highest value was recorded in 2008 but this was approximately twice that for the previous three years. The value for 2010 (23%) was lower than 2008 and is the lowest recorded since 2005. The proportion of barn owls with detectable SGAR residues over the period 1990-2010 was twofold higher in England than in Scotland and Wales. Within England, the proportion of owls with datable residues was higher in the Defra Eastern region than elsewhere