Anticoagulant rodenticides in predatory birds 2007 & 2008: 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), kestrels (Falco tinnunculus) and red kites (Milvus milvus) that were found dead in 2007 and 2008 and presents long term trend analysis for barn owls and kestrels. Since 2006, anticoagulant rodenticide concentrations have been quantified using the more sensitive Liquid Chromatography – Mass Spectrometry (LC-MS) method. This has resulted in lower concentrations of these compounds being detected than was previously possible. Consequently, for samples from 2006 onwards, the proportion of birds in which anticoagulant rodenticides have been detected has increased compared to previous years. SGARs were detected in 81% of barn owls and 68% kestrels and the most prevalent compounds were difenacoum and bromadiolone. The majority of the residues were low and not diagnosed as directly causing mortality. Most of the red kites (91%) had detectable liver SGAR concentrations, again mainly difenacoum and bromadiolone although brodifacoum was also detected in over half the birds. A quarter of the red kites analysed showed signs of haemorrhaging thought possibly to be associated with rodenticide poisoning. 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 (<0.025μg/g wet weight), which are now easily detectable, are not included in the time trend analysis. The proportion of owls with detectable SGAR residues was found to be two-fold higher in England than in either Scotland or Wales. 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. Kestrels have been monitored between 1997 and 2008. Over this period the proportion of birds with detectable SGAR residues was higher in kestrels than in barn owls, although this was not the case in the last two years. There has been no progressive increase or decrease over time in the proportion of kestrels with detectable SGAR residues. Continued monitoring is required to determine whether the high detection rate for SGARs in barn owls is anomalous and perhaps due to random variation in sampling of owls. The high proportion of red kites exposed to SGARs and the relatively large number of birds with signs of haemorrhaging suggests that this species remains at particular risk from anticoagulant rodenticides