Guest blog by Staffan Roos (previously Senior Conservation Scientist at RSPB Scotland) and Jeremy Wilson (RSPB's Director of Science).
A scientific paper, just published in the journal Ecotoxicology, written by scientists from the RSPB Centre for Conservation Science, the Predatory Bird Monitoring Scheme at the UK Centre for Ecology and Hydrology and SASA (Science and Advice for Scottish Agriculture) shows that the common kestrel is often exposed to very toxic rodenticides ("rat poisons") and that this may have contributed to population declines of kestrels in the UK. Here, the two RSPB scientists involved in the study explain what the study discovered.
Kestrels in peril
When did you last see a kestrel? Had we asked this rhetorical question 20 years ago, many people would have said that they very often saw kestrels hovering over fields and road verges. Back then, this small falcon was probably the most numerous raptor species in the UK with around 60,000 pairs.
Newly fledged kestrel © Ben Andrew (rspb-images.com)
However, these days, kestrels are much scarcer in the UK, with an estimated 31,000 pairs (in 2016), and a hovering kestrel is a less routine sight over much of the UK. Results from the national BTO/JNCC/RSPB Breeding Bird Survey (BBS) show a 61% decline in Scotland and a 21% decline in England between 1995 and 2018, and the decline is echoed across Europe, with the continent seeing a 24% population decline between 1980 and 2016.
The overall decline of kestrels in the UK between 1994 and 2019. Graph from BTO/JNCC BirdTrends Report (Woodward et al. 2020. BirdTrends 2020: trends in numbers, breeding success and survival for UK breeding birds. BTO Research Report 732. BTO, Thetford. www.bto.org/birdtrends). Blue dots are the annual population index estimates, the dark green line is the smoothed population trend, and the light green band is the 85% confident intervals around the smoothed trend.
Possible reasons for the declining kestrel numbers
Several suggestions have been put forward to explain the population decline of kestrels. Some studies have suggested that the intensification of the agricultural practices may have contributed to the decline. Also, at a local scale, the establishment of a goshawk population contributed to the decline in kestrel numbers (via predation of both adults and newly fledged kestrels).
However, there have also been suggestions that widespread usage of second generation anti-coagulant rodenticides (known as "SGARs"; read more about these acutely toxic rodenticides here) may have contributed to the decline in kestrel numbers, because both kestrels and other rodent-eating raptors and owls may eat rodents that have consumed the rodenticides. As a result, secondary poisoning may lead to internal haemorrhaging, which can be lethal. But could this have played a role in the decline of kestrels in the UK?
With that question in mind, scientists from RSPB Centre for Conservation Science, the Predatory Bird Monitoring Scheme (PBMS) at the UK Centre for Ecology and Hydrology in Lancaster and SASA (Science and Advice for Scottish Agriculture) teamed up to investigate if there were any links between rodenticides and the population decline of kestrels in the UK. Fortunately, over many years, volunteers have submitted kestrels that they have found dead to the PBMS and SASA. These organisations had then analysed samples from the kestrels' livers to test whether they contained any rodenticides.
Kestrel adult hovering looking for prey below © Ben Andrew (rspb-images.com)
Many kestrels are exposed to rodenticides
Our statistical analyses of the datasets from the PBMS and SASA revealed that many kestrels had detectable levels of rodenticides in their livers. In fact, two-thirds of the 241 kestrels that had been analysed had detectable levels of at least one type of rodenticides in their livers. Interestingly, adult kestrels were more likely to have rodenticides in their livers than juvenile kestrels. This suggests that rodenticides may accumulate over time.
Rodenticides are associated with lower kestrel numbers
Our most important result was that the annual kestrel abundance, taken from the BTO/JNCC/RSPB Breeding Bird Survey (BBS), was lower in years when the concentration of one type of rodenticide (bromadiolone) and the total concentration of all types of rodenticides in the livers of dead kestrels were high. This suggests that rodenticides may have a role in suppressing kestrel numbers at a national scale in a way that had not been described previously.
The cause of the especially steep population decline in Scotland remains uncertain because the proportion of kestrels with detectable levels of rodenticides and the number of rodenticide compounds found was lower for birds found dead in Scotland than in England. So, it seems likely that other factors may be playing a big part in the declines seen in Scotland.
Kestrel declines in Scotland may be due to other factors © Louise Greenhorn (rspb-images.com)
Future work
This study is a starting point for more detailed work on the effects of rodenticides on kestrels and other predators. For example, a new PhD project starts this autumn with the University of York, RSPB, CEH and the Health & Safety Executive, which will continue the work on rodenticide effects on kestrels, assessing the fine-scale spatial exposure risk of rodents and foraging kestrels in relation to the local distribution of SGAR sources on farms, and the fitness and population level consequences to kestrels from SGAR exposure.
The importance of volunteers
This study would not have been possible without the dedication of many volunteers, who have submitted kestrels to the PBMS and SASA, as well as monitored birds by taking part in the annual Breeding Bird Survey. We are very grateful for their efforts.
Time for reflection
Sadly, during the final phases of working on this paper, two of our co-authors died. Both Dr Gill Hartley (SASA) and Professor Richard Shore (CEH) had long and successful careers in ecology and ecotoxicology, and they are missed by their families, colleagues and friends. In a tribute to Gill and Richard, the paper is dedicated to them.
To read the open access paper in Ecotoxicology: Annual abundance of common Kestrels (Falco tinnunculus) is negatively associated with second generation anticoagulant rodenticides
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