Many factors are to blame for the worrying Kestrel population decline, including human behaviour
ONCE OUR MOST common bird of prey, Kestrels are still perhaps the most visible. Their hunting technique of hovering over open landscapes – black-tipped tail fanned out, head held perfectly still – makes them one of the easiest predatory birds to see and identify. In the 1970s, there were thought to be up to 100,000 pairs of Kestrels across Britain and Ireland, while, today, the UK population is estimated at about 46,000 pairs. Over this period, the steady increase in Buzzards, now thought to number up to 79,000 pairs, has pushed the smaller bird into second place. The 1970s figure was calculated at something of a boom time for Kestrels; their populations had been increasing due to recovery from long-term poisoning by organochlorine pesticides. However, BTO population monitoring showed that between the mid-1970s and the late 1980s numbers declined; this was followed by two decades of fluctuating populations before a further period of decline over the last 10 years. Maps from the Bird Atlas 2007–11 show that while Kestrels remain widespread, present in 90% of 10km squares across Britain and Ireland, they are most abundant in eastern and central England. While they are known to adapt to a variety of prey, Kestrels mainly hunt small mammals, and they favour areas of rough grassland that support good numbers of voles. Unfortunately, agricultural intensification is thought to have decreased and degraded areas of rough grassland, and, while the population trends of small mammals are hard to monitor, this is likely to have resulted in a decline in vole numbers. BTO research into Kestrel productivity and survival showed that the number of chicks successfully fledged from each nest remained steady during the period of severe population declines in the 1970s. This suggests that the birds weren’t struggling to find suitable prey to feed their nestlings during the breeding season; rather, adult birds were not surviving over the winters, possibly due to lack of food. There have been other reasons suggested for the decline in Kestrels over the past 40 years. They were once severely affected by organochlorine pesticides, such as DDT; while these are DECLINE The lovely Kestrel faces a range of threats to its population numbers The BTO runs volunteer surveys to monitor and explain changes in bird populations. To find out more about the Waterways Breeding Bird Survey visit now banned, other harmful substances are still widespread. Poisons put down for rats and mice often find their way into the food chain. No evidence has been found so far that this ‘secondary poisoning’ causes enough direct mortality to be driving the population declines. A long-term study of Kestrel numbers in Kielder Forest in northern England, linked declines in this area with increases in Goshawks, supported by observations of Kestrel remains around Goshawk nests. While it’s unlikely that predation has affected numbers nationally, in certain areas, losses of breeding adults to larger predators may have had an effect. Finally, Kestrels are known to be attracted to roadside verges by the undisturbed rough grassland; it’s possible that this has led to an increase in deaths of adults owing to traffic, although, once again, there is no evidence that this has driven the national declines. Overall, however, the picture is clear; these familiar birds of prey are facing a range of threats, and it’s likely that the losses of thousands of breeding birds from our skies and countryside are due to a range of human impacts.
Kate Risely is the British Trust for Ornithology’s Garden Birdwatch Organiser
A long-term study of Kestrel numbers in Kielder Forest in northern England linked declines in this area with increases in Goshawks