Many fac­tors are to blame for the wor­ry­ing Kestrel pop­u­la­tion de­cline, in­clud­ing hu­man be­hav­iour

Bird Watching (UK) - - Your Birding Month -

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

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