Crows Never Had It So Good

News from the BTO
12th June 1996

Richard Gregory and John Marchant have looked at the population levels of the common corvids (crows) in Britain over the 30 years 1964 to 1993. Their data were the unrivalled information provided by the Common Birds Census run by the British Trust for Ornithology. All four species, Jay, Magpie, Jackdaw and Carrion Crow had increased in farmland but only Magpie and Carrion Crow had increased in woodland.

The study was funded under contract from the Joint Nature Conservation Committee to asses the population of a number of 'pest' species in the United Kingdom. Growth was highest in Magpie and Carrion Crow although the rate of increase declined during the second half of the period under review. These increase seem, in part, to be due to the decline in gamekeeping activities although, during the last few years, the use of Larssen traps has increased their efficiency at taking Magpies and Carrion Crows. On farmland increases were weakest on arable farms and stronger on mixed on grazing holdings.

This study strongly supports the theory that current declines in seed-eating birds, most pronounced on arable farmland where intensive methods are practised, are not caused by increase in corvids. Indeed it is on these farms that the crows have gained in numbers less than elsewhere. This is not what one would expect if the loss of seed-eating birds were primarily due to the crows. Further work is in progress to see whether there are any links at all.