RAPID MICROEVOLUTION OF MIGRATORY BEHAVIOR IN A WILD BIRD SPECIES

THE Blackcap, Sylvia atricapilla, a widespread Palearctic migratory bird, rarely wintered in Britain until the 1950s. The winter population has since increased to several thousand birds1,2. Ringing indicates that these are not British Blackcaps forestalling migration, but birds breeding in Continental Europe reaching Britain on a novel westerly migration route3,4. The proportion of northwestern migrants among Blackcaps ringed in parts of Germany and Austria has increased from 0% before 1960 to currently 7-11%5-7. We bred British wintering Blackcaps in captivity and determined the migratory direction of their offspring. Here we report that these birds migrate west-northwest in autumn, a direction genetically distinct from the British breeding population and the predominantly southwestern migratory population of west-central Europe. The novel route must have evolved within the past 30 years with selection favouring birds wintering some 1,500 km further north than most of their conspecifics. To our knowledge, this is the first case in any vertebrate in which a drastic and recent evolutionary change of behaviour has been documented and its genetic basis established.