Early displacement experiments and more recent experimental studies with hand-raised Blackcaps Sylvia atricapilla are reviewed in order to describe our current state of understanding of the genetic basis and recent evolutionary changes of migratory directions in birds. Hand-raised Blackcaps from east and west of the Central European migratory divide, when tested under identical conditions, exibited population-specific migratory directions in orientation cages. Cross-breeding of birds from these two populations demonstrated an intermediate mode of inheritance of this behavioural character. New data on the mitochondrial genetic population structure of the Blackcap indicate that population differentiation at the behavioural level is not necessarily related to long periods of geographic isolation (e.g. during ice ages). Migratory adaptations may have evolved recently, in some cases rapidly, as is illustrated by the establishment of a new migration route of central European Blackcaps to winter quarters in the British Isles. This new route is shown (in a captive breeding experiment) to be based on a navel, genetically determined WNW migratory direction that must have spread from almost zero to 7-11% frequency in parts of central Europe within only three decades. The inheritance of this novel trait also follows a phenotypically intermediate mode and is not influenced by the origin of the female parent (i.e. non-genetic factors can be excluded). The evolutionary flexibility of migratory adaptations is discussed in relation to changes in the environment, both natural and accelerated by man. The need is stressed to study the population-genetic mechanisms of such adaptation with the help of molecular markers as well as large-scale ringing on the breeding grounds.
