ii. The role of environmental cues

Migration is not controlled simply by genetic instructions. Birds also rely on environmental stimuli to cue stages of the migration. For instance, birds are known to time some of their migration according to day length, presumably since it is the most reliable environmental predictor of seasonal change in temperate climates. The exact way in which genetic instructions combine with other cues to control migration remains unknown. It could be that, in response to a change in day length, a bird's central nervous system produces hormones, which catalyse or halt migratory restlessness. In this way, migration is controlled by both genetic programmes and environmental cues, which combine to produce a reliable but adaptable system.

iii. Other factors

Follow the leader: Juveniles can find their way to their destinations alone, but they can also be influenced by the adult’s choice of route. In Europe, two adjacent populations of White storks take different migration routes, one around the southeast of the Mediterranean, and one around the southwest. When seven juveniles were displaced from east to west and subsequently released, they followed their genetic instruction and migrated southeast. However, when juveniles were displaced from east to west and released with adults already based in the west, they followed the adults southwest: here, social bonds modified their innate compass direction⁴.

Learning and experience can also influence a bird's migration route. For instance, the juvenile starlings displaced to Switzerland that migrated to Spain, (section D.c.i.2), returned to their breeding grounds in the Netherlands. The following year, rather than following their innate compass direction to their traditional feeding grounds, they returned to Spain, an area they knew from the year before.

The influence of learnt behaviour could explain why, in many species, older birds migrate to their feeding grounds faster than juveniles². Whether or not older birds still rely on genetic programmes to control migration remains unknown. See section D. e for how adults may navigate.