How do birds know when to migrate?

The timing of migration for longdistance migrants is regulated by a range of factors, which may vary depending on the species involved.
Studies by Peter Berthold and others on passerine species (ie perching birds or song birds) have shown that birds have an innate response to changes in day length, which is a highly reliable indicator of the changing seasons.
And WWT’s detailed studies of the Bewick’s swans wintering at Slimbridge have similarly found that day length is important for regulating when the Bewick’s swan migration happens. Back in the Seventies and early-Eighties, prolonged floodlighting at the swans’ roost site at Slimbridge meant they set off on their spring migration relatively early.
Once day length has signalled the onset of the migratory season, more immediate factors such as weather conditions (particularly wind direction) determine the precise day on which they migrate.
Day length is particularly useful for birds occurring at relatively high latitudes. However, waterbirds that breed in tropical and subtropical regions, where day length is more or less the same all year round, are more likely to respond to regular cycles of rainfall and to exploit temporary wetlands created by flooding.

Answered by Dr Eileen Rees, head of UK Waterbird Conservation Programme at the Wildfowl & Wetlands Trust (WWT).