Migration is the perilous seasonal journey undertaken by many bird species.

In the northern hemisphere it is prompted by the of food.

SCARCE

Migrants are also programmed to respond to the changing

GENE

length of the day as autumn approaches. Nevertheless, in the tropics,

where there is little variation in the amount of daylight, migration is still

a surprisingly common .

OCCUR

Many birds will display considerable restlessness before beginning their

journeys. Their to the earth's magnetic field helps them

SENSE

navigate, but inexperienced birds may get things wrong

SPECTACLE

and end up far from their intended destination.

In the past, the return dates could be predicted with great precision but

climate change makes this harder. Although it is for birds to

ADVANTAGE

return earlier than their rivals so they can establish territories, getting back

too early could have consequences for their long-term survival. However,

some birds are reducing the distances they migrate in response

PROGRESS

to milder climates. Their adaptability in such a short period in

EVOLVE

terms has greatly surprised scientists.