It allows them to travel to areas of greatest resources to survive.
When birds fly South for the winter it is called migration.
The birds flew South for their migration in the Winter.
A migration.
Differential migration is simply a phenomenon especially found in birds where different sexes of birds migrate different distances.
E.P Ramsay has written: 'Notices on the migration of birds in Australia' -- subject(s): Birds, Migration
Janice M. Hughes has written: 'The migration of birds' -- subject(s): Migration, Oiseaux, Birds
flying with birds
Phenology
6000000...0000000milllion trillion birds
use it as the first word e.g. migration is what birds do to survive
the massive migration has begun. Birds also show migration to survive.
H . Elliott McClure has written: 'Migration and survival of the birds of Asia' -- subject(s): Birds, Migration, Parasites