Bird migration is the regular seasonal movement, often north and
south along a flyway between breeding and wintering grounds,
undertaken by many species of birds. Migration, which carries
high costs in predation and mortality, including from hunting by
humans, is driven primarily by availability of food. Migration
occurs mainly in the Northern Hemisphere where birds are
funnelled on to specific routes by natural barriers such as the
Mediterranean Sea. Historically, migration has been recorded as
much as 3,000 years ago by Ancient Greek authors including Homer
and Aristotle, and in the Book of Job, for species such as
storks, Turtle Doves, and swallows. More recently, Johannes
Leche began recording dates of arrivals of spring migrants in
Finland in 1749, and scientific studies have used techniques
including bird ringing and satellite tracking. Threats to
migratory birds have grown with habitat destruction especially
of stopover and wintering sites, as well as structures such as
power lines and wind farms. The