Bird migration is the regular seasonal movement, often north and south along a flyway, between breeding and wintering grounds. Many species of bird migrate. Migration carries high costs in predation and mortality, including from hunting by humans, and is driven primarily by availability of food. It occurs mainly in the northern hemisphere, where birds are funnelled on to specific routes by natural barriers such as the Mediterranean Sea or the Caribbean 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.
Which half of the earth has more bird migration?
How long have people been observing and recording migration?
Who was one early writer recording migration?
And another?
What is one species that was recorded in early records?
And another?
What scientist in the 1700’s studied migration?
In what country was he observing?
What is a modern day method used for observing migration?
Do man made items disturb bird migration?
What is one object that does?
Any others?
Other than breeding grounds and wintering grounds what are birds seeking when they migrate?
Is migration a low risk activity for birds?
What might cause their death then?
Has man changing the landscape helped birds?
the northern hemisphere
mainly in the northern hemisphere
3,000 years
as much as 3,000 years ago
Aristotle
 by Ancient Greek authors including Homer and Aristotle
Homer
by Ancient Greek authors including Homer 
swallows.
Book of Job, for species such as storks, turtle doves, and swallows. 
storks
 Book of Job, for species such as storks
Johannes Leche
Johannes Leche began recording dates of arrivals of spring migrants in Finland in 1749
Finland
 Johannes Leche began recording dates of arrivals of spring migrants in Finland 
satellite tracking
used techniques including bird ringing and satellite tracking
yes
Threats to migratory birds have grown with habitat destruction especially of stopover and wintering sites, as well as structures such as power lines
power lines
structures such as power lines
wind farms too
structures such as power lines and wind farms.
food availability
 and is driven primarily by availability of food
no
Migration carries high costs in predation and mortality
hunting
 including from hunting by humans
no
Threats to migratory birds have grown with habitat destruction