Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat, or weather. Sometimes, journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or in only one direction (dispersal, movement of young away from natal area). Migration is marked by its annual seasonality.
In contrast, birds that are non-migratory are said to be resident or sedentary. Approximately 1800 of the world's 10,000 bird species are long-distance migrants.
Read more about Bird Migration: General Patterns, Historical Views, Long-distance Migration, Short-distance and Altitudinal Migration, Irruptions and Dispersal, Physiology and Control, Adaptations, Evolutionary and Ecological Factors, Ecological Effects, Study Techniques, Threats and Conservation
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“Some say that ever ‘gainst that season comes
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So hallowed, and so gracious, is that time.”
—William Shakespeare (1564–1616)