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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“The bird is not in its ounces and inches, but in its relations to Nature; and the skin or skeleton you show me, is no more a heron, than a heap of ashes or a bottle of gases into which his body has been reduced, is Dante or Washington.”
—Ralph Waldo Emerson (1803–1882)