Bird migration is the regular seasonal movement, often north and south, between breeding and wintering grounds. Many species of bird migrate, typically by flying. Migration carries high costs in predation and mortality, and is driven primarily by the availability of food. It occurs mainly in the Northern Hemisphere.
A flock of barnacle geese during autumn migration
Minoan fresco of swallows in springtime at Akrotiri, c. 1500 BC
The Rostocker Pfeilstorch, found in 1822, demonstrated that birds migrated rather than hibernating or changing form in winter.
Flocks of birds assembling before migration southwards (probably common starling)
Winter is the coldest and darkest season of the year in polar and temperate climates. It occurs after autumn and before spring. The tilt of Earth's axis causes seasons; winter occurs when a hemisphere is oriented away from the Sun. Different cultures define different dates as the start of winter, and some use a definition based on weather.
Forest covered in snow during winter
In the mid-latitudes and polar regions, winter is associated with snow and ice.
In the Southern Hemisphere, winter extends from June to September, pictured in Caxias do Sul in the southern highlands of Brazil.
Sea ice in the Port of Hamburg, Germany
