Modern mid-latitude ecological
Six seasons can be distinguished which do not have fixed calendar-based dates like the meteorological and astronomical seasons. Mild temperate regions tend to experience the beginning of the hibernal season up to a month later than cool temperate areas, while the prevernal and vernal seasons begin up to a month earlier. For example, prevernal crocus blooms typically appear as early as February in mild coastal areas of British Columbia, the British Isles, and western and southern Europe. The actual dates for each season vary by climate region and can shift from one year to the next. Average dates listed here are for mild and cool temperate climate zones in the Northern Hemisphere:
Prevernal (early or pre-spring): Begins February or late January (mild temperate), March (cool temperate). Deciduous tree buds begin to swell. Migrating birds fly from winter to summer habitats.
Vernal (spring): Begins March (mild temperate), April (cool temperate). Tree buds burst into leaves. Birds establish territories and begin mating and nesting.
Estival (high summer): Begins June in most temperate climates. Trees in full leaf. Birds hatch and raise offspring.
Serotinal (late summer): Generally begins mid to late August. Deciduous leaves begin to change color. Young birds reach maturity and join other adult birds preparing for autumn migration.
Autumnal (autumn): Generally begins mid to late September. Tree leaves in full color then turn brown and fall to the ground. Birds migrate back to wintering areas.
Hibernal (winter): Begins December (mild temperate), November (cool temperate). Deciduous trees are bare and fallen leaves begin to decay. Migrating birds settled in winter habitats.