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Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activity or even survival difficult or near impossible. In some cases "winter" is characterized not necessarily by cold but by dry conditions; passing through such periods could likewise be called overwintering.

Hibernation and migration are the two major ways in which overwintering is accomplished.

Overwintering occurs in several classes of lifeform:

  • In entomology, overwintering is how an insect passes the winter season. Many insects overwinter as adults, pupae, or eggs. This can be done inside buildings, under tree bark, or beneath fallen leaves or other plant matter on the ground, among other places. All such overwintering sites shield the insect from adverse conditions associated with winter. Activity almost completely ceases until conditions become more favourable. One example is the mourning cloak butterfly, which experiences advantages to overwintering in their desired locations by being one of the first butterflies to emerge after a cold winter. Another example are the eggs of the forest tent caterpillar moth which overwinter tightly packed on tree branches. Other insects, such as the monarch butterfly, migrate and overwinter in warmer areas. Another unique butterfly, the large white, will only overwinter in southern Eurasia; however, they are not seen overwintering elsewhere. The queens of the yellow-faced bumblebee (Bombus vosnesenskii) will over-winter, and then emerge early in the flight season to obtain the best available subterranean nests. Lastly, many species of Lasioglossum, including L. hemichalceum (which is a common sweat bee), will overwinter in underground nests before emerging in the spring to start new colonies.
  • Many birds migrate and then overwinter in regions where temperatures are warmer or food is more readily available.
  • Plants are sometimes said to overwinter. At such times, growth of vegetative tissues and reproductive structures becomes minimal or ceases completely. For plants, overwintering often involves restricted water supplies and reduced light exposure. In the spring following overwintering many plants will enter their flowering stage.
  • People are also described from time to time as overwintering. This was especially true in the past during the exploration of the planet when people had to pass the winter in places not ideally suited for winter survival, and even today in the polar regions. Today people may be said to overwinter when they temporarily move to warmer areas during the months of prevailing cold weather in northern latitudes, such as people from various parts of North America staying in Florida, Arizona, or New Mexico (among other places) for parts of November to March.
  • In plant pathology, overwintering is where a plant pathogen survives the winter, during which its normal crop host species is not growing, by transferring to an alternative host, living freely in the soil or surviving on plant refuse such as discarded potatoes.