Many people may think, as I used to, that bird migration is a fixed, inflexible event, and that all birds migrate to escape cold weather. However, migration is a complex phenomenon influenced by many different variables. Researchers still are not sure how these variables are connected to or influenced by one another.

Availability of food, in many cases, is the deciding factor in whether a species will migrate. Even birds that can tolerate harsh temperatures migrate if they cannot find enough food. This is known as irruptive migration, in which a species vacates its normal winter range, “invading” other areas as it desperately searches for food.

One of the most frequently cited invasions was that of the great gray owl during the winter of 1979-80. More than 100 of the owls were reported seen in the northeastern United States that year; they had seldom been seen at all prior to that.

Many members of the finch family, such as pine siskins, redpolls and crossbills, are also irruptive migrants. Sightings of one of these birds – the siskins – have been reported recently. A flock of a dozen showed up at my feeders.

Such movements are not timed to seasonal changes, as are the two other types of migration: complete and partial. Complete migration is just that: the bird leaves its breeding range entirely and travels to its nonbreeding range, often thousands of miles distant. This is a risky undertaking. Many birds die along the way, so the benefits (exploiting immense seasonal blooms of food to ensure reproductive success) must outweigh what it costs a bird to migrate.

Even birds that don’t appear to migrate at all, such as chickadees, nuthatches, and blue jays, have been found to be more migratory than was once thought. Banding has shown that local birds may move south, to be replaced by birds from farther north.

Partial migrants are birds that vacate only a portion – in most cases the most northerly – of their breeding ranges come winter. In this scenario, researchers have determined that in some cases, individuals who end up migrating the farthest south are often the juveniles, or the smaller gender (in raptors, many of whom are partial migrants, this is the male). Remember, migration is costly; if a bird can still find enough food on part of its breeding range during the winter, it makes sense for the bird to stay put. That bird is usually the dominant member of its age or gender class.

Even this strategy is not fixed in stone. If the season is unusually harsh or long, or if there is a shortage of prey items, all birds will move farther out of their ranges. Birds will also extend their ranges in response to a greater availability of food. This has been noticed in a few species as a result of an increase in backyard bird feeders.

Chris Corio, a volunteer at Fields Pond Audubon Center in Holden, can be reached at fieldspond@juno.com