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Why

(Courtesy NEBRASKAland Magazine/NGPC)

Ornithologists (biologists who study birds) know that, over time, populations of birds can acquire or lose the habit of migrating. This can happen for different reasons.

Migratory habits and patterns can change in relation to changing climates and ecological circumstances. For example, there might be a loss of habitat for wintering, breeding, or stopovers.

The evolution of a migratory species has three stages:

• partial migration (for example, a prolonged drought forces birds to move a short distance to find water or food);
• division of the species into migratory and resident populations; and
• fully disjunct (separate) migration due to the gradual elimination of resident populations.

Why don't birds just stay where it's warm all year long?
When migrant birds leave their wintering ranges in the spring to go to breeding areas, they're assured of reduced competition for space as well as plenty of food for themselves and their offspring.

Harris' Sparrow (Courtesy NEBRASKAland Magazine/NGPC)

Migrants are attracted to the good nesting opportunities found in temperate latitudes like in North America and Canada during the summers. These lush areas contain highly dispersed (spread out) breeding spots with a low density of birds.

On the other hand, species that stay year-round in the tropics sometimes wait several years to get a good breeding territory because of the high density of birds using the area.

Migrating from one temperate climate to another allows birds to remain active year-round instead of spending part of the year dormant or hibernating, as some mammals do, like woodchucks and some bears.

The cost of migration can be great. It takes radical physical and biological change combined with sustained fine tuning for birds to survive these extended migration journeys.

The difficulties and hazards of the annual migratory journey of birds are usually balanced by the benefits gained. Migrating enables a species to inhabit or take advantage of two different areas during seasons when each region provides plenty of food and warmth. Upland Sandpipers breed in the grasslands of Nebraska and winter on the pampas (plains) of Argentina. These birds never experience winter. If it were not advantageous to make the trip twice a year, birds would not have evolved the migrating behavior.

For many species, moving to higher latitudes provides plenty of time for feeding their young. Because of the longer days, their offspring can grow rapidly and spend less time exposed in the nest to predatory animals.

But the higher the latitude, the shorter the breeding season. The summer days may be long but the summer season is short, and migrants in more northerly areas may have only one chance to breed before they must travel southward again. At lower latitudes, breeding seasons are longer, allowing multiple attempts to produce young. But this longer breeding season means it's more likely that nests will suffer losses to predators.