The most straightforward answer is: to find food.
When birds reproduce and need to raise their young, they require large amounts of food to give them the best chance of survival. In spring and summer, many northern regions provide an abundance of insects, seeds, and other natural foods that are ideal for feeding growing chicks.
For example, Scarlet Tanagers travel from the tropical forests of South America to North America, where newly hatched insects provide an excellent food source during the nesting season.
Birds of prey will also journey north to nest in the spring where an increased population of mice, voles, and small animals will support their offspring during the breeding season.
Complete Migration
Some birds are known as complete migrators, meaning they follow a regular migration schedule every year.
They leave their northern breeding grounds to winter in their southern destinations at nearly the same time each year, and in the spring, they return to the same breeding grounds they visited the year before when the food and nesting conditions are again favourable. The actual journey for complete migrators may be thousands of miles; some individuals will fly over 15,000 miles in a single year.
Complete migration doesn’t always involve heading to the tropical regions. Some birds simply move from colder northern areas to locations with milder winters while still remaining within North America.
For example, some Dark-eyed Juncos live year-round in the Pacific Northwest, while others migrate south from Northern Canada and the northern United States.
Not All Birds Migrate the Same Way
Migration is timed to coincide with breeding. In many complete migratory species:
-
Males arrive first in spring
-
They search for suitable nesting sites
-
They establish and defend breeding territories
-
Females arrive several weeks later
But in autumn, the migration sequence will flip. Females and young birds may begin migrating several weeks before the adult males. This staggered timing helps each group take advantage of changing food supplies and breeding opportunities.
Partial Migration
Not every bird follows a fixed migration schedule. Some species are partial migrators, meaning they wait until food becomes unavailable or extremely scarce before moving to another location. Instead of travelling thousands of miles, they often move only as far as necessary to find better feeding conditions.
Since these movements are entirely dictated by the availability of food resources and the weather, partial migrations are sometimes also referred to as seasonal movements. Anna’s hummingbird is an example of a partial migrator.
Irruptive Migration
Birds that engage in irruptive migrations only make them when necessary to overcome adverse conditions. Instead of traveling a specific path every year, the birds might migrate only every few years or maybe several years in a row, depending on the availability of food.
When the environment doesn’t have enough to go around, the birds may depart from their typical range and take off to seek a more favourable food supply. For example, Red-breasted Nuthatch travels occasionally beyond its usual range when food is limited.
Birds That Stay All Year
Some birds do not migrate at all. They just adapt to the seasonal changes as food sources change rather than making long seasonal journeys.
The American Crow might stay within its home range and move locally to find available food whenever necessary.
Different Birds, Different Journeys
Every species of bird has its own method for finding the resources necessary to survive the year and care for its young, whether through migration or remaining in place, taking on thousands of miles a year or just moving a few hundred miles to find food resources.
Regardless of the specific mode of migration, each species of bird has one ultimate goal: food. So next time a familiar friend disappears from your bird feeder only to turn up in a few months, you’ll know the remarkable journey of migration is the reason.