When Do Birds Migrate?
Millions of birds of many different varieties migrate at the end of summer. With unfailing regularity they leave the regions where they were born to fly to warmer climates for the winter. The following spring they return to their breeding grounds.
Each year these migratory birds travel as much as 20,000 miles, finding their way back on time with extraordinary precision. Some travel as individuals.
Migration is triggered off by the length of daylight, which apparently affects the birds’ nervous systems. When the days get shorter the birds receive the signal to leave for their warmer winter grounds, and, when the daylight increases to a certain level, they receive another signal to return home.
Food may be the key to a regular migration, but birds migrate for other reasons related to helping their offspring survive, including…
- Climate: Birds have evolved different types of plumage to survive different climates, and changes in those climates can affect migration. Many birds leave the Arctic breeding grounds, for example, when temperatures begin to dip and they need more temperate habitat. Similarly, the hottest tropical regions can be a harsh environment for raising delicate chicks, and it is advantageous to lay eggs further north in cooler areas.
- Predators: Habitats that have abundant food sources year-round also attract a greater number of predators that can threaten nests. Birds that migrate to different habitats can avoid that onslaught of predators, giving their young a better chance of reaching maturity. Many birds even migrate to specialized habitats that are nearly inaccessible to predators, such as coastal cliffs or rocky offshore islands.
- Disease: Any large group of birds crammed in one type of habitat is susceptible to parasites and diseases that can decimate thousands of birds in a short period of time. Diseases can and do occasionally devastate breeding colonies. Birds that disperse to different locations, however, have less chance of spreading a disease to their entire population, including their new offspring.
In the end, the reasons why birds migrate all come down to survival – not of the migrating birds themselves, but of the chicks they will raise. Finding richer food sources, seeking safer habitats and avoiding predators are all migration behaviors designed to ensure breeding success. Good migration allows birds to survive for another generation and allows birders the pleasure of witnessing another year’s migration.