For a bird to journey hundreds or thousands of miles between its breeding and non-breeding ranges is a difficult, perilous journey, one that not all birds survive. So why do birds migrate? What reasons send millions of birds into the skies every spring and fall? There is more than one single reason for different birds to migrate, but it all comes down to survival, not just for each individual bird, but also for the families they hope to raise.
If No Birds Migrated
Without a reason to migrate, birds would have even more challenging lives than making these excruciating journeys. If no birds migrated, food supplies in their ranges would be rapidly depleted during the nesting season, and many chicks and adults would starve. Competition for nesting sites would be fierce, and predators would be attracted to the high concentrations of breeding birds and easy meals of nestlings. It is for those two reasons – food and breeding – that birds migrate, but those reasons are far more complicated than they seem.
Migrating for a Meal
For all birds, one of the principle driving forces behind migration is food scarcity. If all birds were to stay in the same tropical regions year-round, food would become scarce and breeding would be less successful. But as food sources are regenerating in the north each spring, millions of birds migrate to those areas to take advantage of the abundance. As the food supplies then dwindle in the fall, they return to tropical regions that have replenished in the meantime.
This pattern of migrating for a meal is true not only for neotropical migrants, but also short-range migratory birds that may move only short distances to pursue a food source. Bird irruptions are also the result of changes in the food supply, with greater irruptions occurring in years when food supplies are low for northern birds. That scarcity forces them to seek adequate food further south, well outside their typical range.
Migrating for Family
Over millennia, birds have evolved different migration patterns, timing and destinations to disperse around the world to breed. This helps birds take advantage of a wide variety of suitable conditions to raise their young, increasing the chances of healthy, viable offspring. The best breeding conditions can vary for every bird species, and may involve many factors. Specific food sources, habitats that provide adequate shelter and breeding colonies that offer greater protection than a single pair of bird parents are all important for breeding dispersal.
It may seem contradictory to argue that birds migrate to help their offspring survive. Many of those same bird parents quickly abandon their young as they mature, leaving the immature, inexperienced birds to make their dangerous first migration without adult guidance. It is exactly because the birds have raised their chicks in a relatively rich, safe environment, however, that gives them the advantage of being prepared to make that journey.
More Reasons Birds Migrate
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 because they cannot survive the cold. 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 steep 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 just the survival of the migrating birds themselves, but also the survival 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.