There is more than one single reason for birds to migrate, but it all comes down to survival. 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 replenished tropical regions.
Although the ice of winter makes it hard for some birds to find food, birds probably do not migrate just to find food. It is more likely that the length of day and the amount of sunlight cause birds to migrate. In the spring, many birds fly northward, where the days have grown longer than the nights.
The longer daylight hours give the adult bird more time to gather the extra food needed to raise its young. Once the nesting season is over, many birds return south, long before the arrival of cold weather.
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 chicks, and it is advantageous to lay eggs further north.
Birds that migrate to different habitats can avoid that onslaught of predators, giving their young a better chance of reaching maturity. While diseases can and do occasionally devastate breeding colonies, birds that disperse to different locations have less chance of spreading a disease to their entire population, including their new offspring.
Finding richer food sources, seeking safer habitats and avoiding predators are all migration behaviors designed to ensure breeding success, allowing the birds to survive for another generation and allowing birders the pleasure of witnessing another year’s migration.
Content for this question contributed by Steve Benevides, resident of East Taunton, Massachusetts, USA
