These northern wetland areas are rich in food and lean in predators. Continuous sunshine and a rich supply of insects and plant material make them ideal for breeding. Some birds, like the blackpoll warbler, make long journeys to South America, first gathering on the east coast of North America to feed for the long trip. After feeding for ten to twenty days they will have over 95 hours of flying time before they arrive at their destination. As birds return from Central and South America competition becomes greater for available food supplies.
Competition is also a reason for birds to migrate north in summer.
When summer ends, these migrants must return south before the ground and estuaries freeze, shutting out their food supply. Sources Bird Behavior. Wetlands in Danger.