The types of foods that migratory birds requires vary according to the seasonal energy demands of migration, raising young, and molting. The birds begin to arrive in North America just as the first generation of spring caterpillars begins to nibble on the newly unfurled tree leaves. These tiny larvae, full of protein and water, become a major food source for birds during their stay at the breeding grounds. A pair of warbler parents can remove caterpillars from thousands of leaves in the ten days it takes to fledge a nestful of young. In fact, a recent study documented that if the birds were excluded from an oak sapling with a wire cage, the tree lost about eleven percent more leaf area to caterpillars than if birds were allowed to forage there.
Berries are an important source of carbohydrates and fats in the diet of migratory birds.
In the northeastern United States alone, nearly one hundred native plant species produce these small fruits. One group, the wild grapes, provides weeks of nourishment for more than fifty species of birds. Migrants rely on berries most heavily on the late summer and early fall. Some berries stay on the bush all winter, providing emergency rations for returning migrants in the spring and cold winter days. Providing the variety of food sources that birds require is the most fun and challenging aspect of landscaping for migrants.