Most of the year, the hermit thrush, a North American songbird, eats a diet consisting mainly of insects, but in autumn, as the thrushes migrate to their Central and South American wintering grounds, they feed almost exclusively on wild berries  Wild berries, however, are not as rich in calories as insects, yet thrushes need to consume plenty of calories in order to complete their migration
One possible explanation is that berries contain other nutrients that thrushes need for migration and that insects lack
Along the hermit thrushes' migration routes, insects are abundant throughout the migration season.