Most swallow species that nest in North America eat almost nothing except flying insects. When the bugs die off in the fall, the swallows head south to winter in the tropical zones of Central and South America and the Caribbean. However, Tree Swallows can also eat small fruits. If Tree Swallows arrive in the north before the insects are out, they’ll supplement their diets with fruit, giving them a competitive advantage for limited nesting sites.

More info and transcript at BirdNote.org. 

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