Tree Swallows and Myrtle Warblers are the only birds that can digest these berries, and this might help explain why Tree Swallows winter farther north than any other North American swallow. Tree Swallows forage mainly near their nest sites, either alone or in loose flocks. They are solitary nesters, but they may nest in loose aggregations with nests 10 to 15 meters from one another. They frequently compete for nest sites with House Wrens and House Sparrows. Generally, older birds arrive at the breeding grounds before younger birds, and males arrive before females.
Females arrive up to a week later, and pair bonds form shortly thereafter.
Pairs begin to defend their sites immediately. Both sexes defend their territory and their nest. The birds are most territorial before the eggs are laid, and they defend a large area. Although they initially defend more than one nest site on their territory, only one nest site is actually used. They become less territorial and defend a smaller area as the nesting cycle progresses.