The young are fed soon after they hatch, and both parents share this duty equally. In situations where one adult has disappeared, a single parent can raise the young alone. Nevertheless, retarded nestling growth, brood reduction, and starvation are typical outcomes. Fecal sacs are removed from the nest until the young are 14 days old. Nestlings fledge after 16 to 30 days, and many are virtually independent immediately upon their departure.
Tree Swallows have one brood per breeding season.
Pairs will nest again if the first attempt fails, but true second broods are extremely rare. The same nest sites are used for more than one breeding season, and pairs will often nest on top of old nests. In fall and winter, Tree Swallows form migratory flocks that can contain anywhere from a few thousand to a million individuals. They head to the southern coast of the United States, Mexico, and Central America. In early spring, a small percentage of the young return to their birthplaces.