The tree swallow is susceptible to a wide range of predators. Eggs, nestlings, and adults in the nest fall victim to black rat snakes, American crows, American kestrels, common grackles, northern flickers, chipmunks, deermice, domestic cats, weasels,Winkler, D. W.; Hallinger, K. K.; Ardia, D. R.; Robertson, R. J.; Stutchbury, B. J.; Chohen, R. R. (2011). Poole, A. F., ed. "Tree Swallow (Tachycineta bicolor)". The Birds of North America. Ithaca, New York: Cornell Lab of Ornithology. American black bears, and raccoons.Chapman, L. B. (1955). "Studies of a tree swallow colony". Bird-Banding. 6 (2): 45–70. While flying or perched, predators to the tree swallow include American kestrels, black-billed magpies, barred owls,Errington, P. L. (1932). "Food habits of southern Wisconsin raptors. Part I. Owls". The Condor. 34 (4): 176–186. great horned owls, merlins, peregrine falcons, and sharp-shinned hawks. Whilst evasive flight is the usual response to predators in free-flying swallows, mobbing behavior is common around the nest, and is directed not just towards predators, but also towards nest site competitors, who might be scared off by it. This behaviour involves the swallow swarming and diving towards (but not actually striking) the intruder from around above the ground, usually giving soft ticking calls near the end and coming within about of the predator. It seems to alter the intensity of its attacks based on which predator approaches; a 1992 study found that ferrets elicited a more vigorous defence than black rat snakes, and a 2019 thesis similarly discovered that black rat snake models were dived at the least and eastern chipmunk models the most. It is suggested that the snake prompted a weaker response because defence behaviours may be less effective and more dangerous to perform against it.
