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Published on Jan 13, 2015
The device on the front of this nest box is a Hironbec Pendulum, which allows light-weight tree swallows to enter and exit nest boxes but prevents heavier house sparrows entry by tipping to one side, blocking the box entrance. The two parent tree swallows have a brood of 5 half-grown nestlings inside, and are kept busy bring them food and removing fecal sacs.