Older Tree Swallow nestlings flutter their wings in the nest cavity.

Tree Swallow nestlings begin to exercise their wings beginning about nestling day 15, and will continue trying to flutter in the nest cavity for the next 4-7 days before the fledge. The more room in their nest cavity, they better able they are to flutter, and those that can are probably better prepared for survival after fledging.
Video from Francois Paquette of QC. Edited by Chris Gates of NY.
For more on behavior of older tree swallow nestlings see:
treeswallowproj...

[@ureasmith3049]

Good video thanks for sharing. I've been building roomier birdhouses. Have had bad experience with Swallows using my Peterson bluebird house.

[@M00Nature]

Thank you for the tip about box size; I now understand why tree swallows need larger boxes. I've been using smaller boxes (standard bluebird boxes) and these have been working. However, since I have never observed a fledge as it occurred, I really don't know how the young fared after they left the box. (I only have two years of experience with tree swallows.) My boxes are in northwest Connecticut and most of the time, the tree swallows are fledging at Day 18. Maybe that is because they are overcrowded in the boxes?

[@jameslomenzo1139]

Tree Swallows like to use my 9 inch PVC gourds, which makes sense. I've even seen them choose a 4 1/2× 4 inch bluebird house only to leave it, to move to the gourd. I'm trying to use the 5×5 inch wooden houses, that is what they really need plus the 9 inch gourd works great.