Tree Swallows and how they find a place to nest

This is an immature female Tree Swallow.  The female retains the brown immature plumage throughout her first year and well into her second.  Males and fully mature females have beautiful iridescent blue coloring.  Tree Swallows are related to Purple Martins, but about 2 inches smaller.

Tree Swallows tend to appropriate bluebird nest boxes - even when occupied by nesting bluebirds! Tree Swallows arrive later in the spring than Eastern Bluebirds, and if they can't find a vacant nesting spot, they force the smaller bluebirds out and take over their nests.  Tree Swallows will kick out the whole bluebird family, babies and all!  This is probably a year old female based on the specks of blue appearing on her feathers.

There is no blue to be seen on this female, at least in this light.  We got quite close to this Tree Swallow at Broadmoor Audubon Sanctuary in early May.

This nest box is a bluebird box no longer! It seems to be quite well guarded, actually.

A female on top and a male (immature or adult) peeking out from inside.