Today my husband got the last of the nest boxes down from the trees and posts where he put them up in April. This box was loaded with twigs by a House Wren, then possibly appropriated by a swallow. The wrens typically load the box deep with fir twigs, and sometimes quit before finishing the job. They leave a dip towards the back, where the eggs lie.
This one has feathers added in a cup form, which is what swallows use as a final layer on top of the dry plant material they pack in. HOWR will use also some feathers, but not always, so it’s uncertain what the story is here. But however it was built, it wasn’t used, which is why the feathers are curled and fresh-looking. After a family of swallows have been shifting around on them, the feathers aren’t fluffy any longer.
One similarity between House Wrens and Tree Swallows- they fledge and scram! The little wrens all hop out on the same day, moving through the grass with their parents,and the swallows depart one at a time, with no adult supervision.