Before I leave the topic of bird nests:
Tree swallows (Tachycineta bicolor) build their nests in open areas or wetlands. Their nests are usually constructed in existing tree cavities (hence their common name). However, tree swallows routinely use artificial nest boxes or will occasionally inhabit eaves, other cavities or even holes in banks. The female uses grasses or other vegetative materials for her cup-shaped nest.
This year the outdoor decorations on the deck of the Fall River Mills Golf Course restaurant (Shasta County CA) included a large red bird box. Ignoring the close proximity to diners, tree swallows moved into the decorative bird house. It was a delight to watch the birds build their nest and raise their brood throughout the summer while enjoying a meal.
Tree swallows will remain mated for several years, but appear to be more faithful to a particular nesting site rather than a particular mate. I do hope the red nest box is on the deck next season and that the tree swallows return.