Birds have been expanding a nest in my barn for 15 years; I wonder if they're the same birds?

For at least 15 years, phoebes, a drab songbird common in the Northeast, have been living on a beam of the first floor of our barn. Their nest, made of mud lined with grass, sits empty all winter, and in the spring they build a new layer atop it, then lay four or five eggs.The babies are currently overflowing the nest; one of these days I'll go into the barn and it will be empty.

This process means the nest grows slowly every year. About 5 or 6 years ago, the nest got so close to the ceiling that they started a new nest, attached to one side. It looked like an in-law apartment. This year, the new nest has gotten as tall as the original one; I wonder if we'll have another extension next year? A triplex, perhaps.

The question is I have, though, is whether it's the same birds, or rather (since they only live a few years) the same extended family, returning each year? Or maybe it's just random birds; the first one who finds the nest each spring moves in? Do phoebes return to the nest where they're born, like salmon? And if so, which of the four or five babies gets to move in?

A bit of Google-ing finds no answer ... maybe I'll have to bug NH Audubon.