I took Cornell's Courtship & Rivalry in Birds course this winter. A young male sneaking into an established pair and adding himself into the gene pool is quite common in smaller birds. Before so many pefas were banded, who knows how often this might have happened.
I've been reading a book called The Nesting Season (subtitle: Cuckoos, Cuckolds, and the Invention of Monogamy) by Bernd Heinrich that discusses this and many other facets of bird reproductive habits. He's an academic, but I find the writing accessible to the lay-armchair-naturalist.
Thanks...adding it to the list...