“The three parents share incubation responsibilities for the eggs — three this year — as they have in previous years,” according to Out There With the Birds, the blog of Bird Watcher’s Digest. “Like their relationship, their history is complicated.”
So… two eagles, Valor I (male) and Hope (female) raised some chicks in a nest. Then Valor II (another male) came along and tried to displace Valor I, but he wouldn’t go, so the pair of them both ultimately cooperated in raising Hope’s chicks, even after Hope was driven away by some other eagles. Later, another female, Starr, turned up and Valor I and Valor II are collectively incubating three eggs of hers in the nest.
I’ve known (human) polyamorous networks with origin stories less-complicated than this.