Bald eagles stay hitched until death do they part, often returning year after year to the same nest. While there, the pair continuously adds to the structure, so that after many seasons it assumes gargantuan proportions and stands as a symbol of their fidelity. In Vermilion, Ohio, one behemoth, used for more than three decades, measured 9 feet across and nearly 12 feet high, and was estimated to weigh more than 2 tons.
Though the birds' courtship rituals are spectacular in their display of aerial acrobatics, it is nest building that cements the bond between male and female. Dad's contribution to his progeny doesn't end at conception: He sticks around to help incubate the eggs and feed the offspring. Bald eagles, which are capable of breeding at about 4 years and have been known to live to 28 in the wild, are not unique in their sexual liaisons. According to Frank Gill, Audubon's senior vice president of science, more than 95 percent of bird species are monogamous, making them among the most loyal members of the animal kingdom. Recent research, however, is yielding evidence that philandering may be more prevalent than previously supposed. "The general rule emerging," says Gill, "is that sexual infidelity is common in socially monogamous species."
Melissa Opar May 2011; https://www.audubon.org/news/do-eagles-remain-faithful-one-mate-their-entire-lives
