“Birds are classically among the most monogamous of all organisms,” wrote Frank B. Gill in his textbook Ornithology. 90% of bird species form a pair bond in which they commit to work together to raise their young. Mammals are famously poor at this. Only 5% of mammalian species form pair bonds. Humans are among the few.(*)
For many years, ornithologists thought that birds were both sexually and socially monogamous but DNA studies have shown there is not always a sexual commitment. Extramarital copulations occur but they don’t dissolve the social bond. For instance, this happens among chickadees and …
Between one in 10 and one in three eggs in a female cardinal‘s nest has genes that don’t match her partner, and less commonly, they don’t even match her own. But because of that pair bond to rear the young, they are considered socially monogamous.— Bird Watcher’s Digest: Do Birds Mate For Life?
Even social monogamy comes in a range of time spans depending on the species. Avian pair bonds may last for one nesting, for one season, or for a lifetime.
Those who mate for life include Bewick’s swans (above), wandering albatrosses (pictured below), blue jays and barn owls (videos).
Other lifelong pairings include Canada geese, bald eagles, peregrine falcons, red-tailed hawks, ravens, scarlet macaws, sandhill cranes and many more.
But as I said, social monogamy among bird species is a spectrum from lifelong to short commitments. Humans are like this, too.
In the U.S. the human divorce rate is 40%. There’s a bird with the same track record. The masked booby.
(*) p.s. Here’s why humans started forming pair bonds: Humans evolved monogamous relationships to stop men killing rivals babies