Monogamy is surprisingly common among birds. About 90 percent of bird species are considered monogamous -- but, to be fair, a bird's definition of monogamy is surprisingly liberal. Ornithologists have amassed enough data to debunk the romantic notion that most birds pair up faithfully for life.
Types of Monogamy
There are two types of monogamy in birds: social monogamy and sexual monogamy. Socially monogamous birds select a "spouse" to help raise their young, but will mate with other birds. The pairing is frequently short-lived. Most monogamous birds are socially monogamous. Sexually monogamous birds have one mate during the breeding season, and sometimes for life. The pair mates and parents together. Rarely do they mate with other birds.
Forming a pair bond during the breeding season helps ensure the young will thrive. In some species, both parents take turns sitting on the eggs, guarding the nest and territory, and feeding the young. Two parents are better than one.
Social monogamy is much more common than sexual monogamy. Birds will pair up to share the responsibility of raising a clutch of eggs, even though some of the babies will not belong to the parents who raise them. Socially monogamous birds will mate with other birds, and females sometimes lay their eggs in other couples' nests. After raising a nest of babies together, socially monogamous birds frequently "divorce" and pair up with other individuals for the next breeding cycle.
Sexual monogamy is rare. Zebra finches are sexually monogamous for life. They form permanent breeding pairs. Scientists believe it's more beneficial for these birds to "mate for life" because of their short lifespans. Permanent pairing eliminates the time needed to find and court a mate. Other sexually monogamous species include most geese, eagles and swans. These birds form bonded pairs for life, but both males and females may occasionally mate with other birds. Like socially monogamous birds, the bonded pair raise all the offspring in the nest, regardless of whether or not they are the biological parent.
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