One of my favorite sights of spring is seeing bird pairs around my yard. Throughout April the cute American Robin pair would take evening walks across the lawn looking for food. The Northern Cardinal pair frequents the feeders together and I even saw the male feed the female some seeds. It’s a common sight to see pair-bonded birds in the spring because many bird species are considered monogamous. In his 1968 book Ecological Adaptations for Breeding in Birds, British ornithologist David Lambert Lack estimated more than 90% of bird species are monogamous.
Merriam-Webster defines monogamy in zoology as “the condition or practice of having a single mate during a period of time”. In 2002, a study by Griffith, Owens, and Thurman found that “extra-pair offspring” occurs in 90% of bird species. If 90% of species are monogamous, how can 90% also have offspring sired by another male who is not in the pair-bond?
Well, it turns out monogamy in birds is a little more complex than a Merriam-Webster dictionary definition. From an evolutionary standpoint, the idea of monogamy occurring in animals at all is a bit of a strange idea. If an individual has the opportunity to mate with multiple partners and doing so can increase their fitness (aka: their reproductive success), then why would they choose monogamy? Biologists are learning that when it comes to animals, three types of monogamy come into play: social, sexual, and genetic. Lack’s assertion that 90% of species are monogamous refers to social monogamy. This means that bird pairs work together to raise offspring, but there is no guarantee that they are sexually or genetically monogamous. In fact, the Griffith et al. (2002) study contributes to the idea that genetic monogamy (that the two partners only have offspring with one another) is quite rare in birds. True genetic monogamy does occur in some species, such as Common Loons, Dovekies, Florida Scrub-Jays, and some populations of Red Crossbills. However, at least some females in about 90% of bird species will mate with a male who is outside of her pair bond, a behavior called extra-pair copulation.
Why would a female seek out a mate outside of her social bond? First, her social mate may have genes that are inferior, so mating with a more desirable male can allow a female to pass on better genes to her offspring. Secondly, a female is more likely to get her eggs, which are a limited resource, fertilized just in case her social mate is infertile or incompatible. Lastly, extra-pair copulation allows a female to have genetically diverse offspring by multiple fathers. It’s the “don’t put all your eggs in one basket” idea.
So yes, a vast majority of bird species are monogamous, but it’s not as clear cut as it seems. Many of our cute little bird pairs have something going on “on the side”. Many birds will only stay together for once breeding season and find a new mate the next. There are some species that do mate for life, but that’s not as common as we once thought.
Infidelity, divorce, and even same-sex pair bonds (although rare) occur in birds. And what about the other 10% of birds that don’t practice monogamy? What mating system do they use? Over the next few weeks we’ll explore some of these interesting avian behaviors on Mating Monday. Stay tuned!
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