I regularly hear that birds like eagles, hawks, vultures, swans, and puffins mate for life. There are documented cases of birds mating and maintaining a long -term pair bond that certainly fit the mate for life pattern. At the Wildfowl Trust in Slimbridge, England, swans studied for more than 50 years never showed a single case of divorce among the thousands of Bewick’s swan pairs that have successfully raised young although this winter a pair returned with new mates, raising questions about the typically monogamous birds.
There are good reasons why mating for life is advantageous. Many larger birds only produce one brood of chicks a year and they tend to take longer to incubate and fledge. Finding a mate requires a lot of time and energy. Parents who stay together are ready to breed earlier in the season so they have plenty of time to raise their young. The more broods the pair raises together, the more experience they gain and the better they get at raising them, so the chicks are more likely to survive. Large migratory birds, such as geese and swans, seem to prefer to save their strength for their long journey, so it makes sense to stay with one partner.
There are lots of other examples of long-term monogamous pair bonds in which a pair stays together until one dies. But is that characteristic of the species or just the occasional pair? And if a pair stays together but “cheats” by having extramarital sex, is that “mating for life?” There is considerable evidence (DNA) to indicate that the majority of bird species cheat; sometimes 75 percent of a clutch of eggs are fertilized by a male other than the female’s mate. But the female cheats, too, in fact, often more so than the male.
Cheating provides two advantages: greater genetic diversity in the chicks, and thus more resistance to disease, and yet the male will remain to help raise the brood, unaware of his mixed gene offspring. Variation among offspring is one of the tenets of the evolutionary process. The more variation there is the more likely one or more of the offspring are more fit than the others, so it appears that cheating by both the male and female of the species is advantageous, at least in some cases.
There is no question that some bird species, such as albatrosses, cranes, eagles, and owls form long term pair bonds and effectively mate for life. But so do humans, as least in most modern societies, intend to mate for life with some sort of moral or legal commitment, but don’t always follow through -almost 50 percent of first marriages end in divorce. Among birds, one study discovered a 3.7 percent divorce rate among Black-browed Albatrosses, known for their mate fidelity, increasing in warmer years due to a lower food supply and difficulty in raising a chick.
Mallard ducks split up only nine percent of the time while it’s 85 percent for Emperor Penguins. An eight-year study of Eurasian Blue Tits found that 64 percent of breeding pairs called it quits over that period. So, mate for life? Well, sort of.