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IT is said that the tradition of Saint Valentine’s Day began in medieval times, probably in France, and was regarded as the date on which birds start to choose their mates. From this came the idea of choosing a sweetheart. Doves feature strongly in this idea and many love potions of the time contained a dove’s heart. Some birds may indeed begin to pair up around this time, and many of those may be doing it for the first time. Pairing up is a serious business as breeding success will depend on finding a suitable mate.
There is also an urban legend that most birds mate for life and while it is true that most birds are monogamous, this is not the same thing.
Also, while monogamy may mean pairing up with one mate during the breeding season, it does not imply fidelity, and lots of birds will indulge in extra-pair mating. What most birds practice is social monogamy, rather than sexual monogamy, and if those medieval observers had looked more closely, they may have found that male pigeons will canoodle with several females. About 90% of birds are thought to be socially monogamous, which differs from mammals. Of around 6,500 mammal species, less than 5% are socially monogamous. These include nearly a third of primates. While long-term monogamy is very rare, it can be found in bats, wolves, beavers, foxes, otters and humans.
One alternative to monogamy is polygamy, where one member of a pair will have more than one partner. For a male this is called polygyny and about 2% of the world’s birds practice this. Great Reed Warbler is one of the few bird species that is polygynous and males have a harem of females. For females it is called polyandry; fewer than 1% of birds do this, mainly rails and shorebirds. New research looked at the genomes of
150 bird species, which were analysed for genetic diversity. While polygamous species were hardly more diverse than monogamous ones (the few polygamous females excepted), surprisingly polygamous species had fewer damaging mutations.
There are some bird species in which males or females may have multiple partners, and so no pair bond is formed.
Male hummingbirds will find one female and mate, then immediately go off and find another female, and in cases like this the female brings up the young alone. In North America, some 97% of female Saltmarsh Sparrows studied had eggs by mating with more than one male. Promiscuity, however, can have its downsides, especially when practiced by a socially monogamous species. A study of 232 bird species found that two factors resulted in a higher ‘divorce’ rate – promiscuity and migration distance. Promiscuous birds have less commitment, which makes them less attractive and so less likely to keep a mate the following year, while longdistance migration may result in pairs not being present together at the start of the breeding season, leading to a new mate being found.