Some birds, individuals and species, are monogamous. But divorce does happen - and, in truth, fidelity between partners is rare.
Mute swans famously 'pair for life. This does not preclude the possibility of divorce - which, when it occurs, usually follows nesting failure - nor, if one dies, that the surviving bird will find a new partner.
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A 1960s study estimated that more than 90 per cent of bird species have a monogamous mating system, the remainder being polygamous - either polygynous (one male, several females) or, more rarely, polyandrous (one female, several males).
Mating systems evolve according to each species ecology. Polygyny is a mixed bag: it's seen in lekking species such as black grouse, in which males display in groups and females prefer one or a few males; and others, such as red-winged blackbirds, in which certain males defend the best territories and females prefer to share a male than to be monogamously paired to one with a poor territory.
Polyandry often occurs in species in which the females can produce lots of eggs in a short period - for example, the jacanas (one of the world's weirdest birds).
In the 1980s, the development of molecular methods (such as DNA fingerprinting) that enabled unambiguous assignment of parentage overturned the belief that most birds are monogamous - indeed, many offspring in a brood are not fathered by the male that helps to raise them.
In fact, true sexual monogamy is the exception rather than the rule among apparently monogamous birds. So the mute swan is unusual in living up to its reputation: no extra-pair offspring have been reported.
The mid-1970s revolution in behavioural ecology ('selfish gene thinking) made sense of extra- pair copulations - for males. Being paired to one female and fathering her offspring, but also engaging in matings with females paired to other males, maximises a male's genetic representation in the next generation.
Of course, for such a copulation to occur the 'other female usually has to be complicit. But why? How do females benefit from mating with males other than their true (social) partner? Suggested explanations include the idea that such extra-pair matings represent insurance for a female in case her partner is infertile - but infertility in birds is so rare that this cannot be the answer.
Another theory is that females seek genetic quality for their offspring by having a 'fling. To breed, a female needs a mate - and in most, 'monogamous' species this means a long-term bond. But not everyone can pair up with Mr Fantastic - some have to make do with second best for their' husband'. So it might make sense for a female who didn't obtain the 'best' partner to have an extra-pair mating with a higher-quality male so that their offspring have his 'fantastic genes.
A straightforward idea - but what evidence would you need to show that that's what happens?
We glibly talk about high-quality partners, but how does quality manifest itself? An obvious way of measuring it would be to count grandchildren: a high-quality individual leaves many descendants. A handful of studies have now provided enough information to test this but, frustratingly, there is precious little evidence that extra-pair offspring or even their offspring are any higher quality than any other bird in the population.
Indeed, there is no consensus about why females engage in extra-pair copulations. Some behavioural ecologists, including me, are wondering whether, from a female's perspective, it makes little difference who the dad is - and that, rather than seeking extra-pair copulations, females merely acquiesce to save time and hassle.
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