Good genes
How birds choose the best partners to guarantee breeding success.
SPRING is here and resident birds are getting prepared to breed. Most will already be paired up and starting to build nests, or ready to declare and defend territories. Choosing a partner is probably one of the most important actions that a bird will take, and keeping that partner while breeding successfully is essential to a bird fulfilling its existence.
A large proportion of our birds are monogamous and for most of them it is the female who chooses her mate. This will usually be the male who is the best singer or the biggest and brightest coloured, as both songs and plumage improve with age, so reflect experience, good health and a better chance of breeding success. In other words, ‘good genes’.
Studies of Great Tits have shown that while a male may have a good territory, it is his appearance which is the clinching factor when choosing a mate. A male with a wide black breast stripe is most desirable as he will be more likely to defend the family well and result in heavy young, which have the best chance of survival. In short, he will be the best at parenting. Other studies demonstrate that the brightness of the male’s yellow and green colours – a result of the amount of carotenoids in the feathers – is linked to food provision. The brightest birds feed their offspring earlier in the morning and choose the best foods to produce the fittest young.
But appearance isn’t everything. Researchers at Oxford University have recently found that the individual personalities of male Great Tits influence how they bond with their future breeding partners. Bolder, more proactive males find their future partners sooner and also devote more time to her before the start of the breeding season. A male who is ‘shy’ and puts less effort into forming a strong pair bond will spend more of his time with other females and so ‘play the field’ perhaps less successfully.
Personality also plays a part in the territory a male Great Tit may set up. As successful male birds will strongly defend their territories and compete with other males nearby, the ‘shyer’ birds tend not to set up home next to a boldly aggressive neighbour, meaning that birds will associate with others like themselves.
Having a group of strongly defensive adults will mean better all-round protection against intruders.
But there is a drawback, as female Great Tits are known to ‘eavesdrop’ on interactions with neighbours. If a female likes the sound of a neighbouring rival, even if she has a good partner, she may well have an extra-pair ‘fling’ with the neighbour.
One final thing a female bird looks for in a mate is genetic difference. Pairing with a close relative leads to inbreeding and produces less fit offspring. But how can a bird tell that a potential partner is not related? Appearance may play a part and another likely way is smell. Studies into birds’ preen oil show that females are attracted to male preen oil, and especially to oil from a bird which is not related; males react to female preen oil similarly. Chris Harbard