Sex in the shrubbery

The last few days have seen my very own soap opera being acted out in the garden. The heady mix of casual sex, illicit affairs and multiple partners has been performed by a small, and rather unobtrusive bird, known as the Dunnock. Rather drab in appearance, this familiar bird has an extraordinary social system. Both male and female Dunnocks have their own territories and, since those of the males are the larger in size, this may provide a male with access to more than one partner – a behaviour known as polygamy. Such behaviour is seen in a range of bird species.

Things begin to get interesting when two males share a territory. The dominant male (known as the alpha male) is really the territory holder but he is joined by a subordinate male (the beta male), who establishes himself through sheer persistence. The alpha male grudgingly accepts this interloper since it does offer something of advantage; when it comes to defending the territory two males are better than one. The downside is that the beta male also tries to secure matings with any females present within the territory. Interestingly, females deliberately solicit matings from the beta male, a behaviour that should help them when the chicks are born. If a male feels that he has fathered a brood of chicks then he is more likely to help provision them. By mating with multiple males, the female is actively trying to increase the number of males that will help rear her chicks – a decent breeding strategy when living in a habitat where food may not be that abundant.

Mind you, there is a competing factor here, in that each male wants to ensure that he makes the biggest contribution to the next generation. As such, the alpha male spends a lot of his time guarding his mate from other males. As a further safeguard the male indulges in some rather unusual courtship behaviour prior to mating. A male will approach his mate from behind; she will quiver her wings and raise her tail to indicate that she is receptive to his advances. The male will then peak at the female’s cloaca (effectively the external part of her reproductive system). This becomes enlarged and then exhibits strong pumping actions that eject some of the sperm from any previous matings. Satisfied that his own sperm will now make a greater contribution the male will mate with the female.

This complex system results from a series of competing factors; the need to secure help rearing the chicks, the need to defend access to a female and the urge to maximise your contribution to the next generation. Soap operas seem a little tame by comparison!