Why do I sometimes see Dunnocks pecking each others' bottoms?
Dunnocks may look dull but have an amazing social system. While many Dunnocks adopt a monogamous relationship with a would-be partner, others engage in more complicated arrangements. It is quite common for one female to engage in a relationship with two males (something that is given the rather grand title of ‘polyandry’). Somewhat less commonly, a single male may establish bonds with two or more females (known as ‘polygyny’) and there are even cases of groups of two or three males consorting with three or four females (‘polygynandry’)!
A series of pre-copulation displays have evolved alongside this bizarre social system and these displays are geared towards attempt on the part of the male to guarantee his parentage of any offspring resulting from the union. Immediately prior to copulation, a female Dunnock will crouch low in front of her prospective mate, fluff up her body feathers, raise her tail and quiver. The male, positioned behind the female, will then hop from side to side before pecking at the female’s vent (also known as the cloaca). The male’s pecking of the cloaca may continue for several minutes and, over this period, it may become pink and swollen. As it swells, it makes strong pumping movements and these result in the ejection of sperm from previous mating attempts. In this way, a male can increase his chances of fathering any young resulting from the union.