The fascinating love life of the Dunnock

I watched a Dunnock today, feeding under the garden table, with that characteristic half hopping half walking way Dunnocks have, pecking here and there things too small to be seen at a distance, maybe seeds or small invertebrates. Dunnocks, or Hedge Sparrows (Prunella modularis) are little birds, which live their lives mostly unnoticed amongst the undergrowth and are easily overlooked or taken for House Sparrows. They have a grey chest and head and chestnut backs with dark stripes, a thin beak and orange legs. Both males and females look similar, females just a bit smaller than males. In gardens they often feed on the ground, under bird tables when there is some cover, and they prefer to skulk than to sit out in the open. Only in the spring, where males sing their weak, warbling song from a prominent perch they are somewhat more likely to be noticed (above). Contrasting with their modest attire and retiring habits the Dunnock shows a variable mating system - including a common arrangement of two males and a female, a system called polyandry- , and a courtship behaviour that can only be described as peculiar. I was lucky enough to witness courting Dunnocks a few years back. This is a sketch of what I saw and my description.

Female appears paler, less extensive grey markings than the male. Both individuals are on the ground. The male hops behind the female and she stops a moment with her tail slightly cocked and vibrating her wings, that are dropped.  The male pecks her cloaca repeatedly. Female hops away a little, male follows and the same behaviour starts again. Then the intensity increases, the male pecks her cloaca again and the female stops, then the male jumps on the female and there is a flutter

 What could be the purpose of this bizarre cloaca pecking behaviour? Has it got anything to do with the presence of polyandry? Nick Davies, in a classic paper published almost 30 years ago, provided some answers. He followed a Dunnock population in Cambridge Botanical Gardens. Males outnumbered females due to higher female mortality during winter, and therefore, there was intense male-male competition. He observed several combinations of breeding partners per territory, including male-female pairs, two males and a female (what he called trios), and some more rare cases including two males defending jointly the territories of several females.
Monogamous males guarded the female, following her around closely for a few days before she was due to lay and gaining almost exclusive copulations with her. When there are more than one male in a territory the larger male was dominant to the smaller one and fights were common, with the dominant male trying to chase the subordinate away from the female. Although the dominant male got the best share of copulations, the subordinate also got them as he was usually very persistent and mated with the female unnoticed by the dominant male, or when the male lost track of her - often after a fight. In all events of courtship, be monogamous males or not, the female always exposed her cloaca, often a pumping action was noticed and the male pecked it. After cloaca pecking, the female was seen to eject a droplet of fluid. Davies managed to collect three of these droplets and when he examined them under the microscope he found them to contain bundles of sperm. When he watched the female ejecting the drop of sperm, the male copulated with her immediately after. The purpose of the cloaca pecking behaviour appeared clearer: the male stimulates the female to eject stored sperm from the previous mating, allowing the suitor a shot at paternity. This was confirmed by the fact that the more other male spent near his female, the more a male pecked the female and copulated with her, as the male pursued to increase their chances of paternity. Interestingly, the female played an active role in being part of a trio:  she tries to obtain copulations from the subordinate male, escaping the guarding of the dominant, despite his efforts: obviously ejecting sperm from the previous mating will offer both males a share of paternity. When a female was observed to have mated with two males, the brood raised was fed by both males. In contrast, if the subordinate male failed to mate with the female, he did not contribute subsequently to raise the chicks. As nestlings fed by two males have a better chance of survival, it appears that is in the female interest to mate with both males.

A subsequent study using DNA fingerprinting confirmed what behavioural observations had strongly hinted: monogamous males got 100% paternity, and both dominant and subordinate males fathered chicks (surprisingly more or less equally). The observant reader will notice that the mean paternity of dominant and subordinate do not add to 100% in the polyandry system. This was because an outsider gained access to the female and fathered a chick.

(modified from Table 1 from Burke et al 1989).

Furthermore, this study showed that the chances of males helping the female rear the brood were dependent on them having sired some of the brood. It appears that the male is able to judge if he has had sufficient access to female to gain some paternity and to be worth the effort of helping her rear the chicks.

Although cloaca pecking can be seen as the male bird trying to ensure his paternity, the elaborate courtship of the dunnock also reflects that females are active participants and, that, as they need more than a single male to rear her chicks, this unusual courtship is the way females ensures that both males help her raise her chicks and that she achieves more reproductive success.

A pair of Dunnocks in the garden. What would they be up to?

More information
Davies, N. (1983). Polyandry, cloaca-pecking and sperm competition in dunnocks Nature, 302 (5906), 334-336 DOI: 10.1038/302334a0

Burke, T., Davies, N., Bruford, M., & Hatchwell, B. (1989). Parental care and mating behaviour of polyandrous dunnocks Prunella modularis related to paternity by DNA fingerprinting Nature, 338 (6212), 249-251 DOI: 10.1038/338249a0