The Dunnock (Hedge Sparrow) and Its Odd Mating Behaviour
More Than Just a Brown Bird
It wasn’t long ago that the dunnock, or ‘hedge sparrow,’ was considered to be a rather uninteresting, small, brown bird. But since the 1980s, when it was first studied in detail, its exploits have become a red hot topic of conversation across garden fences and attracted more smutty headlines than those of a bed-hopping soap star. The dunnock, you see, hides a distinctly lively lifestyle behind its veneer of ordinariness and homeliness. It’s all to do with sex, of course. What else could it possibly be?
Things start to subvert right from the earliest days of spring. The females (not the males as expected) start to mark out borders and skirmish over territories in an unusual demonstration of girl power. The winners sit defiantly in their territories and invite the males outside to move in with them. Not surprisingly, the invitees oblige with delirious enthusiasm and fight amongst themselves for this unexpected privilege.
Now sometimes, just sometimes, the latter dispute is a simple one, and a single male dunnock wins the battle to move in with a single female. If so, they share the territory completely and the male takes over its defence. They become a monogamous pair and go about the breeding season without interruption from the outside. Roughly a third of dunnock unions turn out this way.
But more frequently, there is a two-pronged problem. In the average dunnock population, there are fewer females than males, and logically enough, with a reduced density of population, the females have relatively large territories. Unfortunately, the males, with their much higher population density, have more pressure on their borders and cannot normally defend as much area as the females. So, although a certain male might take over the defence of part of a female’s patch, he normally finds himself unable to defend all of it. To solve this problem, he reluctantly allows another male in to keep the baying hordes outside.
It’s hardly an ideal arrangement. Two males are, metaphorically speaking, flat-sharing with the same female. They defend the territory as a team using the same boundaries, but they both view the female through the same primeval, steamed-up lens of intense desire. They become bitter love rivals.
The Dunnock's Odd Mating Behaviour
Alpha and Beta Males
Their arrangement leads to an immense power struggle, resolving eventually into a dominance relationship between the two of them—a kind of personal pecking order, with the slightly stronger and fitter bird coming out on top. (On the rare occasions, male birds match each other with almost total equality, and in these cases, they often fight to the death instead.) The dominant bird—the alpha male—holds a certain sway over his subordinate—the beta male—in regard to access to the female. Having won his rights, he tries to monopolise the female’s company and keeps her as his exclusive sexual partner.
But it rarely works out as the alpha male intends, not the least because of the unexpected motivations of his female. The female seems reluctant to be monopolized and actually goes out of her way to encourage the beta male to go behind his master’s back and, shall we say, meet her behind the bike sheds. She actively seeks sexual relations with both birds, even as both males are firmly in competition against each other. The garden lawn is the floor, then, for a curious dance in which a female dashes back and forth between her suitors, one relationship open and freely acknowledged, the other enigmatic and merely suspected.
Why should she do this? The motivations of the males are clear enough, each bird wishing to promote its paternity by keeping the female’s attention to itself. But why should the female be so solicitous to both admirers? The reason is surprisingly straightforward—in return for copulation, each male effectively signs a contract stating that he will help with feeding the young. In a situation where food might be difficult to find, the help of an extra shopper could be a significant boost to a female’s chicks’ chances of survival.
Enter the Gammas
So, here is a pertinent but explosive question: if two, why not three? Could they? Yes, they could. Some dunnock territories contain three males: an alpha, a beta and a gamma, all competing for a single female’s affections. Chicks from such households are mightily well-fed, and the female could almost go part-time, but for the males, it means that they are condemned to a stressful and perhaps disappointing breeding season.
Sometimes, though, the dance follows a different step. From time to time, ‘alpha-plus’ males arise in a population—birds that can indeed monopolise a female’s territory and not just one, but two. Perhaps these super males result in years when the female population density is greater than usual and hold consequently smaller territories, or perhaps some males are just born to be superior.
Whatever their origin, these birds find the defence of a single female’s territory so easy that they have a go at defending two, and if they succeed, they monopolise both females. That’s a great arrangement for them, but it naturally reduces the help that each female can expect in the task of looking after the young. Although that would appear to give the females a more difficult time, at least they have the compensation of knowing that their young will come from strong, vigorous stock. Forget the help; feel the genes.
But not all potential ‘alpha-plus’ males are quite as superior as they might seem at first. At the beginning, they can exclude all other males from two territories, but over the weeks, their stranglehold begins to loosen, and they are eventually forced to share the defence of the two territories with a second or even third male.
The arrangement is the same as in the flatshare described above, with each of the males involved in a dominance relationship: an alpha, beta and gamma male. The only difference is that there are now two females involved, and things begin to get complicated. Both females attempt to mate with each male, but at the same, the alpha male will try to prevent this from happening and monopolise both females for himself. But he is now acting against the wishes of not only the beta male and the possible gamma male but also both females! So he rarely succeeds. Four or five in a flat allows for plenty of playing around.
Finally, in rare cases, two males might find that joining forces to defend the territories of two females is so easy that they might as well try defending three, or even four adjacent female territories. If this happens, each female can be fairly guaranteed two helpers, and the males can bicker between themselves as to who mates with whom.
What is so remarkable about dunnocks is not that they frequently have multiple partners—lots of birds do that—it’s because all of their sexual liaisons are formalised, with both sexes having a role, rather than being quick flings outside an essentially monogamous relationship. These are not extra-pair copulations; they are copulations with extra mates.
When all of this was discovered, the scientists struggled with the terminology. A combination of one male and one female has long been known as monogamy, and the combination of one sex with two or more members of the opposite sex as polygamy (polygyny when it was one male with multiple mates, and polyandry when it was one female with multiple mates).
The dunnock, though, has forced a new word to find its way into the dictionary: polygynandry, the system in which, for example, a male can be polyandrous with females that are themselves polyandrous. There are not many garden birds, small, brown or otherwise, whose escapades have been so singular as to require the invention of a new word and a new concept. Every February, this dance goes in back gardens across Britain, and there isn’t a thing you can do about it.
The Dunnock Singing
More On The Dunnock And Its Strange Behaviour
- The RSPB: Dunnock
The RSPB'S guide to the bird that is all too often overlooked.
- British Garden Birds: Dunnock
This website offers a general guide to the dunnock and other birds likely to be found in British gardens.
- Dunnock Behaviour and Social Evolution by Nicholas B. Davies
An interesting book that explores the dunnock's strange behaviour in amazing detail.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.