There is nothing quite like the sex life of the Dunnock.
This small bird, sometimes known as the Hedge Sparrow, at first glance appears quiet, hushed and their unremarkable feathers leave much to be desired. Yet their sexual arrangements range from monogamy all the way to groups whose characteristics resemble that of some sort of 1970s free-love hippie commune with quick-fire orgies lasting hours.
Males may have harems of females each of which he has his own way with. The reverse may also be true, a single female may have access to multiple males all of which operate as house husbands and help to raise the female’s brood. What is most remarkable is that until it became the subject of genetic fingerprinting and paternity analysis, Dunnocks were regarded as the exemplar species of married fidelity. This incredible diversity of sexual behaviour marks out these birds as far more interesting than they appear to be.
What mating systems arises works on a case-by-case basis and ultimately is dependent on ecological conditions. This being quality and accessibility of food, risk of predation and amount of competition, all which inform the dispersal of the female whist the habitat quality informing her home range.
Where the girls go the guys follow and males disperse accordingly to try and profit the most out of the situation. What remains are three overlapping systems:
1. The Guy gets the Girls
When food is abundant and spread out in patches, females will inhabit smaller ranges. This dispersion allows a single male to monopolise multiple females. This system lends itself to a high level of success to males as he is permitted to sire multiple broods and spread his parental care throughout them.
This system is known as polygyny and, whilst rarer in birds, is the most common mating system that animals adopt. The underlying mechanism for this is that males have higher reproductive potential than females and so are not limited by gestation periods and sometimes parental care and will therefore to try produce as lots of offspring with lots of females. This highly attractive proposition for the males gives rise to a large amount of competition for the access to multiple males and therefore males will proactively guard females after mating with them a and ward off other eligible bachelors.
2. The Girl get the Guys
When a female Dunnock finds herself in a habitat that is perhaps of poor quality or where resources are spread over a large area she will occupy a large territory and will have access to more than one male. This number rarely ever gets higher than two, with each male having a slightly a different role in the arrangement.
Females will solicit matings with multiple males whenever she can, in order to control the quality and diversity of her offspring.
The alpha male will have access to the majority of the matings but will be required to invest more in parental care than the beta male who will have to be slightly more stealthy in his quest to sire offspring. The male sacrifices paternity but reduces his parental investment and any energy expenditure that comes with it.
Polyandry, as this system is known, is very rare and only arises when there is an entire sex role reversal or harsh habitats where young can only reach sexual maturity with the help of multiple males. An appropriate example among humans occurs in mountain communities in Tibet where brothers may marry one woman together to raise a family which requires both men to provide in order to produce any children at all. Like the Dunnocks it seems even we are able to stomach the loss of exclusivity if it means we can pass on our genes.
3. The “Christian” Model
When the attributes of a Dunnock’s habitat dictates that females and males share similar sized territories males and females will form a pair bond and operate monogamously. Both parties will share the parental investment and therefore share in the reproductive success of their offspring. Whilst not incorrect in describing three distinct systems that the Dunnock chooses depending on conditions, the reality is that the boundaries are often blurred. Dunnocks are one of the few species have a polygynandrous mating system where numerous males and females will all mate.
The Messy Reality
All systems come under intense completion both between and within the sexes. Females will solicit matings with multiple males whenever she can, in order to control the quality and diversity of her offspring.
Males, as you can imagine will not tolerate any female they mate with copulating with other suitors and will attempt to mate with their sexual partners multiple times every hour. Each time a mating takes place, males will first peck at the female’s cloaca (the avian reproductive tract) in order to entice her to eject a jet of sperm of any previous matings with other males, thus ensuring that any chicks hatched are his and will then happily mate with the female. The mating lasts less than a second (hence its name the ‘cloacal kiss’) and the male is content that any offspring reared will be his. However the females have the last say in the matter as they are capable of accessing a store of non-viable sperm to display, fooling the male and ensuring she retains the genetic information of previous males and will hatch a brood with mixed paternity.
This extraordinary behaviour permeates all Dunnocks and this process will happen over and over in the competition to ensure the survival of their genes to the next generation.
What we see here is how natural selection is not necessarily survival of the fittest but the amount of offspring that one produces. This diversity of sexual behaviour is brought about by sexual selection, a component of natural selection in which fitness is sacrificed in order to gain access to more mates. In the same way that the bright colouration of other birds makes them more visible to predators whilst allowing them to sire more offspring, the sexual displays and rituals of the Dunnocks take time and can be highly visible to predators but evolution deems all of this investment worthwhile should it provide the opportunity produce more offspring. Ultimately, this ability to adapt a mating condition to a surrounding habitat has allowed these little birds to populate the whole of Europe including right across the UK, throughout Scandinavia and even into North Africa.