Never forget that animals familiar to you – the sort you see and hear every day, or every other day – may be exotic and exciting creatures to various of your fellow humans. And it’s for this reason that I’ve sometimes chosen to write about familiar, commonplace species I see every day, since I know that other people won’t be familiar with the animals concerned, nor even (in cases) be aware of their existence. Today I want to discuss a passerine bird I’ve long planned to write about: a cryptic, mostly brown species known generally and most commonly as the Dunnock Prunella modularis but increasingly as the Hedge accentor.
Actually, the ‘old’ name for this species here in the UK is ‘Hedge sparrow’. This name has mostly had its day. It’s naïve and quaint as well as wrong – we’ve mostly given up on the idea that ‘sparrow’ means ‘generic small brown bird’ – and it’s dying out because you look far smarter and more knowledgeable about birds if you know what an accentor is. Accentors are unique to northern Africa and Eurasia (excepting their introduction to New Zealand); all extant 13 species are included within the genus Prunella, though an argument has sometimes been made that Laiscopus should be recognised too (for the large, mountain-dwelling Alpine accentor P. collaris and Altai accentor P. himalayana). They’re mostly birds of mountainous places and temperate woodland, the Dunnock also occurring in suburban gardens and parks. Where do accentors belong within the passerine radiation? They’re part of Passeroidea and – ironically – very close to sparrows proper, but are outside the big passeroid clade that includes finches and New World nine-primaried oscines, termed Emberizoidea (Selvatti et al. 2015). Yes, they have a fossil record, but it only extends back to the Pliocene…. so far.
Dunnocks are mostly insectivorous but also eat worms and seeds, and mostly forage at ground level among leaf litter. Like so many birds that occur in western Europe, the Dunnock also occurs in part of northern Africa and in such parts of western Asia as the Caucasus and Iran. Some populations – those of the UK and elsewhere in western Europe, among others – are essentially sedentary while those of Scandinavia and western Russia migrate to the Mediterranean fringes and Asia Minor during the winter. Several subspecies have been named. These differ mostly in how dark they are, the form of Ireland, western Scotland and the adjacent islands (P. m. hebridium) being darkest, that of England and eastern Scotland (P. m. occidentalis) being palest.
Flexible sexual systems. These days one of the things that most people interested in birds know about the Dunnock is that it’s notoriously flexible in breeding strategy. Some populations are monogamous (one male defends a territory inhabited by a single female), others are polygynous (where one male territory overlaps that of a few females, all of which mate with him and are defended by him from other males), and yet others are polygynandrous (where two males work together to defend the same territory, that territory containing several females, all of whom mate with the two males).
Females are often polyandrous and mate with the several males who share the same territory (these males have a dominance hierarchy of their own, but since they all mate with the same female even the ‘top’ male doesn’t necessarily father the greatest number of offspring). Seemingly because males know (or suspect) that the female in question has been mating with other males, females engage in a striking precopulatory display where she droops her wings, raises and vibrates her tail, and exposes her cloaca… which the male pecks, causing her to eject the contents (Davies 1983). The male will then guard the female to (in theory) ensure that she doesn’t mate with another male again.
Despite the familiarity of the Dunnock as a European garden bird, this weird and memorable behaviour wasn’t documented until 1933 in the book Evolution of Habit in Birds (this reporting an observation actually made in 1902), and even then by someone considered an outsider to technical ornithological research, namely Edmund Selous (Birkhead et al. 2014). The realisation that the precopulatory display and cloacal pecking was linked to sperm competition (Davies 1983), that extra-pair copulations were commonplace in ‘monogamous’ species, and that scientists might be able to test parentage of the resulting chicks via DNA analysis (Burke et al. 1989) didn’t arrive until the 1980s, and the Dunnock studies concerned occurred at about the same time as similar studies were documenting post-copulatory sexual selection and extra-pair copulations in birds and other animals.
Some of you might remember seeing cloacal pecking in Dunnock featuring on TV for the first time in the 1998 BBC series The Life of Birds.
Female-female competition. In polygynous Dunnock populations, females compete for male attention and vie for territory with other females, at least some (and not the majority) of these competing females using complex songs to help attract ‘their’ male when he’s spending time with other females (Langmore & Davies 1997). They might sing as many as 60 times over the space of two days, and bouts of intense female-female competition can cause the male to move “to and fro in response to their trills, sometimes as often as every 10 or 20 seconds” (Langmore & Davies 1997, p. 887). In male passerines, elevated testosterone levels are linked to an increase in singing more. Could the same thing operate in females? Langmore et al. (2002) found that aggression among competing polygynous and polygynandrous females caused a rise in their testosterone levels, with this rise being linked to female calling and singing.
Use of complex, competitive singing by females is not unique to the Dunnock but was first documented in another accentor, the habitually polygynandrous Alpine accentor (Langmore et al. 1996). It’s increasingly well known that female-female competition is present and even important in animals (it’s key to the work I and colleagues have published on mutual sexual selection), but the case studies where it’s well documented aren’t all that familiar among biologists at large. Accentors, it turns out, are among the best of case studies.
Having mentioned variation in female vocalisations, it’s worth noting that male Dunnocks are variable too, their singing changing (‘switching’, to use ornithological parlance) to an increased rate when they’re searching for fertile females. Rapid song switching appears to be liked by females, who are more likely to solicit matings when they hear a male produce multiple song types (Langmore 1997).
So many copulations. Perhaps unsurprisingly in view of all this, Dunnocks are sexually active little animals with a high reproductive output, by which I mean that they can mate over 100 times in a day, each copulation taking less than a second. A thousand copulation events might have occurred over the span of time in which a single egg clutch was produced, the high number of solicitations by females seemingly being more to do with securing male interest in provisioning the clutch than in winning successful fertilisation (Davies et al. 1996). In polygynandrous populations, it therefore makes sense – as a male – to turn down at least some female solicitations, and to help less at the nest than males do in monogamous and other populations.
The possibilities open to these birds are diverse, and all have different knock-on effects as goes which sex has the ‘upper hand’ and what these strategies could mean in evolutionary terms. I haven’t covered half of the complexity here anyway – you could literally write a whole book on this stuff, and in fact Nick Davies did exactly this, back in 1992 (Davies 1992).
That’s where we’ll end for now. This is yet another of those TetZoo articles that’s been planned and in a partially written state for years. Big thanks to Matt Wedel for helping to collect the relevant literature – something he did back in 2006! Yes, a lot of slow-burn stuff here at TetZoo.
If you enjoyed this article and would like to see me do more, please consider supporting this blog (for as little as $1 per month) at patreon. The more support I receive, the more financially viable this project becomes and the more time and effort I can spend on it. Thank you :)
For previous TetZoo articles on passerines, see…
For The Love of Crows, October 2015
Thoughts on the Passerine Tree, 2016, October 2016
A Battle Among Blue Tits, February 2018
Birdwatching in Suburban China, May 2019
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Burke, T., Davies, N. B., Bruford, M. W. & Hatchwell, B. J. 1989. Parental care and mating behaviour of polyandrous dunnocks Prunella modularis related to paternity by DNA fingerprinting. Nature 338, 249-251.
Davies, N. B. 1983. Polyandry, cloaca-pecking and sperm competition in dunnocks. Nature 302, 334-336.
Davies, N. B., Hatchwell, B. J. & Langmore, N. E. 1996. Female control of copulations to maximize male help: a comparison of polygynandrous alpine accentors, Prunella collaris, and dunnocks, P. modularis. Animal Behaviour 51, 27-47.
Langmore, N. E. 1997. Song switching in monandrous and polyandrous dunnocks, Prunella modularis. Animal Behaviour 53, 757-766.
Langmore, N. E., Cockrem, J. F. & Candy, E. J. 2002. Competition for male reproductive investment elevates testosterone levels in female dunnocks, Prunella modularis. Proceedings of the Royal Society, London Series B 269, 2473-2478.
Langmore, N. E. & Davies, N. B. 1997. Female dunnocks use vocalizations to compete for males. Animal Behaviour 53, 881-890.
Langmore, N. E., Davies, N. B., Hatchwell, B. J. & Hartley, I. R. 1996. Female song attracts males in the alpine accentor Prunella collaris. Proceedings of the Royal Society, London Series B, 263, 141-146.
Selvatti, A. P., Gonzaga, L. P. & Russo, C. A. de M. 2015. A Paleogene origin for crown passerines and the diversification of the Oscines in the New World. Molecular Phylogenetics and Evolution 88, 1-15.