The Superb Fairy-Wren. Its name makes it sound delicate, yet majestic, and – well – sweet. But, names can be misleading… Beneath this rather lovely title lies the most promiscuous bird of all, with 95% of broods and 76% of young linked to extra-pair paternity (yep, that basically means that the offspring are the result of some avian ‘cheating’)1. Add to this the fact that Superb Fairy-Wrens are cooperative breeders (yes, that’s right – the parents have a whole group to help them raise their young), and this infidelity seems downright unfair. But come on, let’s not anthropomorphise these birds, they’re not to be judged. This is just one example of the extraordinary array of mating systems that exist in the animal kingdom, and the theory behind this promiscuous behaviour is pretty interesting.
Let’s set the scene: It’s pre-dawn. We’re in South Eastern Australia, and it’s September – the start of the peak breeding season2. A dominant female sets off for a neighbouring territory. (As an aside – why go before dawn? So she isn’t spotted leaving? One reason suggested by Double and Cockburn is so she isn’t followed…1) Here she copulates with an extra-group male, before returning to her own nest where she copulates with her within-group mate1,3. There’s plenty of topics I could talk about here – sperm competition and cryptic female choice being obvious ones. But let’s focus on why the females behave in this way, and which extra-group males particularly take their fancy.
Those of us who have studied sexual selection before know that females are choosy. But what benefits can they gain from mating with a particular male? Firstly, they could receive a direct benefit, such as the chance to forage in a new area1. Secondly, they could benefit indirectly, by producing “good quality” offspring with “good genes”, which go on to show high levels of reproductive success themselves5,6. Of course, you might think, ‘what if there’s already an attractive male in her own group?’ Well, the introduction of genetic diversity into her brood, and incest avoidance, are two more good reasons for her pre-dawn forays1.
On their early morning missions, female Superb Fairy-Wrens are pretty picky. In fact, Dunn and Cockburn found that the chosen males can be predicted based on how early they display their breeding plumage4. Birds which moult sooner, thus showing off their plumage for longer, are preferentially chosen4. This suggests that the indirect benefits of extra-group copulation are key – being able to sustain their breeding plumage for a long time is an ‘honest signal’ of male fitness and the “good genes” that will be passed on to their offspring4. Furthermore, not only have females been shown to revisit the same territory multiple times, but they can even choose the same partner1. One female stayed true to her extra-group male over the course of seven years, despite her moving to a new territory twice1. It seems there’s some loyalty after all!
So there you are, the Superb Fairy-Wren – promiscuous, but all in the name of biology.
- Double, M. & Cockburn, A. (2000). Pre-dawn infidelity: females control extra-pair mating superb fairy-wrens. Proceedings of the Royal Society of London B: Biological Sciences 267(1442) pp. 465-470.
- Kleindorfer et al. (2014). The cost of teaching embryos in superb fairy wrens. Behavioral Ecology 25(5) pp. 1131-1135.
- Calhim, S., Double, M.C., Margraf, N., Birkhead, T.R., & Cockburn, A. (2011). Maintenance of sperm variation in a highly promiscuous wild bird. PLoS One 6(12) e28809-e28809.
- Dunn, P.O., Cockburn, A. (1999). Extrapair mate choice and honest signalling in cooperatively breeding superb fairy-wrens. Evolution 53(3) pp. 938-946.
- Hamilton, W.D. & Zuk, M. (1982). Heritable true fitness and bright birds: a role for parasites? Science 218(4570) pp. 384-387.
- Fisher, R.A. (1930). The Genetical Theory of Natural Selection. Oxford: At the Clarendon Press.