The function of colourful plumage and song in female lovely fairy-wrens

A project undertaken at the Department of Zoology, The University of Melbourne, and supervised by Raoul Mulder

Background

Darwin’s (1871) theory of sexual selection was inspired by his observations of the remarkable sexual dimorphism apparent in nature. Since then, the elaboration of ornamental traits like complex song and conspicuous plumage has been almost universally attributed to sexual selection. However, recent work on bird song has challenged this view with evidence that song is common among female songbirds, and was present in the ancestor of modern songbirds (Odom et al 2014). What selective forces drive the evolution of elaborate female ornaments is a topic of immense current interest and controversy (Clutton-Brock 2007). The traditional view that female ornamentation is a non-adaptive by-product of selection on male ornaments has been challenged by suggestions that female ornamentation could also evolve as a result of sexual selection, or through broader processes such as social selection (Tobias, Montgomerie & Lyon 2012). However, no study has yet investigated the function of elaborate song and plumage in females and males simultaneously to test hypotheses for trait elaboration in one system. A general understanding of the evolution of elaborate traits – in both sexes, not just one, and in a diversity of social contexts, not just mating systems – requires that we significantly broaden our scope of investigation and consider a range of hypotheses.

Aims

This project will use the lovely fairy-wren (Malurus amabilis) to test three key hypotheses for the evolution of elaborate ornamental traits such as complex song and colourful plumage in females:
1) Genetic correlation: The apparent exaggeration of female traits has often been explained as a result of selection on males (Lande 1980). Much of the genome is common to the two sexes, so the genes coding for trait exaggeration may be the same in males and females.
2) Inter-sexual sexual selection: Elaborate female signal traits could have evolved because discriminating males prefer ornamented females, in a process similar to that demonstrated for the evolution of many male traits (Clutton-Brock 2007).
3) Social selection: Female signal traits could function as aggressive signals used against same-sex rivals in competition over mates, over reproductive control, or over ecological (non-sexual) resources such as territories or food (Chenoweth, Doughty & Kokko 2006).

Outcomes to Date

We monitored more than 50 groups of 118 individuals in lowland forest, saltmarsh and mangroves in Cairns the region, QLD over the duration of the project. Territories ranged in size from 0.8 to 4.6 ha, and birds were found at a density of 0.92 birds per hectare. Our study shed light on the ecology and breeding behaviour of this previously unstudied species, as well as the role of song and plumage ornaments.

Lovely fairy-wren ecology and breeding behaviour
  • Year-round territoriality and breeding, with nesting peak coinciding with dry season in Northern Queensland.
  • High annual survival of breeders and long-term pair bonds.
  • Low rates of cooperative breeding compared to other fairy-wrens.
  • Females build nests and incubate eggs (clutch size 1-3 eggs). Eggs hatch after 12 to 14 days, and nestlings fledge 13 days later.
  • Males and subordinate helpers assist the female with provisioning of nestlings and fledglings.
  • Nests suffer high predation rates, and we identified some nest predators using camera traps: butcherbird, lace monitor and snake.
Sexual dimorphism and visual and ornament signalling
  • Males and females are dichromatic, and males have a more ornamented cheek patch than females.
  • Males are larger than females, and male patterns of moult differ from other fairy-wren species.
  • Ornaments (colour and song) may be indicators of individual differences in female and male attributes. Data are currently being analysed to test this.
  • Plumage colour is important in a competitive context in both sexes: in a controlled experiment, variation in cheek patch colour was related to aggressive behaviour towards a simulated same-sex opponent.
  • Both sexes sing complex songs and in an experiment simulating territorial intrusion, both male and female responded to male and female playback songs.
  • Genetic parentage data are currently being analysed to test whether plumage and song play a role in mate choice by males and females.
References

Chenoweth SF, Doughty P & Kokko H (2006). Can non-directional male mating preferences facilitate honest female ornamentation? Ecology Letters, 9: 179-184.

Clutton-Brock T (2007). Sexual selection in males and females. Science, 318: 1882-1885.

Darwin C (1871). The descent of man, and selection in relation to sex. London: John Murray.

Odom KJ, Hall ML, Riebel K, Omland KE & Langmore NE (2014). Female song is widespread and ancestral in songbirds. Nature Communications 5: 3379, doi: 10.1038/ncomms4379
Lande R (1980). The genetic covariance between characters maintained by pleiotropic mutations. Genetics, 94: 203-215.

Tobias JA, Montgomerie R & Lyon BE (2012). The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition. Philosophical Transactions of the Royal Society B-Biological Sciences, 367: 2274-2293

Figure 1. Female and male lovely fairy-wrens (Malurus amabilis) colour-banded to allow free-living individuals to be distinguished. Females have the most colourful plumage of all Malurus females in Australia. The genus Malurus has been well studied, but there have been no previous studies of colour-banded lovely fairy-wrens, and the function of female plumage and song is unknown. Photo credits: Michelle L Hall.

Figure 2. Striking male displays may reveal which females they prefer. Photo credit: John White.

Figure 3. Unbanded female incubating. Photo credit: Philip Chaon

Figure 4. Newly hatched lovely fairy-wren hatchlings