Many types of dabbling ducks have iridescent wing patches called specula (singular: speculum). These flashes are usually blue or green, although the colour can vary somewhat according to the angle and the lighting conditions. Ducks can also control the extent to which these flashes are visible when their wings are folded (see below).
In Mallards (Anas platyrhynchos), which are probably the easiest duck to photograph in the UK, the flashes are blue in both males and females. The blue patches occur on the secondary feathers and are very apparent when the duck stretches its wings (below) and here.
The colours are structural rather than pigmentary. Structural colours are produced by the coherent scattering of light by highly ordered nanostructures. In this case the spectacular blue iridescence is produced by microscopic hexagonal lattices on the surfaces of the feathers. These which scatter the light in the blue part of the colour spectrum
“speculum colour is produced by a photonic heterostructure consisting of both a single thin-film of keratin and a two-dimensional hexagonal lattice of melanosomes in feather barbules” (Eliason and Shawkey, 2012).
The bright, iridescent green head feathers of the male are also produced by light interacting with nanostructures in the feather barbules (Stavenga et al., 2017).
During the breeding season, mallards carry out courtship displays. The detailed behaviour was first described of these displays were first accurately described by the famous German ethologist Konrad Lorenz. Three courtship displays are especially frequent and conspicuous and can be seen in these videos, here and here. These displays allow the female to observe the performance of males and to evaluate them as potential mates. Males also make choices, and the colour and brightness of the female’s flashes (below) is also thought to be be important in attracting the male in the first place (Legagneux et al., 2010).
Mallard courtship displays
Head-Pumping: Males and females rhythmically bob their heads.
Head-Up-Tail-Up: The drake pulls his wings and tail up, shows off his purple-blue secondaries (specula) and compresses his body.
Nod-Swimming: A male or female swims rapidly for a short distance with its neck held low, just grazing the surface of the water. Females probably use this display to express that they are interested in courtship and stimulate the nearby males to display. Female Mallards and other female ducks often demonstrate (inciting displays) and call to provoke males to attack other males or females.
Some studies have suggested that the female mallard makes her initial choice of a mate, based on the male’s plumage characteristics (Klint, 1980). Another study, which observed the courtship behaviour of mallard ducks breeding in a large pond for three successive years, came to the conclusion that females preferred males ‘with a high social display activity, a high plumage status, a small body size and of intermediate age’ (Holmberg et al. (1989). The suspected relevance of the plumage quality of drakes, for the choice of mates by ducks, has also been confirmed in a series of experiments by Weidmann (1990).
So plumage is important for mate selection, but what parts exactly? Surprisingly (to me at least) another experiment found that removing the iridescent specula had no effect on pairing success in male mallards (Omland, 1996). Does that mean that the blue flashes play no part in courtship? I think the jury may still be out.
Other work has suggested that the courtship performance of the male mallard is more important than his plumage colouration. One study showed that females paired with males who courted them ‘most intensively, regardless the colour type’ (Bossema and Kruijt, 1982). In other words, it’s not so much what you look like, but how you strut your stuff! The colour of the male’s bill – bright yellow (see below) – has also been shown to be a significant factor affecting female choice (Omland, 1996).
The blue specula are complex structures and may be a costly ornament – costly in terms of energy – for the duck to produce and maintain. Therefore, they may be an indication of the quality (health and vigour) of the animal, and research has shown that their brightness was ‘condition related’ (Legagneux et al., 2010). However, the flashes might be less important for mate choice, and more important for species recognition? It is very apparent that both sexes have these lively blue flashes, which might help them recognise their own kind?
In practice, animals – like people – make choices about partners based on the total package: behaviour, looks, age, quality and vigour! Male mallards (drakes) have a wonderful set of colours to choose from, or assess: their bright yellow bill, their iridescent green head and neck feathers, their bright blue speculum, and their overall appearance. Perhaps the female specula are more important to males because they do not have the other colours?
However, the final choice of a mate will probably be based on a combination of physical appearance and behaviour. Persistence and stamina can also be a good guide to how fit an animal is. As animals, we are all highly skilled at evaluating another individual of our species, especially a member of the opposite sex. Courtship is a way – albeit rather stereotyped in ducks – of showing ourselves off and allowing the opposite sex to assess whether we would make a good partner for them. Specula may not be the key factor in this process, but they are probably part of whole package that ducks use to recognise, attract and assess each other.
Bossema, I., & Kruijt, J. P. (1982). Male activity and female mate acceptance in the mallard (Anas platyrhynchos). Behaviour, 79(2-4), 313-323.
Eliason, C. M., & Shawkey, M. D. (2012). A photonic heterostructure produces diverse iridescent colours in duck wing patches. Journal of The Royal Society Interface, 9(74), 2279-2289.
Holmberg, K., Edsman, L., & Klint, T. (1989). Female mate preferences and male attributes in mallard ducks Anas platyrhynchos. Animal Behaviour, 38(1), 1-7.
Khudiyev, T., Dogan, T., & Bayindir, M. (2014). Biomimicry of multifunctional nanostructures in the neck feathers of mallard (Anas platyrhynchos L.) drakes. Scientific reports, 4(1), 1-6.
Klint, T. (1980). Influence of male nuptial plumage on mate selection in the female mallard (Anas platyrhynchos). Animal Behaviour, 28(4), 1230-1238.
Legagneux, P., Théry, M., Guillemain, M., Gomez, D., & Bretagnolle, V. (2010). Condition dependence of iridescent wing flash-marks in two species of dabbling ducks. Behavioural Processes, 83(3), 324-330.
Miller, D. B. (1977). Social displays of Mallard Ducks (Anas platyrhynchos): effects of domestication. Journal of Comparative and Physiological Psychology, 91(2), 221.
Omland, K. (1996). Female mallard mating preferences for multiple male ornaments. Behav Ecol Sociobiol 39, 353–360 .
Stavenga, D. G., Van Der Kooi, C. J., & Wilts, B. D. (2017). Structural coloured feathers of mallards act by simple multilayer photonics. Journal of The Royal Society Interface, 14(133), 20170407.
Weidmann, U. (1990). Plumage quality and mate choice in mallards (Anas platyrhynchos). Behaviour, 115(1-2), 127-141.