Males and females are different – we know this. In the bird world, this difference can sometimes be seen between two sexes of the same species by certain characteristics that the birds will display, either by behaviour, size, or the most visually obvious one, what colour they are.

These differences are called sexual dimorphism; birds (and indeed any species) who do not display noticeable differences exhibit sexual monomorphism.

Theories abound, but little proven facts exist

Sexual dimorphism is present in many other animals and plants throughout the world, but its purpose is not all that well understood. The most commonly accepted principle behind the existence of differences is that it is a direct product of evolution by natural selection: the struggle for reproductive success has resulted in different evolutionary paths.‍

But why this particular species and not that one from the same family?

Theories abound, but little proven facts exist. Some scientists have discovered what appear to be links between monogamous and polygamous birds, in that those who tend to have several partners during breeding season also tend to be those who exhibit the least differences, and those who have many differences will keep the same partner. But like all theories, it doesn’t apply everywhere.

Technicolour dream coat

Pick up any bird field guide worth its ink and you should see pictures of both male and female birds when there is a difference in plumage colours, or at least some text describing the differences.

If there is only one picture with the word ADULT underneath, chances are that species is sexually monomorphic. Pictures of any colour variances are invaluable, so do read the reviews online or take a look through one before buying it first.

This simple but very useful addition can make the world of difference in helping you on your journey to successfully identifying birds.

We could find no fixed figure that says how many of the 11,000 or so officially recognised species of bird that exist today have plumage variations across the sexes, a condition known as dichromatism. But if there is such a number, it would probably turn out that it would be wrong anyway.

‍Our human vision istrichromatic, we see only the three true colours of red , blue and green. When we think we are looking at yellow, we are in fact seeing a frequency of white light that is part red, part green, but we call it yellow.

In the 1970s a researcher looking at how pigeons see colours accidentally discovered that birds have tetrachromatic vision: they can see ultraviolet light as well as what we can see. As we cannot perceive UV light, and we also know that feathers reflect it, it stands to reason that some bird species we think are monochromatic do in fact have many differences in their plumage that we simply can’t see.

The females chase and fight over males

When dichromatism does occur, it is often assumed that the males will always be the more vivid, brighter and more diverse of the sexes; look at the vibrant royal blue, cyan and turquoise of the male Superb fairy wren of Australia, the blood-red-soaked colouration of the male northern cardinal of the US, or the hypnotic display of the Indian peafowl, the shimmering greens, blues and purples of the peacock in stark contrast to the browns, buffs and whites of the peahen.

‍It is thought that the colouration preference in males is, among other things,a survival tactic; females need to be visually downplayed as they will most often be the ones on the nest incubating the eggs and tending to the young chicks, and blending in to the natural earth colours of the environment is essential to avoid detection and predation. But, as with most of the world, there are exceptions.

There are a handful of species where vibrancy favours the females, and the males are, by comparison, bland. Such is the predominance for vivid plumage to favour the males that this situation is referred to as reverse sexual dimorphism.

They look so dissimilar that they were originally thought to be completely separate species

When I first saw red-necked phalaropes and their sex annotation in a field guide, I thought there had been a misprint. These birds are small waders who breed in the Arctic regions, and spend their winters on open seas far from land in the tropics.

The female has a dark grey almost black back shot through with yellows and oranges, and pointedly sports a vibrant ochre-hued wide band extending from behind each eye, through the cheek, down under the throat and across the chest, whereas the male’s overall colours are muted versions, and the band stops by his cheeks.

The females chase and fight over males, who will then perform all incubation and chick-rearing duties. Females may also head off on their migration before the males have finished raising the kids.

A similar reversal can be seen ineclectus parrots: the males are bright green all over with orange beaks and a small red eye iris around a black pupil, but the females are a stunning blend of reds, maroons, lilacs and purples with jet black beaks and a white iris.

‍They look so dissimilar that they were originally thought to be completely separate species. Another mating “quirk” is thought to be the reason why this happens - although the female does tend to the nest, often for 11 months of the year,both sexes pursue multiple partners.

One size fits all?

If a species doesn’t exhibit plumage differences, a way of telling them apart could be with theirsize, but here you also have to be careful. Yet another typical assumption is that males are bigger than females, and that is mostly true, even if it is not that noticeable as with many songbirds, unless they stay still side by side long enough for you to see.

However, there are cases where the female birds are noticeably larger than males, such as with shorebirds. Again, the females of these birds will often mate with more than one male.

Mating habits determining size discrepancies aren’t the case with raptors and owls, though, where a number of theories try to explain why females are larger than males.

The most plausibleexplanation revolves around the scarcity of live prey in certain environments, in that males evolved to be smaller because small live prey is more abundant than large prey, and the male does more hunting when the female is incubating. Females may also be larger to produce those massive eggs, and then have all the fun of sitting on them.

Behave yourself

For birders who want to know the sex of the bird they espy, and where no visual clues offer a certainty, we can try to be aware of the behaviour that certain birds exhibit during courtship.

Many males will feed females, and they tend to have elaborate dances, posturing or other actions to try and entice females to watch their displays. Males also tend to be more aggressive, chasing away intruders and fighting off other birds and sometimes even non-bird predators.

‍It was long thought that males were the only birds that sang; since the dawn of Darwinism, it has been thought that any females that sang was a hiccup in nature, an anomaly that couldn’t be explained so may as well ignore it.

It is mostly true, male songbirds will often be the only ones who will trill out the territorial and mating calls and tunes during breeding season. But studies carried out on tropical species in the early 2000s showed that females were just as likely if not more likely to vocalise during breeding, throwing another long-term conception into the blender of vague, perhaps hastily sought conclusions. Research continues.

Identifying male and female birds in one species may not be the ultimate achievement, but being aware of differences helps us narrow down our identification techniques and can open up a whole universe of behaviour and breeding tactics that help us understand birds and the world better.

But ultimately, even if all starlings are identical, and male carrion crows and female carrion crows are black and it is impossible to tell which one is which, the birds will know the difference, and that is really all that matters.