Birding in Coimbatore: Why do male and female birds look so different?

Sexual dimorphism is a term used to explain the differences seen between male and female birds seen in the same species. There might be differences in size, plumage, colouration, physiological and in some cases, behavioural differences. In general, male birds are bigger (this rule is not without exception as in Jacaunas, the females are bigger and in some hawks, the females are twice the size of males). Males in general are more colourful.

Nowhere is sexual dimorphism in birds more apparent than in Indian peacock and a pea fowl

Sexual dimorphism in a little understood phenomenon. In some cases, as in House finches, the male breeding plumage is an indication of the health of the bird. (In this case, darker the male, the healthier it is.) In some species, the male's contribution to reproduction ends at mating, while in other species the male takes care of the eggs and chicks.

Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. In other words, the more colourful or conspicuous a bird is, the more prone it is to be attacked by birds of prey. Other than this, the exaggerated colours and plumages sometimes come at the cost of immunity.

Sexual Dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the Blue-footed Booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This them results in the maximization of parental lifetime reproductive success.

Some extreme examples of sexual dimorphism in birds:

Huia (Heteralocha acutirostris), Now a an extinct bird of New zealand) was remarkable for having the most pronounced sexual dimorphism in bill shape of any bird species in the world. The female's beak was long, thin and arched downward, while the male's was short and stout, like that of a crow. The sexes were otherwise similar, with orange wattles and predominantly black plumage with a green sheen.

Huia (Male) & Female (Below)

The Great Bustard (Otis tarda) is one of the heaviest flying birds and the most sexually dimorphic living bird. Adult males weighed 2.48× more than females, and their linear measurements were 18–30% larger.