Boy, you're lookin' fine! Sexual dimorphism in birds

Ostriches (Struthio camelus) are sexually dimorphic. The male (right)
has a pinkish neck and legs, and black and white feathers. The female
(left) is mostly brown. 

When males and females of the same species have different physical forms, it is called sexual dimorphism. These differences can be in body parts, color patterns, and size.  If you are trying to identify a species, recognition that the sexes may appear different is key. I once met a woman who swore she had two species of cardinals in her yard, but after a bird walk she discovered the Northern Cardinal males are bright red whereas the females are greyish red. Same species, but different looking. 

Why?

Black-and-white casqued hornbills (Bycanistes subcylindricus).
The male on the left has a much larger casque on his bill and
less red bare skin around his eye than the female on the right. 

On the surface, it would appear that the bright colors, long tail feathers, different size, etc. are ways of attracting mates, like how humans may dress up to impress others. But birds do not choose to look the way they do. These differences are rooted in their genes. So why do the males and females of the same species look different?

I re-post this photo of southern red bishops (Euplectes orix) because
it shows the strong difference between the male (left) and the female
(right).

Individual differences among members of a species, like how much force a bill can exert to crack a seed, the strength of the wings, and other characteristics that increase a bird's chance of survival, affect which birds pass on their genes. Only the individuals who can survive long enough to breed actually get to pass on their genes (natural selection). But color, tail length, and other “ornamentation” differences in birds are not so obviously related to survival. They have no apparent function other than attracting a mate.

A male pin-tailed whydah (Vidua macroura).

Males (and in a much smaller number of species, females) are driven to differences by the opposite sex that select them to breed with; then the genes of the ornamented male are passed on (sexual selection). So for example, if a male bird with a brighter red color (than other duller red males of his species) is chosen to breed with by more females, then he gets to have more offspring and more of his genes are passed on than other individuals (assuming the color is genetic and not an environmental factor like how flamingos intake carotenoids in their food).  With more genes for the brighter red color in the population, more individuals are the brighter red color. If females continue to select the brighter red males, then males continue to be the brighter red color over the duller red plumage.

Female pin-tailed whydahs (Vidua macroura).

We really do not know much about why birds select their mates when it comes to ornamentation. The fantastic details may help females distinguish their species from another similar one. The showy details may announce an individual’s presence so loudly that to defy getting eaten by a predator is to imply genetic superiority. It is possible that what we see as fantastic details are byproducts of some unforeseen linked trait; for a hypothetical example, it may be that the gene for long tails is just passed on because the birds who had them have stronger bones (but through no connection to the longer tail itself).

A male black cuckoo-shrike (Campephaga flava) is dark and glossy with
just a small yellow gape behind the bill. 

Whatever the reasons may be, sexual dimorphism is a defining aspect in the appearance of many bird species. People sometimes ask me why the males are more beautiful, but I remind them that beauty is in the eye of the beholder. Also, it is the female birds that do the choosing so they are responsible for driving the gaudiness of the males. Inspired by slogans from the early 20^th century women’s rights movements (like “equal pay for equal work”), Michele and I joke about creating a feminist-birding blog dedicated to females birds called “Equal viewing for equal birds!” Although not as showy, female birds (and non-breeding plumage males and juvenile birds) have a subtle beauty that is worth seeing too. 

A female black cuckoo-shrike (Campephaga flava) is beautiful on her
own.

For more images of sexual dimorphism in birds, see the sunbirds and weavers.

Works Consulted

• Dawkins, R. The Greatest Show on Earth. London: Random House Group, 2009.
• Ehrlich, P, et al. "Sexual Selection." Birds of Stanford. Accessed: October 17, 2011. http://www.stanford.edu/group/stanfordbirds/text/essays/Sexual_Selection.html
• Fanshawe, J. and Stevenson, T. Birds of East Africa. Princeton and Oxford: Princeton University Press, 2002.
• Owens, I. and Hartley, I. 1998. "Sexual dimorphism in birds: why are there so many different forms of dimorphism?" National Center for Biotechnology Information. Accessed October 16 2011 at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1688905/pdf/RQQD3PMMWYMYQD4H_265_397.pdf