Even if you don't know much about birds, you can easily tell the difference between a peacock and a peahen. Expressions like "proud as a peacock" refer to the male's brilliant plumage and carriage, while the female is quite plain in comparison. That long peacock tail and those vivid colors evolved to impress the peahens for reproductive purposes. Still, it's the peahen who makes the actual selection of a mate. As the peacock spreads his tail, those ornamental eyespots make all the difference to peahens. The larger the fan of the tail and the brighter the eyespots, the more likely a peacock will be popular with the girls.
There's a sound biological reason for the female peahen's ordinary plumage versus the splendor of the peacock. Peahens are the ones who nest and set on the eggs. Brown plumage makes it easier for them to blend into their surroundings while nesting, making them less of a target for predators.
The adult Indian peacock sports a tail train that makes up over 60 percent of the bird's body length, with between 100 and 150 feathers making up his impressive display. Each feather contains a blue, green and gold eyespot at the tip, formally known as an ocellus. These eyespots are surrounded by additional iridescent colors, including black and purple. The Indian peacock has a distinctive blue crest on his head. The Indian peahen is predominately brown with a white abdomen. She does have some green feathering on the neck, but that's about it in terms of bright coloration.
Green peafowl, also known as Java peafowl, also have obvious sex differences in appearance. The green peacock has a train containing approximately 200 feathers, with each one sporting a gold, green and brown eyespot. His train contains more gold and is somewhat darker than than of the more common Indian peacock. He boasts a green head crest on his head. Although green peahens are actually green, their coloration is less iridescent and they don't have trains.
Sexing peachicks isn't easy. Male peachicks generally have longer legs than females by the age of 2 months. Look carefully at the tips of the bird's outer primary feathers. These tips are brighter on males. The male's tails don't develop until the age of 3, so if you want to be absolutely sure of your peachick's sex and don't mind spending the money, pluck a few feathers and send them to a laboratory for DNA testing.