Why Do Peacocks Spread Their Tails? The Train Display Explained

The train is not really a tail

What we call the peacock’s tail is a train of more than 100 greatly lengthened feathers called upper tail coverts. They grow from the bird’s lower back and cover the base of the true tail. Most of them end in an eyespot, or ocellus: a dark, heart-shaped center ringed with blue, copper and bronze-green.

The true tail is a set of shorter, stiff brown feathers underneath. During a display these feathers push the train up from behind and hold it in place, like the ribs of a hand fan. The Indian peafowl male grows his first train in his second year, and it reaches full size at about four years of age.

How a peacock displays

The display usually takes place at a lek, a traditional site where several males gather, each holding a small patch of open ground. Females visit the lek and walk among the males. A male courting a peahen goes through a set sequence:

1. He raises the train into an arched fan, sometimes taller than a person.
2. He half-opens his wings and lets them droop, showing the chestnut flight feathers.
3. He often turns his back to the female, then swings around to face her as she comes close.
4. He shivers the train in quick bursts, called train-rattling, which makes the eyespots shimmer and the feathers rustle.

Males also shake their partly opened wings, and some “freeze” over food to attract a female in a form of courtship feeding. Peahens often seem to ignore all this and keep feeding, but they are watching. A 2013 eye-tracking study found that peahens look mainly at the lower part of the train when close to a male, and glance at the upper train mainly from a distance.

What do the eyespots do?

In the classic study from the late 1980s and early 1990s, British biologist Marion Petrie and colleagues watched free-ranging peafowl at Whipsnade Wildlife Park in England. Males with more eyespots in their trains mated with more females. When the researchers cut off some eyespots, those males mated less often. Chicks fathered by males with more elaborate trains also grew better and survived better.

Later studies did not always agree. A seven-year study of feral peafowl in Japan, published in 2008 by Mariko Takahashi and colleagues, found no evidence that peahens preferred males with more eyespots, more symmetrical trains or longer trains. Other researchers point out that almost all adult males have similar eyespot counts, so small differences may matter little, and that what females judge may differ between populations.

Current research suggests peahens use several signals together: the eyespots, how often and how vigorously a male displays, his calls and the angle at which he holds his train toward the sun.

The hidden sound of the train

Train-rattling is not only visual. High-speed video studies show the train feathers vibrate at about 25 times per second, which creates a soft rattling noise. Researchers have also recorded low-frequency sound, near or below the lower limit of human hearing, during peacock displays, and playback experiments in Canada in the 2010s suggested that peafowl react to these sounds. The function of this sound is still being studied.

Why the peacock’s tail puzzled Darwin

To Charles Darwin, the peacock was a real problem. A long, heavy train seems to make a bird easier for predators to see and catch, so natural selection, which favors traits that help survival, should get rid of it. Darwin wrote in a letter in 1860 that looking at a peacock’s feather made him feel sick, because it seemed to contradict his theory.

His answer, set out in The Descent of Man (1871), was sexual selection: a trait can spread if it helps an animal win mates, even if it costs something in survival. Peahens choosing showy males would, over many generations, drive the train to its extreme size.

Later biologists added further ideas:

• Good genes and handicaps: In the 1970s Amotz Zahavi argued that only healthy, strong males can afford such a costly ornament, so the train is an honest signal of quality.
• Runaway selection: Ronald Fisher proposed that female preference and the male ornament can reinforce each other, growing more extreme over time.
• Maintenance cost: A 2010 study of wild peafowl in northern India suggested that keeping a long train in good condition is itself demanding, so a fine train reflects body condition.

Is the train a burden?

The train is less of a handicap than it looks. A 2014 study by Graham Askew at the University of Leeds measured peacocks taking off with and without their trains (after the yearly molt) and found that the train made little difference to take-off performance. Peacocks with full trains still fly up into trees to roost every evening, as explained in Can peacocks fly?

Only males grow trains. The plain brown plumage of the female helps her stay hidden while she sits on eggs on the ground, a contrast described in What is a female peacock called?