Scientists in the US have used eye-tracking cameras to work out exactly what peahens find alluring in a peacock's tail fan.
The male birds grow their trains of iridescent feathers during the mating, or lekking, season, fanning them out and rattling them to attract a mate.
This team of biologists fitted peahens with eye-trackers to find out what they looked at during this display.
Their results are reported in the Journal of Experimental Biology.
The eye-tracking footage revealed how difficult it is to keep a peahen's attention, which helps explain why such a large and elaborately decorated tail fan evolved.
It also gave clues about what a peahen looks for in this tail fan. Side-to-side eye movements suggested that females were gauging the fan's width and that they were most interested in the striking eyespots on the feathers.
The peacock's tail is probably the most famous example of sexual selection - a phenomenon identified by Charles Darwin whereby animals evolve a trait because it is attractive to the opposite sex.
"There are quite a few species that have these elaborate colourful traits that don't serve any survival function," explained Dr Jessica Yorzinski, who carried out this research project while based at the University of California Davis and Duke University in North Carolina.
"[This long train] might actually make it very difficult to get away from a predator."
To find out why the peacock's train is quite so cumbersome and elaborate, the scientists set out to understand what it takes to impress a peahen.
"I wanted to know what it was the females attended to when they were evaluating potential mates," explained Dr Yorzinski.
The researchers trained 12 peahens to wear eye-tracking equipment. This consisted of two tiny cameras on a head mount. One recorded the scene in front of the bird and the other recorded eye movement.
"We were surprised by the results," said Dr Yorzinski.
Rather than looking up at the high crescent of the fan above the peacock's head, the eye-trackers revealed that females looked primarily at the lower portions of the train.
"From the head down was where most of their gaze was directed," said Dr Yorzinski.
"The peahens often looked from side-to-side across the bottom portion of the train, suggesting that they were gauging the width of the train."
The experiments showed that females constantly shifted their attention between the environment and the peacock's tail.
"It is likely beneficial for them to divide their attention among potential mates and the environment," explained Dr Yorzinski.
"If females are not alert and focus completely on a displaying male, they may end up as a tiger's dinner."
The research suggests that the peacock's tail has had to evolve to eclipse all the other things competing for a female's attention.
It also raised the question of why the tail fans are held so high if the females focus most of their attention on the lower portion.
Dr Yorzinski has an explanation for this too. In their natural habitat in India, the vegetation is very high. "All you can see [of the peacock] is the upper train," she said.
"So we think it's a long-distance signal to the hen."
Prof Tim Birkhead, a bird expert from the University of Sheffield, said the research was "very exciting", adding: "It is a wonderfully novel approach."