Springtime in Wyoming. The sagebrush prairie is alive with the dancing courtship displays of greater sage grouse. It's the time of year when birders flock to the giant birds' annual strutting grounds, called leks, to watch one of the greatest avian spectacles in North America unfold before their eyes at sunrise.

"Ba-loomp, Ba-loomp, Ba-loomp…"

That's the strange "booming" sound a male sage grouse makes by puffing air in and out of the big sacs on its neck. If you visit a lek this spring, be sure to listen as well as watch. The female birds certainly are.

While the showy dances of the colorful male birds are a delight to watch, scientists are discovering that the sounds the birds make are actually more important when it comes to mate selection.

"Sound is probably more key than the visual display. In fact a lot of the display is actually designed to generate the sound," said Tom Christianson, sage grouse coordinator for the Wyoming Department of Game and Fish. "We tend to call it 'strutting' which implies the visual, but the sound is actually more important. It's not to say the [visual] display doesn't play a role but the sound plays more of a role than people give it credit for."

Researchers from the University of California-Davis are studying the acoustics of sage grouse courtship at various sites in Wyoming. The scientists are expanding on previous studies that discovered the importance of the amplitude for breeding selection. The posturing of the male as it struts is a way of directing the sound toward females from certain angles to achieve certain amplitudes.

And the females are very picky about those amplitudes. A lek can have up to 300 birds on it but as few as one or two males will be deemed suitable for mating by all the females.

"If you get to a good lek you'll see all the females clustered around, oftentimes in the center of the lek around a small number of males. They're all going for the same guys and very few of those outside males will do any breeding."

A male bird also makes a swishing sound by brushing its wings against the stiff white feathers on its neck. That sound may be inaudible to humans from a distance, but when you see the male make that motion, it's more than just a fancy dance move. It's a sound.

"Those feathers have evolved for that reason to make that sound."

Environmental acoustics - the soil type and topography of the lek itself - probably play a significant role too. How the sound carries at a particular site may be a big reason why the birds return to the same lek year after year - or possibly why they abandon a lek if a noisy road or energy development pops up nearby.

One fairly common sight on a lek is a scrap between two males - a dramatic but brief boxing match with their wings. But fighting doesn't play much of a factor in mate selection, Christianson said.

"I'd say that's more just boys being boys."

When a female grouse decides on a mate, her posture indicates acceptance.

"The breeding itself only lasts a few seconds and it's all over with."