For millennia, greater sandhill cranes have made an important pit stop in southern Colorado's San Luis Valley. They fuel up on insects and barley on their way from New Mexico to the Yellowstone area. The barley is provided by officials at the Monte Vista National Wildlife Refuge.

The 33rd Monte Vista Crane Festival wrapped up Sunday, but the birds, which stand 5 feet tall and have wingspans of 7 feet, will "loaf" (yes, that's the term for it) through mid- to late-April, according to refuge manager Suzanne Beauchaine.

At their thickest, Beauchaine said the biggest flocks of birds can actually darken the sky. That happened last weekend to one of her tour groups when something -- probably a predator like a coyote -- flushed the birds out, she said.

"There were probably 5,000 birds in the air -- and they were all over us," she said. 

The birds pair for life, Beauchaine explained. And in the spring, birds who've had offspring rekindle their relationship after young cranes leave the nest.

"The adults that have been paired [for years], or new pairings, they will start to dance," she said. "And some folks viewed cranes plucking feathers out of each other and throwing them up in the air, catching them and continuing dancing."

Beauchaine said the birds are a sign of spring, which is often a welcome sight in the San Luis Valley. Winter temperatures can fall well below zero on the coldest days. 

"To see this many birds, especially in Colorado ... is an amazing spectacle," she said.

Click the audio player above to hear the full conversation.