The Cornell Lab of Ornithology has just released a two-CD guide, "Voices of North American Owls." Call it a veritable "Who's Hooo" of North American owl sounds.

The CDs give voice to more than 19 species of owls and their repertoires that include more than 200 different calls. These voices of the night come with a detailed booklet and color photos. The benchmark reference guide also features tracks of two rare owls in addition to sounds more commonly within earshot.

Owl calls vary from species to species and from vocalization to vocalization. The birds use sound to proclaim and defend territories, attract and bond with mates, beg for food and express alarm.

For instance: Great horned owls are known for their stentorian hoots. But they also squawk, chitter, bark and emit a curious wac-wac sound.

Screech owls are aptly named but are not limited to their shivering trills: When annoyed they also produce an eerie chuckling and rattling sound, among other sonic curiosities.

"Almost all the other recordings presented to the public are primary songs, like the hoot of the great horned owl or the toot of the northern saw-whet," said Gerrit Vyn, the lab's producer and studio engineer who chose the sounds for the new audio guide. "People just don't realize owls make so many different sounds, or don't recognize what they're hearing."

For years, requests for owl vocalizations have been among the most popular recordings sought by researchers and the public alike at the lab's Macaulay Library, according to Greg Budney, curator of the natural sounds collection and co-producer of the new guide. For a sampling of owl sounds visit http://www.birds.cornell.edu/AllAboutBirds/owl.

Volunteers have recorded many of the library's owl recordings, and some of them date back to the 1950s, said Budney. Vyn sorted through hundreds of these sounds and also contacted outside sources for new recordings. The painstaking project came together with the help of Budney and supervising engineer Robert Grotke in collaboration with scientists who specialize in each individual species.

Vyn said he tried to include as much variety as possible, but he also wanted to represent calls that are similar between species. For example, he said, almost all owls make a chitter sound when they are in close contact with other owls, interacting with people or are uncomfortable.

"And some tracks are just cool to listen to," he added.

Among the hoots, shrieks, barks, moans, trills, chuckles and whines, Vyn said his favorite tracks are those of the barred owl that he and Lab of Ornithology colleague Ben Clock recorded during the recent ivory-billed woodpecker search in Arkansas.

"We got vocalizations made by a female barred owl that no one had heard before," he said.

Budney said he hopes that through the CDs, biologists and researchers will gain a tool to study these birds and will realize the potential for further investigation. "As nocturnal birds, their voices are important in locating owls and understanding them," Budney said. "Despite the superb research, there is still a good deal that is unknown."