Until recently scientists weren't aware of the extent of these calls, because most bird migration takes place at night.

"It really began with amateur bird watchers who were noticing these sounds in the sky and knew that they were birds migrating overhead," says Rosenberg. "A few people became detectives and tried to figure out what these sounds were."

Bird call detectives

The migratory flyways over the United States are crowded with billions of birds every spring and fall.

By aiming special microphones at the night sky during these times and recording the passing calls, scientists realized they could determine what kinds of birds were flying overhead, and learn their migration schedules and flight paths. But there was a lot of sky to cover.

Pat Leonard, also with the lab, says they turned to military bases and amateur birders to help record the sounds. "We couldn't do that any other way if we didn't have everyday birders out there collecting that information for us. There are just too many birds and too many places."

Cornell's Lab of Ornithology is busy unraveling avian mysteries.

And maybe too much data. Each location recorded eight hours worth of tape nightly, all of which had to be listened to in real time. And many of these short calls sound very similar to the untrained ear.

One recording contains the calls of 48 different types of thrushes. The Ornithology Lab teamed up with Cornell University's Bioacoustic Research Program to develop software that would decode the birdcalls.