This is Bird Note. Black swifts are extraordinary birds. They spend most of their lives in the air and migrate thousands of miles every year.

But researchers have found that their lifestyle is even stranger than we once knew.

Scientists place lightweight tracking devices on swifts nesting in Colorado.

This let them study their movements once they reach their wintering grounds in Brazil.

This provided unprecedented access to the swifts world in the skies.

Estonishingly, black swifts spent over 99% of their time in the air during the winter, almost never touching the ground for months.

What's more, the swifts flew to incredible heights, reaching the highest altitudes on nights when the moon was full.

Sometimes over 13,000 feet.

It's the first time scientists have seen birds changing their altitude along with the cycles of the moon.

It's still unknown why the moon seems to draw black swifts high into the sky.

It's possible that insects, many of which can also fly thousands of feet in the air, are attracted to the moon and swifts follow their insect prey upward.

What is clear is that these birds have abilities beyond what our eyes can see.

For Bird Note, I'm Michael Stein.