RENEE MONTAGNE, host:

Every summer, an invasion overwhelms the tiny island of Saint Lazaria off the southwest coast of Alaska. Swarms of seabirds descend to breed. For our series, Sounds Wild, we're going to descend into the middle of this mass of birds with biologist Michael Andersen. Andersen, from the University of Kansas, recorded two kinds of birds: the Leach's storm petrel and the fork-tailed petrel.

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Mr. MICHAEL ANDERSEN (Biologist, University of Kansas): It's nighttime, and so the birds are coming in and flying all about, upwards of half a million birds that we're talking about.

These birds are actually digging burrows in the soft, moist soil in the forested areas, and so they're coming in through the forests, landing on the ground and going underneath the ground, tunneling into their burrows to lay an egg.

This nocturnal shift is sort of a changing of the guard where one bird comes back in and exchanges incubation duties while the other bird goes out to do some foraging, and they're making these vocalizations to help recognize one another individually in this sort of sea or mass of birds.

(Soundbite of bird)

Mr. ANDERSEN: There are two species here, and each gives a unique vocalization. The fork-tailed storm petrel has this one that just gives three to five grating notes, like a yup, yup, yup, yup.

(Soundbite of birds)

Mr. ANDERSEN: And the Leach's storm petrel is giving just more of a purring sound.

(Soundbite of birds)

Mr. ANDERSEN: As they fly through the trees, oftentimes they're crashing through the limbs of the trees, stumbling down to the ground and then stirring about, calling to their mate, trying to find their particular burrow.

There are so many of these birds that they can't help but flying into the microphone or the tripod or, even one time, a bird flew right into my head.

(Soundbite of birds)

MONTAGNE: Those purring and yapping sounds in our series are from the Cornell lab of ornithology. Thanks to NPR science correspondent Christopher Joyce for tracking them down.

If you'd like to see the animals and the birds in our series, visit NPR.org.

(Soundbite of music)

MONTAGNE: This is NPR News.

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