Frogs usually operate under our radar, but in late winter and early spring they leap to our attention as they let loose with their voices.

Usually called Pacific tree frogs (although they aren’t true tree frogs), they’re more accurately called Pacific chorus frogs, reclassified from the genus hyla (true tree frogs) to pseudacris.

Very small (1.5 inches), often green (can wear other earth tones), they sport black stripes running around the eyes and down to the shoulders.

The Washington native is not the spring peeper, a different species that’s found in the eastern United States.

Males are heard croaking in nearby water-filled depressions, ditches and small ponds, sending out their come-hither messages. It’s safer to breed in temporary ponds where there are no fish nor bullfrogs to eat their eggs and tadpoles.

Pacific chorus frogs also provide sound tracks for many movies and cartoons, the familiar “rib-bet, rib-bet, rib-bet!”

A one-syllable call is used the rest of the year, although if another frog intrudes on its mating territory, there’s a one-syllable encounter call, too.

They make sound basically the same way we do, except they keep their nostrils and mouth closed so the air flows over vocal chords and into the vocal sac, puffing it up like a balloon.

The sac amplifies a certain frequency and broadcasts the sound, said Elliot Brenowitz, a professor of zoology and psychology at the University of Washington who studies acoustic communication in frogs and birds.

The vocal marathoners expend massive amounts of energy going on and on, hour after hour, night after night. Just ask a resident who lives near a chorus.

“I’ve heard stories of people filling in ponds to get rid of them. I tell people that they should really appreciate having these guys around. It’s a sign of a healthy environment.”

It’s possible to hear them a mile away on a quiet night, and if, theoretically, you could put your ear next to one, the “rib-bet” would sound as loud as a jackhammer.

“Female frogs actually orient better to a chorus, or recording of a chorus, than to the call of one frog,” Brenowitz said. “It may sound like thousands, but start counting, there’s never that many. If you find 100 frogs, that’s a big chorus.”

The males make shopping around easier by gathering around twilight. It’s the concept of a selfish herd.

“A frog in a chorus has a lower chance of being picked off by a predator,” the professor said.

Then there’s the female’s opinion.

“Females prefer choruses of males over single males. In some species it’s been observed that females walk right around a single male to get to a chorus.

“When they’re singing, they’re right at the edge of the water, usually with the head end sticking out of the water,” Brenowitz said.

“The male calls … a few thousand times a night. The female swims around and samples the calls.”

Loudness reflects the male’s size, attractive to a female. She swims up to her choice, nudging him if he’s caught up in croaking.

He crawls on top and grabs the larger frog with his forearms. She swims down to vegetation at the bottom of the water for the start of a several-hour process, with the pair breathing through their skin.

She lays eggs; he ejects sperm. She sticks the jellylike clusters of 10 to 70 eggs on plants.

A few weeks later, depending on water temperature, tadpoles emerge; two months later, metamorphosis produces frogs, part of next year’s chorus.

Purple martins are the stars of Pilchuck Audubon Society’s 7:15 p.m. April 9 meeting at Cascade Crags, 2820 Rucker Ave., Everett.

Biologist Kevin Li, who has been studying purple martins with other scientists for several years, will speak about these swift-moving bug-catchers.

The meeting is open to the public.

Columnist Sharon Wootton can be reached at 360-468-3964 or www.songandword.com.