For certain female frogs, the high-pitched calls stimulate procreation.

And for local humans lucky enough to live within ear’s reach, that “peep-peep-peep” sound rumbling inland from water’s edge has a more innocent implication: Spring is here!

Known for their chorus of metrical warbles at nightfall, “spring peeper” frogs have tuned up the vocal sacs underneath their chins to put mating season, typically March through June, in full swing.

Spring peepers, most common in the eastern United States, have crawled out from under the mud, bark and loose logs where they spent the winter and are now occupying shallow ponds, swamps and marshes across Cecil County.

The light brown, green or gray creatures with the shape of an X on their backs are about the size of a nickel, said Selena Sampson, naturalist at Fair Hill Nature Center. The frogs — scientifically called pseudacris crucifer — cling to tree bark using adhesive toe pads and eat small insects caught with their long, sticky tongues.

The nocturnal amphibians are usually not seen but heard. As the sun fades into the west on spring nights, especially during warm rains, the camouflage Casanovas tether themselves to tree barks and send their tenor-toned peeps resonating through the forests.

This is the call of male peepers looking for “love.”

“The males have a very loud shrill to attract a mate,” Sampson said. “The male with the deepest, longest peep will attract the female with the most eggs.”

A willing female that responds to a male’s call retreats with him to the water. The couple swim together for hours while the male grips her tight and covers her in a slimy solution that mixes with her eggs. About one week later, this usually results in 200 to 1,000 tadpoles, which will lose their tails by summer’s end.

Because of their tiny size — which makes them easy prey for snakes, birds and other frogs — many peepers don’t live through the first year, said Glenn Therres of the Maryland Department of Natural Resources.

“The important thing about peepers is that they’re very loud,” Sampson said. “A lot of people hear them and think they’re birds, but really it’s just a big group of tiny frogs.”

The spring peepers’ mating calls are said to resemble jingling bells. Though a single peep can resonate at least one-third of a mile, its pitch is much higher than a bullfrog’s, which sounds like the “thump, thump, thump” of a bass. Together, when dusk falls, all the frogs of the forest blend their calls together like a freshwater jug band to form layered rhythms, usually to the delight of humans nearby.

Anna Green, who lives on Elk Neck between the Elk River and Chesapeake Bay, said just when the weather turned warm, she started hearing peepers’ calls coming from the trees.

“I like to step out onto my porch every evening and listen for them going ‘Peep! Peep! Peep!’” she said. “For me, they’re one of the first harbingers of spring.”

She said that lately, when her husband, Robert, goes out to check the mailbox, he sees three tiny sets of frog eyes peeking out from the ditch in front of their house.

“We’re nature lovers,” Green said. “I have a sister in Virginia and every year, she’ll call and say ‘I heard my first peepers!’”

Within the last few weeks, Doris Welch of Russell Road near Grammies Run in Fair Hill has also relished the high-pitched trill of male frogs calling into the wild.

“I always think, ‘Oh boy, spring’s here,’” she said. “It’s just like when you see your first robin.”