If you're out for a walk this month, and you hear something that sounds like ducks quacking, don't expect to see ducks. The call of a male wood frog fools a lot of people. The all-male frog chorus is revving up now, and wood frog males are the first to announce their availability to females.

Wood frogs spend all but a few weeks in woodland habitat while the other true frogs hang out in water. On a rainy spring night, some signal known only to wood frogs triggers their nearly simultaneous mass migration from woodlands to vernal pools. When you hear wood frogs quacking, you know there's a vernal pool nearby, formed by snow melt and spring rains. The males advertise, females respond, eggs are laid and fertilized, and the adults return to the safety of the woods. Their camouflage coloring blends well with leaf litter on the forest floor, so a quick return to safe habitat favors their survival.

As for why they're the first voice in spring's frog chorus: Wood frogs tolerate extreme cold. They're the only North American amphibian found north of the Arctic Circle. They also breed in vernal pools that usually dry up in summer, so an early start increases tadpole survival chances. When winter comes, bull frogs and other pond frogs submerge to spend the winter in the murk and mud well below the ice. Not so wood frogs. Able to tolerate a deep freeze, they winter among the leaves and debris of the woodland floor, frozen quite solid.