WALKING through winter woods at the Schuylkill Center for Environmental Education, executive director Mike Weilbacher heard a few song notes high up in the bare trees and said: "That's a cardinal's mating song. The birds are dusting off their love songs, warming them up for spring."

A few minutes later, Weilbacher heard a woodpecker tapping out a drum solo in the Upper Roxborough forest on Hagy's Mill Road near Port Royal Avenue.

"Woodpeckers are not great singers," he said, "but they are great percussionists. People hear them hammering on hollow trees and think they're feeding on insects. They're not feeding.

"They're telling females, 'I'm a male woodpecker and I've got a great nesting spot, babe.' And they're telling other male woodpeckers, 'I've got mine. You go find your own hollow tree.' "

Weilbacher smiled the smile of a man completely at home in the Schuylkill Center's six miles of trails through 340 hilly acres of forests, butterfly meadows, ponds, streams, rock outcroppings and old farmstead ruins.

"This is not a preserve set aside for nature," he said. "It's a preserve set aside for people to connect with nature. For me, this is like the mother ship of environmental education."

Celebrating the Schuylkill Center's 50th anniversary, Weilbacher exemplified its teaching mission when he saw a bare patch on the trunk of a young tree and identified it as a "buck rub."

"During rutting season in November," Weilbacher said, "the male rubs his antlers against the bark, makes a muddy rut on the ground with his hooves, and pees down his legs into the rut.

"The urine goes over glands in his knees, picks up pheromones, runs into the rut and makes a puddle of this soup that the females find irresistible. They know there's a healthy male in the area. Later, they hook up."

A little farther up the trail, displaying eyesight that would do credit to a hawk, Weilbacher spied a tiny but sure sign that spring is on its way.

"Red maple," Weilbacher said, bending to pick up a red bud from the ground. "In early spring, when thousands of these bloom, there will be a red blush across the forest."

And come spring, he said, the center's 27-year-old wildlife clinic, where two rehabilitators and 80 volunteers care for injured and abandoned animals, will be overrun with baby squirrels and rabbits brought in by the nature lovers who find them.

Weilbacher said the wildlife clinic has handled everything from a bald eagle that a police officer found lying in a pool of blood in a Broomall parking lot to a gigantic snapping turtle that WMMR DJ Pierre Robert found after it had been hit by a car.

"The turtle had a cracked shell," Weilbacher said, "but after we spent 13 months caring for it, Pierre was able to come back here and release it into the Schuylkill River. We saved the eagle and released it, too. That was pretty much a miracle."

Standing in a Philadelphia neighborhood, enveloped in the silence of its deep forest, broken only by a bird practicing its mating call for spring, is pretty much a miracle, too - one that has blessed the city for 50 years.