The annual Cradle of Birding festival started with a birdwatching outing, of course. They headed out at 7 a.m. and were rewarded almost instantly with a pine warbler - likely resting from its migration - in some shrubby woods not far from the parking lot.
Not long after that, they spotted one of the resident bald eagles that began nesting here a few years ago; the pair has produced four young so far.
Croasdale, an environmental consultant who lives in South Philadelphia and travels a lot, considers the refuge "probably the most spectacular urban oasis in any city in the country."
"It brings you a wildlife spectacle," he said. "Where else can you see in a city 30,000 ducks in a day and peregrine falcons overhead and osprey fishing?"
It's a landscape that you'd think you'd have to make a trek to, he said, yet it's accessibly by bike and public transportation.
The refuge, created by an act of Congress in 1972, is one of 550 in the nation. It 1,200 acres include the last portions of the once vast and teeming Tinicum Marsh, which stretched from the Schuylkill River to what is now Chester.
It seems an oddity to some, to be sure. The refuge encompasses a former landfill where toxins were dumped. Nearby, trucks thunder by on an I-95 and jet engines whine at the Philadelphia International Airport. The horizon is dotted with skyscrapers.
But, it's important not in spite of where it is, but because of where it is, its supporters point out. For wildlife, it's a welcome spot of habitat in the Boston-to-Washington megalopolis.
Unlike a pristine national park, the point of a refuge is that the critters are there, even if it means they're nesting in an old tire.
About 1,800 people come to the festival. Saturday, they included an entire cub scout pack from the White Rock Baptist Church in West Philadelphia. "It's all about the outdoors," said cub master Terry Johnson, as the boys gawked at an exhibit that was a cut-away view of a marsh.
Plenty of residents of the nearby Eastwick neighborhood came, too, including Carol Simmons, who called the refuge "a national treasure. We have to appreciate it."
For her and others, this year's event was a way to underscore their support for the refugre and their community. A developer wants to build an apartment complex across the street from the refuge; many residents and refuge-goers oppose it.
The event was an extravaganza of wildlife, including a tiny shark in a tank from the Camden Aquarium, plus live turtles, rabbits, snakes and exotic birds.
That, plus a plethora of mascots, from Smokey the Bear to Woodsy Owl, of give-a-hoot-don't-pollute fame. (That's not counting the guy wearing a stuffed buzzard on his head.)
At the Academy of Natural Sciences booth, 11-year-old Garrett Ku of East Norriton - heeding the challenge of "anyone brave enough to hold it? - stuck out his hand to receive a Madagascar hissing cockroach.
"I think cockroaches are cool," he said.
Outside under a tent, Lorrie Schumacher, of the New York-based raptor exhibit, "Talons: Birds of Prey," introduced the crowd to her charges - three owls, two ravens, a hawk and a falcon.
Wally the barn owl was perched on her gloved hand, and one of the ravens, who had just enough length on his leash to sneak under a table to try to steal some food, squawked loudly.
"Igor still has to learn to find his quiet place," Schmacher told the kids.
After leading a morning bird walk, Croasdale headed back out to the marsh.
Over the course of the day, he expected visitors would log sightings of at least 70 to 80 species of birds.
Contact Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or follow on Twitter @sbauers. Read her blog, GreenSpace, at www.philly.com/greenspace