At Peace Valley Park in central Bucks County, another birder saw a jaeger, a bird that most people don't see unless they're 50 miles out at sea.
And from the parking lot of the Plymouth Meeting Mall, a birder shot a picture of a frigate bird - also found far at sea over tropical oceans - with his smartphone.
Hurricanes are known for ferrying birds along with them - especially, given the track of hurricanes such as Irene, southern birds.
The storms also sweep along seagoing birds, which "can't just duck into a woodland or a field when it gets windy and rest it out," said Pennsylvania Audubon's Keith Russell. But they can stay aloft indefinitely.
No doubt many birds die in storms. But others make it to new worlds. "It just really shuffles the deck," said New Jersey Audubon's Pete Dunne.
So once Irene seemed to be easing, area birders grabbed their binoculars and spotting scopes and headed out.
Birders describe hurricane birding as a rite of passage. In some cases, it's a dicey one.
In Cape May, where late-storm westerly winds will push the birds up along the Delaware Bayshore, more than a dozen serious birders were out at dawn Sunday, scanning waves that they put at 10 feet. The wind was creating a sandstorm that was strong enough to scratch the print from bumper stickers.
One was Vince Elia, research associate at the New Jersey Audubon Society. "I always say, it's the most exciting birding that there is," he said. "If I'm in Costa Rica, I know the birds I'm expecting to see. The thing with a hurricane is, you just don't know the next thing that's going to come around the corner."
He feels a little sheepish at his excitement. "All of us realize what comes with a hurricane - devastation, loss of life." But hurricanes happen, "and there's not a lot anyone can do to change that."
The group saw a few tropical terns, which were nice. Some jaegers went by.
Then Mike Fritz screamed "white-tailed tropic bird" and "we all just about fell over," Elia said. Before that, only one had ever been seen in New Jersey.
Later, fellow birder Tony Leukering called out, "big swift!"
What we have in this area are chimney swifts, which Elia referred to as "a dinky swift," often said to resemble a cigar butt with wings.
This bird was bigger. Could it be a white-collared swift? No one got a good enough look to be sure. But Elia, who later confirmed that it breeds in a particular mountain range of the Dominican Republic that Irene had swept over just days before, cannot imagine it was anything else.
Up on the Delaware River near Pennypack Park, Frank Windfelder, president of the Delaware Valley Ornithological Club, scanned the water.
Forster's terns are the only ones normally seen on the river. But here he was spotting black terns and least terns and maybe 30 common terns - many perhaps swept over from the Jersey coast.
He was trading cellphone calls with Tom Bailey, across the river, who spotted a bridled tern, common along the southeastern U.S. coast.
A few miles downstream, George Armistead and Matt Sharp saw a sandwich tern, another common southern bird, although it does visit New Jersey - and the first confirmed sighting in Philadelphia, according to Audubon's Russell.
In all, they saw about 10 tern species. "These birds were just streaming by," Windfelder said.
They also saw classic Jersey shorebirds such as willets, ruddy turnstones, and even a sanderling.
"These birds are pushed inland," Windfelder said. "Once they find water, they know to head downriver."
On birding listservs, more sightings were coming in - the Peace Valley jaeger, the Plymouth Meeting frigate bird. And more.
Many saw sooty terns, described by some as the quintessential hurricane bird because it flies high over the ocean and is thus more apt to be caught in storm winds.
Alas, by Monday, it was pretty much over.
Windfelder went out by the river again and saw nothing but local plumage.
"When the sky clears," he said, "they get up in the air and they're gone."
Contact staff writer Sandy Bauers at 215-854-5147, firstname.lastname@example.org, or @sbauers on Twitter. Visit her blog at philly.com/greenspace