Serious birders know that the avian set loves places that humans often consider unsavory.

Landfills and sewage treatment plants attract diverse bird species, and the binocular-toting humans who follow in hot pursuit.

The wastewater treatment plant in Northeast Philadelphia, located in an industrial area near the Betsy Ross Bridge, is one such less-than-scenic birder haven. New winged residents this winter have been attracting more attention than usual to the plant.

New bird in town

To make their pilgrimages to the plant, birders park on the side of the street and walk a few hundred feet along abandoned train tracks, stopping at a chain-link fence to peer at four-million gallon holding tanks on the other side.

For years, birders have been lining this fence, occasionally sneaking under the gate, for a close look at the flock of northern rough-winged swallows that have called this spot home since 2005.

The birds typically migrate to the Gulf Coast and southern Florida, and Philadelphia is the only place in the northern U.S. a population has ever over-wintered.

Standing by the fence, visitors can watch from underneath as the swallows swoop from the treatment plant pools over the fence to nearby trees.

"There's these times when they're really moving through in numbers, and then they kind of dissipate and disappear, and then they come back again," said George Armistead, a lifelong Philadelphia birder and events coordinator for the American Birding Association. 

This winter, for the first time, the rough-winged swallows are being joined by a handful of cave swallows, their flashier, southern cousin.

"Their population has been exploding for some time, but to see them in winter was really surprising because there's really no place within hundreds of miles where they can make it through (the winter), " Armistead said.

Cave swallows typically live near the Mexican border and migrate to the tropics for the winter. They have started making late-fall incursions north, a trend conventional wisdom among birders attributes to climate change. Until now, the cave swallows have always flown back south when the truly cold weather strikes. So spotting one in Philadelphia in February was enough to make even an experienced birder like Armistead excited as he tried to point one out at the treatment plant.

"They've got a nice cinnamon rump, all this cinnamon in the face," Armistead said. "They're a pretty dapper looking little swallow, really."

Drawn to a colder clime

Why are these tropical transplants here? Part of the answer: Bugs. Tiny, flying midges, which spend part of their life cycle in water.

The process manager at the sewage treatment facility, Anthony DiGironimo, showed off the inside of the facility, where midges hover over the vast treatment pools.

"You can see the birds are dive-bombing around, getting as many as they can," Digironimo said.

The open pools are the last stop for treated sewage before it is released into the Delaware River. The water is mostly kept underground while it is treated, so it stays at a relatively constant 55 degrees.

That means the midges can stay alive all winter, serving as a bird buffet in a frozen landscape.

Next to one of the pools, DiGironimo opened up an old equipment box to show me midges seeking refuge there on a windy day. Stuck in spiderwebs, they had clear wings and looked like small mosquitoes without the blood-sucking proboscis.

Ongoing mystery

The midges aren't the whole story. The tiny flying insects are found at many water treatment plants around the country. Birds that typically spend winters in the tropics are not.

"This is a real mystery, why, for the last eight or so years, this has been the only place where this appears to be going on," said Keith Russell, outreach coordinator for Audubon Pennsylvania.

Russell is so intrigued by the relationship between bird and midge at the sewage plant that he captured samples of the insects and took them to the Academy of Natural Sciences to get them identified. His plan was foiled, though, when he was told that only midge larvae, not adults, can be identified.

"We weren't able to definitely find out what species this is, to know more about why this is happening here and not other places," Russell said.

Russell thinks something about the midges themselves attracted the swallows — he said he has not seen them at other Philadelphia treatment plants.

Others have different theories as to why the swallows chose this sewage facility over others: maybe it is the plant's proximity to a waterway, since many birds use rivers to navigate. Or maybe there is a warm place to roost at the plant that doesn't exist elsewhere.

Russell hopes swallow appearances at other sewage facilities eventually will produce more clues.

For now, he thinks of the swallows as an important reminder to us humans.

"As we continue to change the landscape and manipulate things for our benefit we also create conditions that can benefit or affect birds and other animals, and these birds are just exploiting some very unusual conditions where flying insects are available all winter long in Philadelphia," Russell said.

Birders around the country will be watching this winter to see if the cave swallows can survive the cold.