Swallows usually go south in winter.

But in recent years, a particular group seems to have picked one place in all of North America, aside from a slim edge of the balmy Gulf Coast, to spend the coldest months.

It's not a pristine forest. Or a pretty field.

Hard by Delaware Avenue, in a heavily industrial area where 18-wheelers rumble by and power lines crisscross the sky overhead, is the spot they've picked: the sewage-treatment plant in Northeast Philadelphia.

Sewage-treatment plants tend to attract birds - at least part of the time - but apparently this one is way off the charts.

Why they are there is a mystery that has remained unsolved for eight years.

In 2005, Peter Kurtz was taking part in one of the regular winter bird counts, and it was not going well.

The Pennypack environmental educator had been to a spot he thought would be rich with birds. But it was not.

So he drove around the corner, which took him past the plant.

There, swooping over some of the industrial piping, were birds that should not have been there. Not even close.

They were Northern rough-winged swallows, which in summer are common throughout the continent. But by midwinter, they should be in Florida or Central America.

Nevertheless, Kurtz counted 21 of the birds at the plant. The next year, there were 95. Some years, the numbers declined, but this winter's recent census tallied a new high of 142.

Nowhere else in North America are flocks of them found this far north - indeed, anywhere north of all but the most southern locales - in winter.

Given the thousands of participants in nationwide winter bird counts, birders would have discovered other Northern populations, if any existed. But they have not, said Geoff LeBaron, who has managed the National Audubon Society's annual Christmas Bird Count since 1987.

Which has many birders scratching their heads. "When we started to get this, it was big news," said Keith Russell, Audubon Pennsylvania's science and outreach coordinator in Philadelphia. "Everybody wanted to know why this was happening. And we're still not there yet."

Meanwhile, the birds have become a local phenomenon. Birders stop along Delaware Avenue, hoping for a glimpse through a chain-link fence.

One clue: The birds seem to be well-fed.

Swallows normally eat flying insects, which are scarce in winter. Other insect-eating birds that stick around switch to seeds and berries.

It turns out that Philly's sewage-treatment plant is cranking out a profusion of "midges" - aquatic insects about the size of mosquitoes. Some species thrive in sewage effluent.

Since migration is mostly about food, perhaps some birds flying past on their southerly fall route stopped to refuel on the insects and never left. With plentiful food, they never had to.

But why wasn't this happening at other sewage-treatment plants? There's the rub.

A Connecticut birding group reported that midges at their sewer plant have attracted some warblers and Northern rough-winged swallows - but just briefly.

Other sewage-treatment plants report a variety of birds - 242 have been documented at the Roanoke, Va., plant, according to a 2006 report - but the unexpected ones are usually just transients.

Having midges in a sewage-treatment plant "is typical and unremarkable," said Jason Weintraub, entomology collection manager at the Academy of Natural Sciences of Drexel University, where Russell took some of the bugs, hoping to have them identified.

It turns out that midges are a large and diverse family, with more than 10,000 species worldwide. Differentiating some species is not merely a matter for microscopes, but requires analysis of their chromosomes.

"A very difficult group," Weintraub said.

Russell has not seen midges in great numbers at Philadelphia's other two sewage-treatment plants, however.

Plant maintenance manager Richard Stasiorowski said the midges have bred in a final treatment pool where disinfectant is added to the wastewater. Because the water flows underground to get there, and the pool itself is below ground level, its temperature stays at a fairly constant 55 degrees.

Stasiorowski wonders if the Philly plant, built in 1980, has just enough differences from other plants to suit the midges and the birds.

"We have the right arrangement, I guess," he said. "We have a mini-ecosystem."

Indeed, the midges are not the only effect. Russell suspects that the swallows survive the coldest weather by roosting on pipes close to the warmer water.

Anomalies turn up often in the bird world. Populations are generally in flux, and now an increase in the number of birders, plus a rising skill level, means people are more likely to spot the odd and unusual, LeBaron said. Advances in digital photography allow them to document it.

To Audubon's Russell, the Northern rough-winged swallows are perhaps a shining example of one of the things birds do best: They adapt.

It has happened in chimney swifts, which once bred in large hollow trees until Europeans came and cut them all down. As it happened, the early settlers also built pretty good substitutes - chimneys - which led to the birds' common name.

Likewise, beach-nesting birds have started building nests on rooftops to escape a proliferation of dune buggies, kids, and dogs. Peregrine falcons have moved from cliffs to bridges and tall buildings.

Barred owls that once shunned the suburbs are sitting in backyards and getting koi out of people's ponds.

"Birds have learned a lot of things," Russell said.

For the swallows, the trade-off in staying in a colder climate means that they do not have to migrate as far, a journey fraught with danger.

"Predators can get you. Storms can get you. It's a lot of work," Russell said.

"I think these birds somehow figured out, 'We can stay here,' " he said. And then, in whatever birdy way they communicated it, "they started bringing more of their friends."