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.
