Sure, it's a miracle of nature, an echo of a time in America, ornithologists say, when flocks of hundreds of millions of birds could darken the sky for hours as they passed over.

But when thousands of crows choose your trees and your neighborhood for a winter roost, the "miracle" can mean raucous evenings and a messy walk to the car the next morning.

"It's fascinating in a way, and problematic in another way," admitted Vicki Hoagland, of the Original Northwood section of Baltimore. A slice of her neighborhood, and of the adjacent Hillen and Ednor Gardens-Lakeside communities, have been a winter crow roost for as many years as residents can recall.

After sundown these days, they drape the tall trees like black tinsel on both sides of The Alameda, from just north of 33rd Street, past the Veterans Affairs Rehabilitation Center and along Loch Raven Boulevard as far north as the Northwood Shopping Center.

"At times they're squawking so loudly, you can't hear somebody talking to you," said Greg Rex, president of the Original Northwood community association.

Longtime residents Stefan Goodwin and Dean R. Wagner, in an e-mail to The Baltimore Sun, described the crows' arrival more lyrically, as a welcome sign "that winter is on the way."

"For a couple of weeks during the fall, large flocks of crows sometimes migrate through the beautiful old growth trees for an hour or so as the sky darkens," they said, adding that the birds "can occasionally be a bit noisy for a half-hour or so," but then move on, "leaving Original Northwood as peaceful as before."

In fact, the birds do quiet down at night, but they stay for the night, and for the winter, Rex said. And their presence is evident each morning.

"They just mess all over everything," Hoagland said. "You never know when you go to your car in the morning whether you can put your hand on the handle, or what's going to be on your windshield."

The big black birds have been gathering each evening since Halloween, filling the tall oaks, cawing, squabbling and flapping around before settling in for the night, or moving to another roost.

They'll fly off in the morning to forage in the country, and return every evening, no matter what residents may do to discourage them.

"You can go outside with pots from the kitchen, and bang on the lid and they will take off immediately," Hoagland said. "But they circle around and come back, and they'll be all settled again within 20 minutes or so. It's kind of a fascinating phenomenon."

Crows have been gathering like this on winter nights "for as long as there have been crows," said Kevin McGowan, a crow expert at the Cornell Laboratory of Ornithology, in Ithaca, N.Y. Some roosts can draw as many as 2 million birds. Scientists have documented sites that have persisted for more than a century.

"Perhaps there's a nice hill, with a bunch of big trees, and it's an attractant; so they come," he said. "I think it's quite conceivable there are honest-to-god traditions - that old crows bring younger crows, or birds from last year remember where they're going. But we don't know about that."

Scientists suspect the gatherings are partly defensive, partly social. "Crows are like people; they don't like to be alone," McGowan said.

There are often staging areas - noisy, busy places where the birds gather and mix before flying off, a mile or so, to an overnight roost, which tends to be quieter.

Ornithologists suspect the large gatherings also enable "information transfer," McGowan said. The birds, by observing each other fly in and out, may gain clues to the direction of good winter forage.

What does seem relatively new is the crows' willingness to winter near people.

The winter roosts "seem to be urbanizing all across most of North America," McGowan said. "There are still some rural roosts, but [urban roosts] are getting more and more common."

Again, scientists can only guess at the reasons. "There are things we know they should like," McGowan said, such as the urban "heat island" effect, which makes cities a few degrees warmer than the country.

And, "they tend to like being around lights," he said. "They don't see very well in the dark," and urban lights may help them guard against their greatest threat - the great horned owl. "A night light is a great thing."

In the past, in a more rural America, crows avoided people, because farmers hunted them relentlessly to protect their crops. Hoagland said she grew up on a farm on the Eastern Shore, "and I remember my grandfather ... shooting one of them and hanging it up" as a warning: " 'You're not wanted here' ... That wouldn't go over too well here in Baltimore City."

As hunting pressure waned, crow numbers increased, McGowan said, and more of them learned they were safer near dense human populations, where firearms can't be fired, legally.

"I always liken it to a tired boxer," McGowan said. "If you're tired, you don't run away from your opponent; you go in and grab him." For crows, cities may now be the safest places to roost for the winter.

Crow numbers were growing until early in this decade, when the West Nile virus arrived in North America. Mosquitoes spread the dangerous West Nile fever among both humans and crows, and dead crows became a primary marker for the virus' advance across the continent.

Crow populations dropped sharply for several years. The most reliable estimates come from the annual Breeding Bird Survey run by the Patuxent Wildlife Research Center, which uses a standardized counting protocol, then converts the data into an index number to reflect population changes.

In Maryland, the crow index reached a high of 57.8 in 2001. Then it fell by almost half, to 31.9 in 2004. It has since rebounded to 41.7 in 2007, McGowan said.

So the crows are coming back. But they have never entirely left the neighborhoods around Loch Raven Boulevard and The Alameda.

Hoagland and her husband, Lou Borowicz, bought their home six years ago, delighted to find a quiet neighborhood with fine old homes from the 1930s, mature hardwood trees and "a little bit of nature" in the city, even foxes.

No one mentioned the crows.

"We were here a few months before we actually saw it happen," she said. "We said, 'Oh my gosh! What's this?' "

"There are people who have been here for a long time, and [for them] it's just part of life in the neighborhood," she said. "I sort of look at it that way, too, although sometimes it's really aggravating."

After a few days during the winter crow roost, she said, a good hard rain is always welcome.

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