PITTSBURGH -- Hawks swoop in and gobble up songbirds. Raccoons feast on nests of eggs they never could have reached before. Salamanders and wildflowers fade away, crowded out by invasive plants that are altering the soil they need to thrive.Like a once-quiet neighborhood cut up by an expressway and laced with off ramps, northeastern forests are changing because of the pipelines crisscrossing them amid the region's gas drilling boom, experts say.Environmentalists have loudly worried that hydraulic fracturing, or fracking, may threaten water and air, though the Obama administration and many state regulators say the practice is safe when done properly.Threats to wildlife, though, have flown largely under the radar. But as studies detail plans for thousands of miles of new pipelines and related infrastructure, the dangers to biologically rich forests that have rebounded since vast clear-cutting in the 1800s are taking on new urgency."If you wanted to create a perfect storm for biological invasion, you would do what the energy companies are doing in north-central Pennsylvania," said Kevin Heatley, an ecologist with the national firm Biohabitats who works to restore areas that have been damaged by human activity. "You can only put so many bloody parking lots in the woods."Energy companies, which say they are being responsible stewards of the land, have rushed to unlock the natural gas lying in the shale beneath Pennsylvania, Ohio and West Virginia. The gas has lowered energy costs, allowed the U.S. to lessen reliance on foreign energy and provided private landowners who sit atop well sites with a gold mine in royalties. New York, which also has large reserves, is trying to decide whether to allow fracking.The new energy development is "almost a spider web coming down to the forest," said Nels Johnson of the Pennsylvania chapter of The Nature Conservancy, which estimates the state could see thousands of miles of new pipelines over the next two decades.Even northeastern states that have put a hold on fracking aren't immune, because many import natural gas.The U.S. Energy Information Administration found that 245 miles of new pipelines were laid in the Northeast last year, and that figure is projected to grow.Wind turbine development poses similar threats, too. The Nature Conservancy says Pennsylvania already has more than 600 of the giant blades, with the potential for thousands more in coming decades.The total acreage taken up by the pipelines, wind projects and related development isn't that large, but the open spaces they create allow predators and invasive species to permeate a canopy of trees that once kept them at bay.It's not hypothetical, scientists say. Studies and observations have documented invasions. And just as with humans, the uninvited guests change the neighborhood.Forest fragmentation opens the door to invasive species such as the cowbird, a type of blackbird that normally prefers open land, said Bridget Stutchbury, a biologist at York University in Toronto who studies forest songbirds."The female cowbird sneaks around the forest, laying her egg in other species' nests," Stutchbury said. Forest birds such as thrushes and warblers don't realize the egg isn't theirs and expend energy raising chicks from another species in what she called a "nifty strategy for child-rearing."A report by the U.S. Geological Survey, released last week, found that in Pennsylvania's Susquehanna County, at the heart of the drilling boom, the number of patches or sections of forest increased by about 156 between 2001 and 2010, with Marcellus Shale drilling and related pipelines responsible for most of the change.The energy industry said that it welcomes original research, but that it should be seen in context.The Geological Survey report, said Patrick Creighton, a spokesman for the Marcellus Shale Coalition, an industry group, found that oil and gas activity affected less than 1 percent of the forest area in Allegheny and Susquehanna counties.