A federal judge has given Valley Forge National Historical Park officials the go-ahead to let sharpshooters start killing deer in the park next month. Animal rights groups filed a lawsuit to stop the shooting, but U.S. District Judge Mitchell Goldberg ruled against them Wednesday, saying the destruction the deer have caused justifies an exception to a federal law aimed at conserving all wildlife in national parks. Valley Forge Park managers say it can support about 35 deer per square mile, and the current population is about 240 per square mile. They say the large and destructive herd is devouring young trees and other environmentally-sensitive vegetation. A contract with the federal agency that employs the sharpshooters calls for eliminating 500 deer this winter, another 500 next winter, and 250 to 300 more in each of the last two years of a four-year-deal.