What was to be a four-year program of killing deer at Valley Forge National Historical Park has entered its sixth season.

The National Park Service said that although the deer population has been reduced to the target level of 185 - down from 1,277 when the shootings started in 2010 - culling still is needed to maintain density.

It said deer-population control is critical to plant life that sustains other species in the park.

Citing safety concerns, the agency would not disclose when U.S. Department of Agriculture sharpshooters in night-vision goggles enter the park, saying only that they show up between November and March on unspecified nights. But it did disclose a total: Through March 2015, 1,628 deer have been killed in the 51/2-square-mile park.

The program does not have an end date. That would depend on coming up with a "nonlethal" means of controlling deer, such as a practical and acceptable contraceptive.

The program has rankled some animal-rights activists, such as Priscilla Feral, president of Friends of Animals.

"The hunters have, in our opinion, put a stranglehold on the wildlife in the park," said Feral. "Our question is: What is off-limits to the gun obsession America seems to have?"

Bruce Davis, a retired lawyer, recalled that when he lived near the park, he would come home to find deer munching on his shrubbery. He said that some of his old neighbors had contracted Lyme disease, possibly from deer ticks, and that the deer were such a hazard that he was "afraid to drive through the park."

Pennsylvania is among the top states in the nation for deer-auto collisions, according to State Farm Insurance.

No statistics were available, but the Park Service said such collisions have decreased with the deer population.

In its deer-management plan, the Park Service concentrated on vegetation rather than motor-vehicle safety.

It said the big problem was the park's forest "understory," wildflowers and shrubs that provide habitats for other animals. "Deer were so dominant in the environment that there was little or no habitat for a whole range of species," the plan said.

Before the shootings began, the deer population was estimated to be 241 per square mile. Coming into this shooting season, it was 31 to 35 per square mile.

Also, it said, more than 49,000 pounds of deer meat have been donated to food banks, pantries, soup kitchens, and other organizations in the state.