Matt Rourke, Associated Press
Josh Balk, a spokesman for The Humane Society of the United States, speaks during a news conference Thursday, April 12, 2012, in Philadelphia. Balk says undercover investigators have found deplorable conditions at a central Pennsylvania egg farm.

PHILADELPHIA — An undercover investigation at a central Pennsylvania egg farm found hens suffering in severely overcrowded cages, along with mummified bird carcasses, thick layers of dead flies and other "deplorable" conditions, officials with the Humane Society of the United States said Thursday.

The group's six-week probe in February and March at a Kreider Farms facility in Manheim found many of the birds did not have enough room to spread their wings and were sick or injured, said Josh Balk, the society's director of corporate policy for farm animal protection.

Investigators found dead hens composing alongside birds still laying eggs, birds partially stuck in cages, and the strong ammonia smell that occurs when chicken manure is allowed to build up, Balk said.

"Their quality of life is miserable day after day after day," said Wayne Pacelle, the society's president. "This situation is intolerable."

Pennsylvania is the nation's third-largest egg producing state. Kreider Farms is among its largest egg producers, supplying eggs, milk and other products to retail stores in the mid-Atlantic region.

A message left by The Associated Press with Kreider Farms was not immediately returned.

Dave Andrews, vice president of sales and marketing for Kreider Farms, told WGAL-TV that the Humane Society was misrepresenting the farm conditions.

"They're taking select footage by going through a facility and finding a dead bird or something that may look unsavory, and they're blowing that up on camera and making this look like a representation of how we treat five million birds," Andrews told the station.

The Humane Society also found instances of salmonella at Kreider. Andrews told the television station that one of the buildings tested positive for salmonella and has been cleaned. He said none of the eggs tested positive for salmonella.

The group also is calling for more support of federal legislation that would improve conditions for hens. Among other things, the legislation would require phasing in new housing for hens that would provide the animals more space and contain perches and nesting areas.