The U.S. Department of Agriculture said Thursday that it considers many factors when determining ways to euthanize large numbers of animals during a disease outbreak, and said it was reviewing an animal rights group’s criticism of a recent proposal for fighting any future outbreaks of bird flu. The agency released the brief statement in response to a letter from the Humane Society of The United States, which criticized a plan outlined last week by USDA Chief Veterinary Officer John Clifford during a Senate committee hearing.