The United States Department of Agriculture's Wildlife Services program kills millions of animals annually - whether it's because they are considered a nuisance or killed by accident - and some think the agency may be overstepping its boundaries.

In 2013, more than 4 million animals were shot, poisoned, snared or trapped by the department, among them bobcats, coyotes, river otters, foxes and black bears, The Washington Post reported.

The agency's primary responsibility is to deal with invasive species such as feral hogs, giant swamp rats called nutria, Argentine lizards called tegus, and starlings. However, according to a release from the Center for Biological Diversity, sometimes native species are killed if they're perceived as a threat.

For instance, birds at about 800 airports around the country are killed so they don't clog the gears in airplanes, Wildlife Services spokeswoman Lyndsay Cole told the Post.

Wildlife claims it works to "resolve human/wildlife conflicts" in a strategic way. "As wildlife damage increases, requests for assistance also increase," said spokeswoman Carol Bannerman.

They have yet to explain why their numbers fluctuate each year, from a relatively low 1.5 million one year to spike to 5 million the next. Since the "rogue agency" doesn't keep detailed data about why and how the animals are killed, two members of Congress have called for an investigation into how it operates.

Rep. Peter A. DeFazio (D-Ore.) has described the agency as "one of the most opaque and obstinate departments I've dealt with," the Post reported.

Last December, the Center for Biological Diversity filed a petition demanding that the agency explain the exact reasons why it makes each kill of a native animal, for whose benefit and the methods used.

"We're really not sure what they're doing. I've asked the agency to give me breakdowns on what lethal methods they're using," DeFazio added, according to Ecorazzi. "They can't or won't do that. We've asked them to tell us what goes into their poisons. They won't say."