addition to the appropriate state hunting license. Harvest of animals other than big game is typically restricted by a bag limit and a possession limit. A bag limit is the maximum number of a specific animal species that an individual can harvest in a single day. A possession limit is the maximum number of a specific animal species that can be in an individual's possession at any time. Shooting Gun usage in hunting is typically regulated by game category, area within the state, and time period. Regulations for big-game hunting often specify a minimum caliber or muzzle energy for firearms. The use of rifles is often banned for safety reasons in areas with high population densities or limited topographic relief. Regulations may also limit or ban the use of lead in ammunition because of environmental concerns. Specific seasons for bow hunting or muzzle-loading black-powder guns are often established to limit competition with hunters using more effective weapons. Hunting in the United States is not associated with any particular class or culture; a 2006 poll showed seventy-eight percent of Americans supported legal hunting, although relatively few Americans actually hunt. At the beginning of the 21st century, just six percent of Americans hunted. Southerners in states along the eastern seaboard hunted at a rate of five percent, slightly below the national average, and while hunting was more common in other parts of the South at nine percent, these rates did not surpass those of the Plains states, where twelve percent of Midwesterners hunted. Hunting in other areas of the country fell below the national average. Overall, in the 1996–2006 period, the number of hunters over the age of sixteen declined by ten percent, a drop attributable to a number of factors including habitat loss and changes in recreation habits. Regulation Regulation of hunting within the United States dates from the 19th century. Some modern hunters see themselves as conservationists and sportsmen in the mode of Theodore Roosevelt and the Boone and Crockett Club. Local hunting clubs and national organizations provide hunter education and help protect the future of the sport by buying land for future hunting use. Some groups represent a specific hunting interest, such as Ducks Unlimited, Pheasants Forever, or the Delta Waterfowl Foundation. Many hunting groups also participate in lobbying the federal government and state government. Each year, nearly $200 million in hunters' federal excise taxes are distributed to state