As the American Civil War ground on, pressure on wildlife from foraging troops must have been intense. Troops need food, and when you have to move 25 miles in a day, sometime food is left behind lost or simply can’t keep up. Any edible animals would be fair game and those men had become very good shots, even though many of them were excellent to begin with.
By 1866, after the conflict was settled, the industrial might of the north had been established. Thousands of men returned to small communities across every part of the country. Production of many agricultural food items including cattle and chickens had been seriously disrupted during the war so many men turned to easy harvesting. The era of market hunting had begun.
Although regulation by states became a protective factor the assumption at the time was that the birds are so abundant we can never kill them all. The same was true for the vast herds of American Bison further west. By the late 1870’s many herds were decimated. Meanwhile, in the upper Midwest, the colonies of Passenger Pigeons were under assault, by 1900 the species was in serious decline and extinct by 1921.
The basic principles of Ecology were not understood, and today we still do not realize how quickly extinction can occur.