A North American bird that once formed flocks of billions…

A North American bird that once formed flocks of billions, darkening the sky for hours as they passed. Nomadic and highly social, it nested in vast colonies in the eastern forests. Commercial hunting on an enormous scale, together with the clearing of woodland, caused a rapid collapse during the nineteenth century. The last known individual, a female named Martha, died at the Cincinnati Zoo in 1914, making the species extinct.

The answer in 15 letters