The passenger pigeon was exceptional for a number of reasons. In modern times, it is the only pigeon species that was not an island species to have become extinct even though it was once the most numerous species of bird on Earth. Its former numbers are difficult to estimate, but one ornithologist, Alexander Wilson, estimated one flock he observed contained over two billion birds. The decline of the species was abrupt; in 1871, a breeding colony was estimated to contain over a hundred million birds, yet the last individual in the species was dead by 1914. Although habitat loss was a contributing factor, the species is thought to have been massively over-hunted, being used as food for slaves and, later, the poor, in the United States throughout the 19thcentury.