The passenger pigeon may have once constituted 25 to 40 percent of the bird population in what is now the U.S., according to the Smithsonian Institution. As many as 3 to 5 billion of these birds were alive when Europeans arrived.The birds' traditional habitats were the large forests of eastern North America. As settlers cleared the forests for farmland, the pigeons turned to the new fields for subsistence."The large flocks of passenger pigeons often caused serious damage to the crops, and the farmers retaliated by shooting the birds and using them as a source of meat," explains the Smithsonian.The 19th century brought widespread hunting and trapping of the birds, which severely diminished their populations. The last passenger pigeon, named "Martha," died at age 29 at the Cincinnati Zoo in 1914.