The Collaborative International Dictionary of English v.0.48:

passenger pigeon \passenger pigeon\ (Zool.),
   A once common wild pigeon of North America (Ectopistes
   migratorius), now extinct. It was so called on account of
   its extensive migrations.
   [1913 Webster +PJC]

   Note: The passenger pigeon presents a striking example of how
         dramatic a negative influence man can have on other
         species. The population of the passenger pigeon is
         estimated to have been at one time as high as five
         billion in North America, but over a period of about
         one hundred years large scale hunting for use as food
         and killing for "sport" reduced the numbers below that
         necessary to sustain existence of the species. At one
         time over 200,000 birds were shipped to the food
         markets in one day. The last wild pigeon is believed to
         have died in 1900, and the last bird in captivity died
         in 1914. It is a rare example of the date of a man-made
         extinction being recorded with certainty.
         [PJC] passe-partout

WordNet (r) 3.0 (2006):

passenger pigeon
    n 1: gregarious North American migratory pigeon now extinct
         [syn: passenger pigeon, Ectopistes migratorius]