Conservation of migratory species.

Were you to find yourself somehow transported to the American Midwest in the 18th century, one feature that might strike you would be a curiously shifting cloud approaching from the northeast. As it drew closer, you might begin to discern some sound, "[like] an army of horses laden with sleigh bells" in the words of a Potawatomi Native American. However, not until it was nearly upon you would it become apparent that this cloud was made up of billions of individual birds. This was the autumn migration of the passenger pigeon (Ectopistes migratorius), the most abundant bird to ever be found in North America and perhaps in the world (Figure 1). By some estimates, the number of passenger pigeons in North America at the time of European colonization was greater than the combined number of all birds of all species found on the continent today. Yet, by the beginning of the 20th century, passenger pigeons had effectively been eradicated, in large part due to the extraordinary keenness with which they were hunted. By 1914, 5 billion birds were gone and the last passenger pigeon, Martha, died in the Cincinnati Zoo. Passenger pigeons represented an extraordinary example of one of Earth's great natural phenomena: the annual migration of organisms. They also highlight the potential vulnerability of migratory species. These seasonal movements are not only awe-inspiring; they are also critical for the health of countless species and ecosystems.