It is believed that the population of the Passenger Pigeon started declining with the European colonisation of North America. However, it seems that the most critical phase was during the 1870's. At the beginning of the decade huge flocks still existed but by the end the populations were so dispersed that their hold on survival was broken for ever.

There are a number of reasons for the birds extinction, such as habitat loss as the deciduous woodlands of North America were ravished as Europeans settled on the land. However, undoubtedly the most destructive factor for the species extinction was due to over hunting. It was reported that for one hunting competition, over 30,000 dead birds were needed to claim a prize!