The extinction of the passenger pigeon is an example of what can happen to a species when conservation efforts are implemented inly after a population falls below its critical density. Until the mid-1800s ,fights of passenger pigeons darkened the skies of the mid-western United states for day at times. One flock was estimated to contain at least 2 billion birds. Hunting drastically reduced the population of the pigeon in the 1870s,and by 1885 only one breeding colony was left. Hunting of the pigeon was eventually prohibited ,but the various groups of pigeon that formed separate population repeatedly failed to reproduce. In just 50 years, the passenger pigeon went from being North America’s commonest bird to being extinct. In hindsight, it is evident that the presence of thousands of bird flying, flocking , and roosting together stimulated breeding . Once flock sizes were reduce below this critical mass, breeding failed. The last passenger pigeon died in the Cincinnati Zoo in 1914.