This species was the most abundant bird in the world...

The passenger pigeon (Ectopistes migratorius) was a species of bird that was endemic to North America. Its name comes from the French word passager, which means passing by. It lived basically anywhere in North America and it is known to have enormous migratory flocks. The population was an estimated 3-5 billion birds all around the world. The flocks could last up to 3 days in a continuous wave, darkening the sky. Feces rained everywhere and the birds often flew only 1 metre above the ground. One flock, recorded 1866 in southern Ontario, showed that it was 1.5 kilometres wide, 500 kilometres long and took 14 hours to pass. If the birds flew single file, it would stretch around the Earth 22 times. 

The passenger pigeon was a good source of food for North American people. When the Europeans arrived, they were hunted intensively. It wasn't even hard to shoot them down; one bullet can take down several birds! Other methods, such as nets and trapping, were also widely used. Some nets could catch birds in the thousands! At a point, 50000 birds were being killed daily. Soon, this started taking a toll on the wild population and it slowly declined since about 1840. Some states tried to protect the bird, but efforts were futile as many people didn't believe that the pigeon would go extinct. Hunters were rarely punished and by 1900, there were no pigeons in the wild. There were only pigeons in the zoos and breeding them did not work because they didn't want to breed. Soon, in 1909, there were only 3 survivors left at the Cincinnati zoo; Martha and her two male companions. Martha was named in honour of Martha Washington since one of here companions was called George. The third pigeon died on April 1909 and George died July 10, 1910. Desperate attempts and huge rewards were offered if anyone could find a mate but no one did. How ironic that only 70 years before, hunters were killing them by the thousands! Over the years, her health slowly declined until September 1, 1914, she was found lying on the floor of her cage. Her body was frozen in a block of ice and sent to the Smithsonian, where she was stuffed and displayed at the Museum of Natural Arts. They had the second largest flock size, beaten only by the Rocky Mountain Locust, which is also extinct. (Coincidence?)