Passenger pigeons went extinct a century ago when the last member of the species passed away on 1 September 1914.

Martha, the last member of her species, died at the Cincinnati Zoo just before her 29th birthday. The bird was named after Martha Washington, wife to the first president of the United States.

Passenger Pigeons once filled the skies of North America, with populations estimated to be between three and five billion animals. During the 18th Century, they may have been the most common bird on the planet. The species developed in eastern regions of North America and migrated west in search of hard nut fruits called mast.

"The number of these birds... was far beyond the power of human calculation. for hours they darkened the sky like a pall of thunder clouds; and that they broke down, by their weight, the limbs of the forest whenever an entire flock lighted in search of food," Philip Bruce, a 17th century historian, wrote.

Along with the dodo, the passenger pigeon is one of the animals being considered for a return from extinction. For the first time, biologists have the technology to bring an extinct species back to existence. Using DNA, researchers would be able to bring back any recently-extinct species, such as passenger pigeons, dodos, or woolly mammoths.

De-extinction is a controversial idea among both scientists and the general public. While some critics are wary of a return of species such as the saber-toothed tiger, the technology also serves another purpose. At current rates of population loss, elephants, zebras and giraffes could all disappear in the near future. Development of the animal's genetic code could preserve the species.

"Techniques being developed for de-extinction will also be directly applicable to living species that are close to extinction. Tiny populations can have their genetic variability restored. A species with a genetic Achilles' heel might be totally cured with an adjustment introduced through cloning," Stewart Brand, creator of the Whole Earth Catalog, said.

Passenger Pigeons were so successful when found in nature, they could prosper following a return from extinction. This trait could also make the birds the ultimate invasive species. With rapid population growth and the ability to thrive across North America, passenger pigeons might radically alter the environment over the entire continent, if they are able to survive.

"Yes, the set of plants alive a century or so ago when the passenger pigeon went extinct are probably still here. Is the pigeon's habitat intact? Surely not: The land use changes since then have been far too extensive," Stuart Pimm of Duke University, said.

The genetic code of animals decays over time, placing a limit on the amount of time a species can be extinct before being brought back through de-extinction. This means dinosaurs will not be walking the Earth again anytime soon.