The passenger pigeon (Extopistes migratorius) was once the most abundant bird on Earth. Flocks of millions of birds darkened the skies of North America, and took days to pass overhead. When they descended on a forest they would strip it bare of nuts and acorns, break branches with their sheer numbers, and carpet the ground with droppings, which eyewitnesses said “fell like snow”. People firing randomly with shotguns or waving sticks would knock down dozens, to be packed and shipped east as cheap food. Yet, incredibly, by the end of the 19th century, the birds had almost disappeared.
The last passenger pigeon, named Martha, died in Cincinnati Zoo in 1914. Her body was immediately frozen in a block of ice and sent to the Smithsonian Institute. Other museums had been collecting specimens as the pigeons approached extinction; there are about 1500 in museum collections around the world, including three in the Whanganui Regional Museum. But it wasn’t collectors or even hunters that ultimately wiped out the passenger pigeon, although they were hunted in staggering quantities, but forest clearance. The birds relied on huge forests of nuts to maintain their enormous flocks, and they needed to breed in enormous communal colonies. Once enough forest had been cleared for agriculture, flocks dropped below a critical threshold for breeding, and during the 1870s the population crashed. Past this point, they were doomed as a species, though a few persisted into the 20th century.
Recently, researchers examining passenger pigeon DNA were surprised to find that despite their abundance the birds had quite low genetic diversity, suggesting that their population had only recently exploded when European settlers encountered them. The best explanation is that pigeon numbers went through periodic booms and busts, in response to regular fluctuations in the availability of nuts and acorns, which occasionally have super-abundant “mast” years. Pigeons had evolved to seek out heavy nut crops and rapidly increase their numbers to exploit them. When Native Americans were devastated by disease following colonisation, they stopped hunting pigeons and collecting nuts, and passenger pigeons quickly increased to enormous numbers. Their population was going through a natural downturn at exactly the time European forest clearance and hunting began to increase, and those combined forces were enough to push them into a death spiral.
To estimate pigeon genetic diversity for this study, the researchers needed DNA, which is hard to get from a species extinct for 100 years. They turned to museum specimens, and were able to take tissue scraped from the toe pads of passenger pigeon skins and use recently-developed technology to extract fragments of DNA and reconstruct long sequences of the bird’s genome. This ability to recover ancient DNA has made museum collections valuable in a new and unexpected way: from them we’ve learned that female moa were twice the size of males, that the kiwi and the Madagascar elephant bird are close relatives, and, now, why the passenger pigeon went extinct so rapidly. Even if they died over a century ago, museum specimens can, in a small way, live again.