Genetics leaves little doubt that humans wiped out passenger pigeons

The eye-catching birds of North American that went extinct during the last century weren’t experiencing genetic decline before their disappearance – which suggests humans were probably responsible for their extinction.

“These charismatic species that went extinct in the early 1900s – and one of them in 1988 – appear to have been doing just fine historically over tens of thousands of years, until European colonisation [of North America],” says Brian Tilston Smith at the American Museum of Natural History in New York.

The only parrot native to the US, along with a woodpecker, the passenger pigeon, the prairie chicken and a warbler, were among the colourful birds that once roved the New World in abundance before recently – and dramatically – dying out, says Smith.

To learn more about why they did so, Smith and his collaborators ran DNA sequencing on 100-year-old skin samples from conserved birds in the American Museum of Natural History, representing five recently extinct species: the heath hen (Tympanuchus cupido cupido), the passenger pigeon (Ectopistes migratorius), the ivory-billed woodpecker (Campephilus principalis), the Carolina parakeet (Conuropsis carolinensis), and the Bachman’s warbler (Vermivora bachmanii).

Specifically, they examined genetic markers from numerous individuals representing various geographic areas of the eastern part of North America, which would help tell the story of the species’ demographic past.

To compare with similar species that haven’t gone extinct, the researchers also sampled century-old museum collection skins of the northern bobwhite (Colinus virginianus), the eastern screech owl (Megascops asio), the mourning dove (Zenaida macroura), and the hairy woodpecker (Dryobates villosus).

Using computerised modelling with machine learning that takes several possible scenarios into consideration, the researchers found similar demographic histories for the birds that went extinct, and those that didn’t, says Smith. Neither group showed any consistent, long-term genetic bottlenecking throughout their evolutionary past, from glacial conditions of the Pleistocene to the present.

It was only in the last few decades before their disappearance that the five extinct birds showed signs of loss of genetic diversity as they headed rapidly towards extinction, says Smith.

This points to human behaviour as the primary culprit, he says. European settlers cut down forests, killed crop-raiding birds, and shot birds for sport, and these activities correspond with the rapid decline in bird numbers.

Why such human activities drove some bird species to extinction and not others remains a mystery, says Smith. Some birds might have created more competition for crops, or have been easier to shoot, or needed larger and older forests to nest in. “The ones that went extinct tended to be larger, but a lot of large species survived,” he says.

Journal reference: Proceedings of the Royal Society B https://doi.org/10.1098/rspb.2020.1945, DOI: 10.1098/rspb.2020.1945

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