Five billion passenger pigeons once spread across North America in flocks so big that they could block out the sun and spread for hundreds of miles. Then, in just a few decades, they were gone. Now, scientists have found the cause for their devastating decline, and it reveals how fragile even large populations of animals can be.

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A new study shows that passenger pigeons have existed since the Ice Age in massive populations. But as large as their numbers were, they lacked genetic diversity. This allowed them to adapt quickly within the population, but it also meant that when a new threat came along – in this case humans – they couldn’t adjust quickly enough. While hungry humans are the main cause of their decline, the genetic shift caused by the shift from massive groups to small groups sped up the decimation of pigeon populations.

Related: The sixth mass extinction is killing off wildlife 100 times faster than “normal”

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“We often think of large populations as a safeguard against extinction,” said the study’s co-author, University of California, Santa Cruz biologist Professor Beth Shapiro. “What this study shows, however, is that we must also consider the longer-term natural history of a species when making decisions about their extinction risk.”

Via The Independent

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