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Being "common" isn't what it used to be. According to a recent study, a whopping 421 million birds vanished from Europe over the past three decades—and the vast majority were common species.

Using data compiled from three decades of citizen scientists' observations, researchers determined that more than 420 million individual birds had disappeared, according to results recently published in the journal .  Records showed that bird numbers are down to about 1.6 billion in Europe, compared to over 2 billion just 30 years ago. That means Europe has lost close to one percent of its birds—about 10 million individuals—annually.

"We suspect that the declines are being caused by agricultural intensification and changing agricultural practices," says lead researcher , associate research fellow at the University of Exeter. In particular, monocultures and increased pesticide use are likely to blame, he says.

Around 90 percent of the lost birds were from the 36 most common species in Europe, such as Common Starlings, House Sparrows, and Skylarks. Rare birds are likely affected as well, but because common birds are so much more numerous, they tend to dominate these types of population surveys. Inger points out that common species are by no means immune to extinction—just look at the .

Common species play vital roles in their ecosystems, such as pest control, and are more unique than their name implies.

"Being common in itself is a rare state—very few species are common," Inger says. "So common species are actually quite exceptional."