Big invasive snails are driving birds of prey to get bigger

Enlarge / A snail kite with its prey of choice. (credit: US Fish and Wildlife)

The term “Anthropocene” was coined to connote humanity’s recent and indelible impact on the Earth. But it is not only geology and the climate that we have altered; our shuffling of species around the globe has thrown wrenches into many an ecosystem. The invasive species we import have been shown to change rapidly upon entering new territory, but how native species respond to these interlopers has not been as well documented.

In central Florida, there is an endangered bird, the snail kite, that, as its name implies, prefers to dine on apple snails. The size and shape of the kites’ curved bills have adapted to be perfect for plucking the snails out of their shells. These native snails are about an inch in diameter. By around 2009, exotic snails—“considered one of the world’s worst invaders,” according to a new study of this particular predation—had taken over. They are about three times as big as the native snails.

Using a combination of demographic data and genetic analyses of the birds, researchers have found that the new, larger snails have ended up facing larger birds with larger bills. The birds’ body mass increased, and the length of their bills increased—just the change required to cope with the larger prey. Not only did bill size increase, it even increased relative to body size, so even though the birds are larger, they still have more pronounced bills.

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