The body and beak size of endangered North American snail kite birds have increased in the ten years since snails larger than those that they usually prey upon invaded their territory. The findings, published online this week in Nature Ecology & Evolution, are an unusual example of rapid change of predator species in response to an invasive prey species.

Snail kites (Rostrhamus sociabilis) use their curved beak and long claws to extract the meat from the shells of apple snails. Populations of these birds have been in severe decline, but research published in 2016 showed that kite numbers are on the rise again. In the same timeframe, an invasion of a different species of apple snail (Pomacea maculata) has swept into their territory. This was surprising, because the invasive snails are much larger than the kites’ usual prey, and the birds typically have difficulty getting at the meat of larger snails.

Here, Robert Fletcher Jr and colleagues report that the overall body mass and the relative beak size of the kites have increased over the past 10 years - which is only about 1.5 kite generations. The authors also find that fledgling birds larger in body and beak size are more likely to survive their first year than smaller or smaller-beaked birds. Statistical tests show this effect is most likely to be a change in the birds’ expressed phenotype rather than a genetic change, at this point in time. However, the authors suggest that this snapshot of the early stages of a predator’s response to an invasive species of prey indicates that evolutionary change is imminent.