We are what we eat, but a new study suggests that waterbirds like penguins and storks are more than that. They are how they eat too.

Research recently published in PLOS ONE suggests that evolution has aided these aquatic birds in their quest for food in their aquatic environment. Be it short and stubby or long and wispy, a tail is the key to how a waterbird operates in its realm.

"Previous research has shown that diving birds evolve specializations in wing and leg morphology to facilitate underwater locomotion,” said Ohio University researcher Ryan Felice, who published the paper with his colleague Patrick O’Conner. “This study puts a necessary focus on the tail, finding that this region of the body also evolves in response to the demands of underwater movement."

The researchers examined the tails of roughly 50 species, including storks, pelicans, penguins, gulls, and puffins. Organizing the species by hunting strategy—aerial, terrestrial, and diving—they compared how the tails matched up with the birds’ hunting styles. Six of the species were members of the distantly related group of shorebirds with similar eating patterns, which the researchers say provided a useful point of comparison to waterbirds in understanding the relationship between form and function.

The distinction between the two groups (diving birds and non-diving birds), Felice says, can be found in one tiny bone—the very last vertebrae where the tail feathers attach to the body, otherwise known as the pygostyle. Birds that feed in the air have a shorter and more bent pygostyle while underwater feeders have a longer version that is straighter.

Take for example the cormorant and the penguin. In appearance, the lengthy and fanning tail of the cormorant looks fairly different from the short and stubby tail of the penguin. Still, the two are unified by a common bone. The same goes for puffins, gannets, and tropicbirds. They have all (more or less) developed the same specialized tail, whereas aerial and terrestrial birds have their own style of tail to suit their needs. However, the researchers said they found no connection linking tail shape and flight style.

Felice plans to expand the research to look closer at waterbird morphology. This initial study, though, is a big step towards making both heads and tails of the way birds live.

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