Appliance of Science: Why do some birds not fly?

Appliance of Science: Why do some birds not fly?
An ostrich meets a crane: Why did some bird species evolve away from flight?

When we think of birds we imagine the few creatures that have actually mastered the art of flying. But it may surprise you to know that there are more than 60 species of bird alive today that cannot fly. Why have wings and not use them?

Some examples

Flightless birds can be found all over the world, from the penguins in the Antarctic to the more northern, flightless cormorants of the Galapagos islands. Ratites (Ostrich, Emu, Rhea, Cassowary and Kiwi) and penguins make up many of the species of flightless birds, but others include steamer ducks of South America, Waka of New Zealand and the smallest flightless birds, the rail bird of Inaccessible Island.

Evolution

Were these birds ever able to fly? There has been much debate about this over the years but many recent studies, including an extensive DNA analysis of species both extant and extinct, report that all flightless birds have evolved from flighted ancestors. This is an example of convergent evolution… all around the world different bird species in different environments lost their ability to fly through independent evolution.

Why lose the ability to fly?

Flight offers birds an incredible advantage, it allows them escape predators, travel large distances and hunt. However, there is one obvious down-side to flight; it requires an incredible amount of energy. Flight also places restrictions on how large a bird can grow and how much it can weigh.

Birds may swap the advantage of flight to develop other abilities. One good example of this can be seen in penguins. By evolving as skilled divers and swimmers they can reach a rich source of food, fish, from deeper waters.

Studies have shown that there is a trade-off though, the better the swimmer, the worse the flight. It seems that it is hard to be both a great swimmer and a good flier.

The more penguins evolved their diving skills, the more their bodies changed, becoming larger with shorter wings; eventually their physiology prevented them from taking off at all.

Other birds have lost the ability to fly simply because they no longer needed to. 

It is no coincidence that so many flightless birds are found on islands. If an island is free of any natural predators, birds will very easily succumb to the evolutionary pressure to give up flight.

Recent studies have even shown that island birds that maintain their ability to fly, such as humming birds, will still evolve heavier bodies, longer legs and weaker flight muscles compared to mainland relatives, and can occurr in just a few generations.

Most species of streamer ducks are flightless as their environment provides them with a good food supply all year round. They have no need to migrate. Even where food supplies are poor, a bird may make a trade-off; swapping the energy sapping activity of flight for a flightless life on a less nutrient food source.

Are flightless birds the same as flighted ones?

In short, no. Most flightless birds have evolved heavier bodies, eventually swapping their light, hollow bones for more dense ones. They will usually develop smaller wings, or none at all. Many grow longer, stronger, more muscular legs and all, bar the penguins, lose the keel of the chest and strong flight muscles.

Evolution pushes for advantage and efficiency wherever it can, even with flight… it is never just about winging it.

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