We’ve all heard the joke (although I’m not sure it should be classed as a joke) about why the chicken crossed the road. Well forget roads, why can’t chickens fly? I was recently asked this question by a good friend who wanted me to investigate; Sherlock was on the case. What about other birds though? Here we’ll discuss some of the most famous flightless birds.
Game birds, such as pheasants and grouse, and the closest ancestor to the chicken (i.e. the jungle fowl) can actually fly, albeit for a very short distance. They do what is called “burst flight”, meaning they exert most of their energy by taking-off almost vertically to flee predators as fast as possible. Chickens however, have been bred to have larger wing muscles which taste better. Breeding has led to such an increase in muscle size, that they are now too big for flight. To fly, birds need appropriate “wing-loading” (i.e. the ratio of body mass to wing area). Domestic chickens are heavier and have smaller wings, making the chance of flight slim. Young chickens however, can sometimes fly but only short distances. If a fence is high enough and the chicken were close to it, they wouldn’t have enough power to take off at a steep enough angle to escape. If they were further away from the fence, so would need a smaller angle for take off to reach the height, they wouldn’t have the stamina to reach it. Chickens are considered very close to being completely flightless now.
Other than chickens, ostriches, emus, kiwis and penguins are amongst some of the many flightless birds, but why are they this way? Ostriches, emus and kiwis, for example, cannot fly as their breastbone lacks a keel (i.e. a large flat bone) which anchors the strong pectoral muscles needed for flight. This leaves their wings pretty useless, unable to lift them and their large bodies off the ground. These flightless birds are known as ratites and have obvious differences to most bird species.
Darwin first predicted that ratites were related, and commonalities were later discovered. For example, the bones in the roofs of ratite mouths are more similar to reptiles than birds (discovered by Thomas Huxley). There is much debate about the origins of flightless birds. It is believed they all stem from a common ancestor who could fly, but independently lost their ability through evolution. For many years, scientists speculated that when the southern section of the supercontinent Pangaea separated 100 million years ago, this split up the ancestors of the ratites onto the various continents in which they’re found today. This theory conveniently explained how flightless birds crossed oceans. However, new evidence shows that the ratites evolved into separate lineages between 45-90 million years ago, not matching the split of Pangaea. It is very possible that ratite ancestors flew to the continents and became flightless later on.
Unlike ratites, penguins do have a keel, but are still flightless. A penguin’s wings aren’t very suitable for flying, and are more adapted to life in the water; more similar to flippers than wings. Their wings are so well adapted to the water that their smooth feathers trap air, making them more buoyant, and protect them from cold water. Alongside added buoyancy, their wings/flippers and tail, help them to stay upright in a delicate balancing act for these wobbly creatures. So why hasn’t this adorable bird evolved to fly? Penguins don’t, and potentially never have, had many predators on land. As a result, these birds never needed the power of flight to flee a predator and instead, focused their evolution on a life dedicated to the sea to increase their chance of survival.
Flightless birds are still of importance in research. Scientists are still studying their DNA to determine the origins of their inability to fly. Recent findings have shown that areas of the genetic code which are usually focused on in such work (i.e. protein-coding genes), may not be the source of an explanation of species diversity and evolution. So ostriches, chickens and penguins may be bound for the ground, but the evolution and genetic code of such creatures is still very much under investigation. Why did the chickens cross the road? Because we made them hench and now they can’t fly. Sorry chickens.