BIG BIRDS DON’T FLY!
The Ups and Downs of Flightless Birds
By Jerome A. Jackson
Why in the world would a population of birds, over a period of millions of years, lose the ability to fly? The freedom to fly opened so many possibilities to birds – long-distance migration, escape from earthbound predators, access to new food resources. But some birds have lost the power of flight. The answer to the question seems to lie in the realm of basic economics – the results of a sort of cost-benefit analysis, the genetic hand the birds have been dealt, and adaptation to local conditions.
If a flock of birds gets carried to a remote island by winds of a tropical storm, finds suitable food and shelter there, and no serious predators, the birds may become established on the island, and flight may become an energy-demanding activity that is no longer needed. With luck, random mutations may genetically reduce their ability to fly and body size may increase, allowing them to store more fat when food is available and thus withstand periods when food resources are scarce. Over time, flying birds may be carried away from the island by tropical storms. On the up side, creatures reluctant or unable to fly are more likely to remain on the island and contribute their genetic information to future generations.
If environmental conditions change, flightless birds can be very vulnerable to extinction. The first flightless bird documented to have become extinct was the flightless dodo of the island of Mauritius east of Madagascar in the Indian Ocean. Its DNA reveals that it was related to the pigeon family and likely became flightless when plentiful food and shelter and a lack of predators made flying unnecessary.
Even today some bird species are at varying stages in the transition from flying to flightless. In the Galapagos Islands 500 miles west of Ecuador in the Pacific lives the flightless cormorant – related to the double-crested cormorant of Florida, but flightless for the lack of a need to fly and the likelihood that it won’t stray far from the isolated islands on which it lives. It lacks native predators and feeds on fish caught in the rich waters around the islands. The flightless cormorant still has wings, but they are somewhat diminished and without the stout flight feathers needed to get it airborne. As with our cormorant, the flightless cormorant still extends its wings in the sun to dry after a bout of diving for fish.
Birds such as ostriches in Africa, emus and cassowaries in the Australian region, kiwis in New Zealand, and rheas in South America are all flightless and collectively known as ratites – a term that comes from the Latin word for raft, referring to the flat shield-like breast bone that is raftlike in appearance in that it lacks
the keel that extends
downward like the keel of a sailboat from the breastbone of flying birds. The keel is where the flight muscles of flying birds are attached. A larger keel is correlated with stronger flight ability of the bird.
As these birds lost the ability to fly, their flight muscles were reduced and the keel lost, and their wings serve today only for balance and use in courtship and aggressive displays.
In adapting to their new mode of life, ratite legs became stronger and larger, not only for running, but also for defense. In the evolution of the horse as a running animal from a five-toed ancestor to an animal having a single toe on each foot, a hoof was formed, creating a more efficient foot for running. In the ratites we see a similar progression in the ostrich, the only bird with two hoof-like toes on each foot.
Most flightless birds are large. The best known, and the largest of living birds – sometimes weighing over 300 pounds – is the ostrich. Native to open habitats north, east and south of the central African rain forests, ostriches have been introduced elsewhere and have been widely raised in captivity for their meat, eggs and feathers. In the 1890s, ostrich feathers on ladies’ hats and clothing were at their height of popularity and ostrich farming was popular in Florida. Farming gave rise to ostrich racing – sometimes with a rider on the back of an ostrich, often with an ostrich hitched to a cart. Ostrich meat (and that of other similar large, flightless birds) is high in protein, but low in calories, fat and cholesterol.
The emu, the second tallest living ratite, is native to open areas of Australia. Since the 1980s, emus have often been raised on farms for their dark green decorative eggs, meat, and oil that is used in cosmetics.
The cassowary, native to rainforests and brushy areas of northern Australia and New Guinea, comes in at No. 3 on the height scale, No. 2 on the weight scale, but No. 1 among birds in the danger they pose to humans. The claw – up to five inches long – on the middle toe of a cassowary is used like a dagger with a powerful thrust of its large legs and is a weapon that can kill a human. Cassowaries are forest and brushland birds that crash through the vegetation head first – with a horny casque on top of the head to protect them. They are also the most colorful of the ratites with brilliant red and blue skin on the head and fleshy wattles.
All of the ratites are declining in range and in numbers as a result of loss of habitat and hunting, dogs, fences and vehicles. Other flightless birds have become extinct in recent centuries as a result of the actions of humans and predation by animals that humans introduced to new areas. Many of these now extinct birds have been island species.
Big birds make big targets and provide considerable meat. A ratite known as the elephant bird was known to live on the island of Madagascar – perhaps up to the 17th century. This vegetarian bird stood to about 10 feet tall and might have weighed as much as 1,000 pounds.
There is another major group of flightless birds: the penguins. They have taken flightlessness in a different direction and maintained very strong wings that allow them to dive, to soar through the water on the whims of the tides and currents in search of their meals.