Life on Earth by David Attenborough

Author:David Attenborough
Language: eng
Format: epub, azw3
Tags: Non-Fiction, Science, Environment, Nature, Biology, Natural History, Animals
Publisher: HarperCollins Publishers
Published: 1978-12-31T13:00:00+00:00

So feathers did not evolve initially for flight and do not define birds. The birds inherited them from feathered ancestors. And this leads to the realisation that all dinosaurs were not exterminated when the asteroid crashed into Mexico 66 million years ago. Some of those with feathers survived. So dinosaurs today are flying around our gardens.

But evolution would have to bring many profound changes before those little feathered dinosaurs became the extremely efficient flying creatures we know today. The overwhelming evolutionary pressure came from the advantages of reducing weight. Archaeopteryx’s bones were solid, like a modern reptile’s. Those of true birds are paper-thin or hollow, often supported inside with cross-struts which closely resemble those designed to strengthen the wings of aeroplanes. Birds’ lungs are extended into air sacs which bulge into the body cavity, so filling space in the lightest possible way. The heavy bony extension of the spine that formed the basis of Archaeopteryx’s tail has been replaced with stout-quilled feathers requiring no bony support of any kind. A weighty jaw, laden with teeth, must have been a particular handicap for any creature trying to fly, for it would tend to unbalance the animal and make it very nose-heavy. Modern birds have lost it and have developed instead another lightweight construction of keratin, the beak.

But even the best beak cannot chew, and most birds still have a need to break up their food. They do so with a special muscular compartment of the stomach, the gizzard, as some of their distant sauropod ancestors did. So the beak itself has to do no more than gather the food.

The keratin of the beak, like that of the reptilian scale, seems to be easily moulded by evolutionary pressures. Just how quickly it can be changed to suit the diet of its owner is vividly shown by the honey creepers of Hawaii. The ancestor of these birds was probably a finch with a short, straight beak that lived in continental America. A few thousand years ago, a flock of them must have been carried out to sea by a freak storm. They eventually reached the Hawaiian islands, and there found lush forests empty of other birds, for the islands are volcanic and were formed comparatively recently. To exploit the many kinds of food now at their disposal, they rapidly evolved into over 50 different species, each specialised for a particular diet with the beak shape that was best suited to gather it. Some have short thick bills for seed-eating, others have hooked and powerful ones for tearing carrion. One species has a long, curving bill for extracting nectar from lobelia blossoms; another has an upper mandible twice the length of the lower, which it uses to hammer bark and lever it off in its search for weevils; yet another has crossed mandibles, a form that apparently enables it to extract insects from buds. Darwin had noted similar variations in the bills of the finches of the Galapagos Islands and regarded them as powerful evidence for his theory of natural selection.

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