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The oldest known bird fossil was dug out of a limestone quarry in Bavaria in 1861. It was given the genus name Archaeopteryx, ancient winged, and since the quarry was the source of the fine-grained limestone used for lithographic printing, the species name became lithographica.

We know from the age of the rocks where it was found that archaeopteryx lived about 150 million years ago during the Jurassic period of the Mesozoic era. Its contemporaries included a variety of dinosaurs, among them the first pterosaurs, the flying reptiles, as well as primitive mammals and the first frogs.

Archaeopteryx was about the size of a crow. It was beautifully preserved in the stone. You can see every bone in its body, and if that was all you could see, you would suspect th at you were looking at the remains of a reptile. The ancient winged one had large, heavy jawbones armed with teeth set in sockets. Its wing bones end in large, heavy claws, and its long, sinuous tail contains no less than 20 vertebrae.

Modern birds have lightweight beaks rather than big jawbones, and no living bird has any teeth. There is a South American bird called the hoatzin that retains a wing claw, which it uses mainly for climbing, but all other birds have clawless wings. And of course the tails of modern birds have been reduced to a tiny stump that scientists call the pygostyle and the rest of us refer to as the pope's nose.

But surrounding the skeleton of archaeopteryx, perfectly preserved in the stone, are the unmistakable impressions of feathers, the only evidence we really need to declare this creature a bird. There are other winged animals and other flying animals, and for that matter, some birds can't fly, and some don't even have wings, but all birds have feathers and only birds have feathers. Feathers are the evolutionary invention that made birds what they are today.

The reptilian scale was almost certainly the starting point for the evolution of the feather. Birds still retain scales, especially on their legs and feet, and these scales are just like those on a snake. They grow from follicles set into the inner dermal layers, which makes them quite different from the loosely attached scales of fish. You could not scale a lizard by scraping a knife over its hide.

Our best guess is that at some point in the Mesozoic, some reptiles began to grow longer and more finely divided scales. Insulation was probably the major function of these proto-feathers. Their use for flight came later. In fact, it seems likely that archaeopteryx was a glider rather than a flier. Perhaps it was an arboreal animal whose feathers allowed it to glide from tree to tree the way flying squirrels do now. The advance to true flight would come only after some further skeletal modifications such as the loss of those heavy jaws and the long tail and the addition of a keel bone at the center of the breast to serve as an anchor for the massive muscles that power a bird's flight.

Feathers grow in five different basic styles: vaned feathers, down, semiplumes, filoplumes, and powder down. The most visible on a living bird are the vaned feathers. As contour feathers, these give the bird its streamlined shape, and as flight feathers on the wings and tail, they keep it in the air.

Vaned feathers have a central shaft with a flat vane attached to each side. These vanes are the most complex epidermal structures known in the animal kingdom. Next time you see a pigeon feather on the sidewalk, pick it up and take a look at it. At first glance, you can see that each vane is made of hundreds of hairlike filaments called barbs. Near the base of the feather, these barbs are loose and fluffy, but over most of the length of the shaft, they are stiff and straight.

Take a closer look with a magnifying glass, and you can see that each barb is fringed with even tinier filaments called barbules. There are hundreds of these barbules on each barb, which means there are as many as a million on a large feather of a large bird.

Subject your pigeon feather to a magnification of 75 to 100 times and you can see that these barbules come in two different shapes. Those on the distal side of the barb, the side away from the bird, have tiny hooks on their undersides. Those on the proximal side, the side nearer the bird, have tiny flanges on their upper sides.

The barbules meet at right angles, and the hooks on the distal barbules grab the flanges on the proximal barbules and knit the vane of the feather into a single, solid sheet.

It's a system that is better than Velcro. It will hold the feather together while the pigeon is flying at 70 miles an hour, and if something does pull it apart, the bird can rezip it just by drawing the barbs through its beak.

Down feathers have no central shaft, just a cluster of fluffy filaments arising from the base of the feather. The barbules on down feathers don't hook together, but they do help trap an enormous amount of air. That ability gives them their value as insulation and explains why a bulky down coat can keep you warm all winter even though it weighs far less than a fur coat. It also explains how a mallard duck can paddle happily around in water that is only a degree or so away from turning to ice.

Semiplume feathers are a sort of intermediate between down and vaned feathers. They have central shafts, but no barbules. Filoplumes are hairlike feathers of obscure function. My grandmother called them pin feathers, and when she dressed a chicken, she used to singe them off by holding the carcass over a flame.

Powder down feathers are unique in that they grow continuously. Their tips are constantly breaking down into a water-resistant powder rather like tale. The powder seems to help waterproof the plumage.

You may wonder how many feathers a bird has. Thanks to our system of graduate education, a system that provides ornithological labs with a constant supply of intelligent, highly motivated young people willing to work for wages well below the legal minimum, I can answer that question. The answer is: it varies.

As you might expect, big birds have more feathers than small birds. One investigator found that a whistling swan had 25,216 feathers (I think the suffering grad student who counted that one should have been given a PhD on the spot). Sparrow-sized birds are likely to carry 2,000 to 3,000 feathers. A ruby-throated hummingbird had 940.

Some of the variation is harder to explain. There is a tendency for birds to carry more feathers in the winter than in the summer, but some counters have found substantial variations between birds of the same species, age, and sex taken at the same time of year. For example, two female song sparrows taken on March 5 in the same location had 2208 and 2093 feathers respectively.

To really appreciate the wonder of feathers, you need to compare birds to bats, the only other flying vertebrates. A bat's wing is skin stretched on a framework of bone. It is living tissue that has to be maintained at body temperature, so it radiates a huge amount of heat. Bats cannot live where it is cold. Bats of the temperate zone must either migrate or hibernate to make it through the winter.

The feathers of a bird's wing are dead, just like our hair. They insulate the wing while requiring no heat themselves. So ivory gulls can make a living in the winter scavenging polar bear kills on the pack ice in the Arctic Ocean and tiny chickadees can maintain a body temperature of over 100 degrees through a winter in northern Minnesota.

If a bat gets blown against a tree branch and pokes a hole in a wing, the wing needs to heal. The animal may get an infection in the wound or the two sides of the tear may not grow back together.

A bird in the same situation may need to do nothing but draw a feather through its bill to refasten the barbules. At worst, it might lose a feather or two, and if it does, it will immediately start growing new ones. Archaeopteryx was onto something good.

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