The Diversity of Birds
Birds are dinosaurs [p. 81 bottom]. Some dinosaurs grew feathers more than 160 million years ago and gave rise to true birds. The meteor impact 66 million years ago wiped out more than two-thirds of all terrestrial species on earth, including all dinosaurs and all but a few bird species [p. 81 middle]. The general consensus is that there are about eleven thousand species of birds on earth today, and about eight hundred are found regularly in North America north of Mexico. These species are incredibly diverse, and a sampling of their remarkable adaptations and abilities are presented in this book.
EVOLUTION—NATURAL SELECTION AND SEXUAL SELECTION
The incredible diversity of birds is the product of millions of years of evolution. Evolution operates by selection on individual birds, similar to the way breeders of roses or dogs select the characteristics they want to enhance in future generations. In nature, mortal threats like disease, weather, predators, and others remove the “less fit” individuals from the population. At the same time, members of the opposite sex select characteristics that are appealing. All of this affects which individual birds survive and reproduce, which then influences the characteristics of the next generation. Over the course of hundreds of millions of generations, this process leads to the entire diversity of life on earth. Natural selection is mediated by survival, Darwin’s classic “survival of the fittest.” This leads to the wide range of bill shapes, wing shapes, nesting habits, and so on, as birds with the best adapted features are stronger and healthier, raise more young, and pass along their traits to more offspring. Sexual selection is driven by mate choice, as each sex selects a mate for particular features. This can lead to extravagant plumage, as we see in the male Wood Duck [p. 177, Wood Duck].
THE FUNCTION OF FEATHERS
When asked “What does a feather look like?” you probably think of an oval shape, a central shaft, and many barbs on each side (like the one shown here), but feathers are extremely diverse in structure and size. Similarly, when asked “What are feathers for?” you might think of flight or insulation, but feathers have adapted to serve a myriad of different functions. Feathers keep birds warm and dry, streamline the body, provide color and ornamentation, allow birds to fly, and more. Two of the key properties of feathers are very light weight and incredible strength.
-Feathers did not evolve from scales. The precursors of feathers were bristle-like and hollow, and gradually evolved more complex structures [p. 33 right].
-The precise multibranched structure of feathers makes possible many of their remarkable properties [p. 11 bottom].
-Feathers resist breakage because fibers run continuously from the tips of the smallest barbules to the base of the feather shaft [p. 11 middle].
-Feathers have evolved many different forms, even on an individual bird, specialized for each part of the bird’s body [p. 107 bottom].
-Owls’ feathers have several adaptations to make them silent during movement [p. 65 middle].
-Bristle-like feathers around the mouth apparently function to protect the eyes [p. 97 bottom].Feathers as waterproofing
-Feathers are water resistant because of the precise spacing of the barbs; water can neither flow through nor stick to the surface [p. 17 middle].
-Water birds have feather barbs closer together, making it harder for water to penetrate, and also more and stiffer feathers than land birds, [p. 17 bottom].
-Feathers wrap around the underside of a swimming bird to create a waterproof shell [p. 11 middle].
-Cormorants’ body feathers have waterproof centers but get wet on the margins [p. 27 middle].
-Owls’ feathers are less water repellent than other birds’, which may be why so many owls seek sheltered roost sites [p. 180, Eastern Screech-Owl].Feathers as insulation
-Down from ducks and geese is still the most efficient insulation known, natural or synthetic [p. 9 middle].
-Feathers insulate birds from heat as well as cold [p. 107 middle].Feathers for flight
-The large feathers of the wings and tail form a broad flat surface that makes flight possible [p. 69 top].
-Details of shape and structure ensure that wing feathers have the right combination of strength and flexibility [p. 103 bottom].Feathers as ornamentation
-Feathers have evolved a myriad of colors and patterns (see col- oration, below), and feathers also create three-dimensional shapes.
-The “ears” or “horns” of some owls are tufts of feathers, for display and for camouflage [p. 63 top].
The crest of a jay or a cardinal is simply feathers, and can be raised or lowered at will [p. 147 top].
-Highly modified feather tips in waxwings have a hard, smooth texture, just for decoration [p. 185, Cedar Waxwing].
HOW MANY FEATHERS DOES A BIRD HAVE?
The number of feathers depends partly on the size of the bird, and also on how much it needs waterproofing.
-Small songbirds generally have about two thousand feathers, fewer in summer and more in winter. Larger birds like crows mostly have larger feathers, not more [p. 161 middle].
-Water birds have more feathers than land birds, especially on the parts of the body that are frequently in contact with water [p. 17 bottom].
-A swan’s long neck is covered with a dense coat of feathers, with more than twenty thousand feathers on the neck alone [p. 7 middle].
Feathers are critical for a bird’s survival, so birds spend a lot of time on feather care. Preening is the most obvious and frequent feather-care activity, and involves using the bill (for body feathers) and claws (for head feathers). Preening effectively combs the feathers into place, cleans out any dirt or debris that might be stuck there, removes parasites like feather lice, and applies protective oil to the feathers. Many other activities are also related to feather care.
-Birds spend at least 10 percent of each day preening their feathers, and there is a routine to it that is similar in all species. Preening is so important that some details of bill shape have evolved specifically for that activity [p. 145 middle].
-A bird can’t preen its own head with its bill, so it must use its feet; some species get around this by preening each other [p. 183, Common Raven].
-Birds bathe frequently, most likely because water helps to rejuvenate their feathers [p. 137 right].
-Dust bathing is common in some species, though the reason is unclear [p. 161 bottom].
-Sunning and anting are two behaviors that are often confused by observers, and neither is well understood; sunning likely has to do with feather maintenance, anting with food [p. 109 bottom].
-Vultures often spread their wings in the sun; here, too, the reason is unclear [p. 59 top].
-The wing-spreading behavior of cormorants is still not fully explained, but probably helps dry the feathers after swimming [p. 27 top].
GROWING NEW FEATHERS
Feathers wear out and have to be replaced periodically— generally once a year— and this process is called molt. Since feathers are so critical to survival, most birds have evolved a very orderly process to molt gradually without hindering their ability to fly or to keep warm and dry.
-Feathers grow from follicles in the skin, rolled up in a cylinder; the tip emerges first [p. 15 bottom].
-The same feather follicle can grow feathers with entirely different colors and patterns at different times, triggered by hormones. Many species take advantage of feather replacement to change their color. They molt twice a year, once to acquire a drabber nonbreeding plumage, and again for a bright plumage for the spring-summer breeding season [p. 165 top and p. 186, Scarlet Tanager].
-Once a feather is grown it is “dead,” like our hair, and can only be changed through wear, fading, or staining [p. 47 middle].
-Each feather only grows a few millimeters a day, so even small birds take at least six weeks to go through a complete molt. Large birds can take much longer.
-Faint dark and light bars indicate each day and night of growth in a feather [p. 175 middle].
-Growing new feathers requires a lot of energy, and makes flying and keeping warm more difficult, so molt generally happens during a warm season and doesn’t overlap with any other demanding activities like nesting or migrating [p. 165 top].
-When replacing wing feathers, most species use a gradual process so the bird can continue flying [p. 99 top].
-Geese and ducks molt all of their flight feathers at the same time, becoming flightless for a few weeks in late summer, putting the birds at higher risk but for a shorter time [p. 5 middle].
-On rare occasions, a bird will molt all of its head feathers at once, with no apparent negative effects [p. 147 right middle].
[illustration of ‘A hypothetical race involving a roadrunner and four competitors’]
In real life, a coyote is much faster than a roadrunner (even without any rockets or other accessories used by the cartoon coyote), but a roadrunner is faster than most humans. If the competitors shown here ran a 100-meter sprint, the ostrich would easily take first place, in under five seconds (with a top speed of about 60 miles per hour, and sustained 45). The coyote would be close behind, in under six seconds (over 40 miles per hour). The roadrunner and Usain Bolt would take about twice as long. A roadrunner’s top speed is said to be about 20 miles per hour, which would get it across the finish line in just over eleven seconds. Usain Bolt’s 100-meter record is under 9.6 seconds, or about 23 miles per hour. The average human runner finishes in fifteen seconds (under 15 miles per hour). So an elite human sprinter would beat the roadrunner to the line—but most of us would not.
[illustration of ‘The feathered dinosaur known as Anchiornis’]
For more than a century, the link between birds and dinosaurs was debated, but recent discoveries of many dinosaur fossils with feathers and other birdlike features, and a better understanding of the evolution of feathers (see p. 33 right), has settled the debate. Modern birds are the descendants of dinosaurs. Anchiornis, shown here, was one of the “proto-birds” from about 160 million years ago; it was smaller than a roadrunner. It probably couldn’t fly. Its feathers were loose and shaggy without interlocking barbules (the third stage in the evolution of feathers), and might have been useful for gliding, but were probably mainly for insulation and display. Many other feathered dinosaurs and true birds evolved in the next 100 million years after Anchiornis, but almost all went extinct after the meteor impact 66 million years ago.
[illustration of ‘Greater Roadrunner running’]
At the time of the meteor impact that ended the Cretaceous period 66 million years ago, there was a great variety of birds on earth, including many species that lived in trees and were fully capable of flight. That event killed most of the large trees on earth as well as all of the non-avian dinosaurs, and ferns became the dominant plants for thousands of years afterward. Only about 25 percent of all species of plants and animals survived the catastrophic global changes, and among birds only a few small ground-dwelling species survived. These included one species that gave rise to the modern tinamou/ostrich group, another species that gave rise to the modern duck/chicken group, and a third species (perhaps pigeon-like or grebe-like or maybe even roadrunner-like) that gave rise to all other modern birds.
Copyright © 2020 by David Allen Sibley. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.