THE WORLD OF LIFE with Douglas Drenkow

Ironically, although birds undoubtedly evolved from reptiles, the most likely ancestor of birds was not a "bird-hipped" dinosaur (such as a stegosaur) but rather a "lizard-hipped" dinosaur, probably a "theropod" (a carnivorous lizard-hipped dinosaur that ran on its hindlegs), such as a little cousin of T. rex. Other theories hold that birds evolved earlier in prehistory, not from a dinosaur but rather from a more primitive reptile, perhaps a common ancestor to crocodilians.

Modern birds exhibit evidence of a reptilian ancestry. The hind legs of a bird, with claws on the toes, are typically covered with reptilian scales and the rest of the body is typically covered with feathers (evolved from reptilian scales).

The fore legs of a bird are typically modified into "airfoil" wings (each including three fused fingers). There is a stout "keel" on the breastbone (to which the flight muscles are attached). The bones are very strong and hollow (and, thus, lightweight), sometimes internally "trussed" (reinforced with criss-crossed bracing, like inside an airplane wing).

The mouth bears a tongue and jaws but no teeth -- instead, there is a horny beak. The eyes bear lids and tear glands. There is typically a well-developed neck (useful in preening feathers) and a short tail (modified for flight).

Primitive modern birds (such as ostriches) have a reptilian "palate" (roof of the mouth), usually no powers of flight, and an often large body; whereas advanced modern birds (typical birds) have a less reptilian palate, usually good powers of flight, and a usually smaller size. However, birds are no smaller than the smallest hummingbird: If these warm-blooded creatures (See below) were any smaller, they would lose precious body heat more quickly than they could re-generate it (hence, hummingbirds, like shrews, have a very rapid metabolism -- constantly replenishing lost body heat -- and a very anti-social nature -- each bird protecting a territory with vital supplies of nectar etc.).

FEEDING HABITS

Scavenging, filter-feeding (ex. flamingos), herbivorous, carnivorous, and even parasitic (Cuckoos, for example, are "nest parasites": They lay their eggs in the nests of other birds; their hatchlings do away with the other eggs or hatchlings; and the bird(s) who built the nest then raise the young of the cuckoo, not realizing it is not their own).

MOTION

The wings of birds evolved from the front legs of their reptilian ancestors. The wing of a bird is supported by three fused fingers and is covered by numerous lightweight feathers (whose "barbs" on the central shaft are easily preened back into shape if disturbed). The basic shape of the wing of a bird is that of an "airfoil" (a design copied by the Wright Brothers): The wing is basically curved on top (and thick in front and thin in back, to reduce drag). With its lightweight, hollow bones and its strong flight muscles attached to the stout "keel" of its breastbone, a bird typically flaps its wings forwards through the air (something like a person rowing a boat through the water); and as it does so, air that travels over the upper, curved surface of the wing covers a greater distance than air flowing straight under the lower surface of the wing: The air above the wing is in effect "thinned out" relative to the air under the wing -- the air pressure above the wing is less than the air pressure under the wing -- so the wing (and the rest of the bird) is lifted upwards, into the sky. The tail acts like a balance and a rudder. Because the shape of the wing and the tail and their "angle of attack" into the wind can be changed, birds -- from soarers to flappers -- are more aerobatic than any aircraft ever made by us human beings.

The wings of penguins have been modified from flying structures to swimming flippers. The feet of many waterfowl are webbed, for paddling (In addition, waterfowl have an especially well-developed oil gland, near the tail, for waterproofing their feathers).

The feet of various other birds are adapted for perching, for seizing prey, for scratching, for kicking, etc.

Because a bird typically supports its entire weight on its rear legs when not flying (and especially when landing), the "pelvis" (hip region) and the bones in the hind legs of a bird are typically very strong (like those of its presumably "bipedal" reptilian ancestors).

DIGESTION

With no teeth, birds typically have beaks that are highly adapted to their various foods. For example, seed-eaters have stout beaks; flamingoes have filtering beaks; hummingbirds have tubular beaks, able to reach deep within flowers to the nectaries; and birds of prey have very strong and sharp beaks.

The gut of a bird usually includes a "crop", which stores foods, and a "gizzard", which has strong, horny walls and often contains stones, consumed by the bird, that grind-up foodstuffs. Because flight requires much power, the gut of a bird is very efficient; and birds typically eat high-energy foods, such as insects, fruits, or seeds, instead of low-calorie grasses or leaves.

RESPIRATION

Many air sacs extend from the lungs of a bird throughout its body, even into the hollow bones; inhalation and exhalation move air through this complex respiratory system very efficiently (Some birds can even fly at very high altitudes, where the air is very "thin") -- flight is a very powerful activity, consuming much oxygen.

Song birds sing by vibrations lower in the throat than the voice box in humans.

CIRCULATION

Birds have a heart with four chambers -- two "atria" (one receiving blood from the veins, the other receiving blood from the lungs) and two "ventricles" (one pumping blood to the lungs, the other pumping blood out into the arteries) -- there is no inefficient mixing of "oxygenated" and "deoxygenated" blood, as in fish, amphibians, and most reptiles. Birds that fly require the rapid circulation of relatively large volumes of blood (carrying food and oxygen into tissues and wastes out of them); they have a very rapid heartbeat and a relatively high blood pressure.

Birds are "endothermic" (so-called "warm-blooded") -- biochemical reactions release heat, carried by the blood throughout the tissues, resulting in a relatively high and constant temperature within the body (which is insulated with feathers, especially fuzzy "down"), thus helping to maintain their "high metabolic rates" (relatively active lifestyles).

Birds sometimes cool down by panting (like dogs)

"Migrating" birds fly to warmer regions, nearer the equator, before winter arrives, and to more temperate (or polar) regions, with longer daylight hours (and thus longer food-growing hours), in spring and summer.

EXCRETION

Nitrogen-rich and other wastes (as in our urine) are very efficiently cleansed from the blood of a bird by a great many tubules within a pair of kidneys and are excreted (although not through a heavy urinary bladder) into a "cloaca" chamber, which also receives digestive wastes and opens as the anus.

Excess nitrogen in birds is typically excreted as "uric acid", which is relatively insoluble in water (an advantage within the egg).

Terrestrial vertebrates typically secrete less precious water from their kidneys than do aquatic species. Sea birds typically excrete excess salt
through their nostrils.

COORDINATION

The growth, development, and behavior of birds is under genetic and hormonal control, influenced by the environment. In particular, seasonal changes in the environment typically trigger hormonal changes, bringing about such activities as migration or breeding.

Birds typically have relatively large brain, although their higher thought processes are usually not well developed: The "brain power"of birds is typically devoted to the complex coordination required for flight (and the considerable powers of navigation for migrating species). Typically flying high in the air (above the vegetation, etc. on the ground below), birds typically have a poor sense of smell but great powers of sight.

Songbirds produce and recognize the calls of potential mates -- experiments have shown that in at least some species this ability is learned to a great extent while a young bird listens to older birds sing.

The colorful feathers of birds often serve to visually identify potential mates. Female birds, however, sit on nests more often and thus tend to be more camouflaged and less colorfully feathered than the males of their species (For example, the peacock is striking; the peahen is drab).

The sociable flocks of "birds of a feather" help improve contacts for mating, and the "courts" of crows often "verbally" scold and at least temporarily banish members of the group who are "found guilty" of engaging in anti-social behavior (Crows have relatively high intelligence).

REPRODUCTION

Birds reproduce sexually, with the sexes separate; and as at least originally terrestrial vertebrates, they "copulate" (mate with physical contact). Birds often have complex mating rituals, often involving elaborate, species-specific dances, to go with their often species- and sex-specific plumage.

The fertilized eggs of birds are typically hard-shelled and laid in terrestrial environments -- the developing "embryo" within is bathed and cushioned by an "amniotic sac", fed from a "yolk sac", and "oxygenated" by an "allantois" membrane. Typically, birds keep their eggs and hatchlings warm, by sitting upon them in specially constructed nests. After the breeding season, adult birds typically "molt" their feathers, although not all at once (they can usually continue to fly until they grow a new set).

Representatives

Higher Reptiles & Their Descendants (Diapsida)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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