Kingdom AnimaliaPhylum Chordata

Subphylum VertebrataClass Aves

A Separate Taxon?

Molecular and fossil evidence has convinced most biologists that birds are actually reptiles.The evidence indicates that crocodilians and birds are more closely related to each other than either one of them is to snakes and lizards.

Characteristics of BirdsEndothermicBony beak, no teethLarge muscular stomach BipedalLarge, yolked, hard-shelled, amniotic eggsThe parent bird provides extensive care of the young until it is grownStrong, lightweight skeleton Specialized adaptations for flight:– Keratinous feathers (modified scales) that

function in both flight and insulation– Specialized wing shape that produces lift– Numerous weight-reducing features

Internal Anatomy

WingsMost birds have airfoil-shaped wings.

Wing shape is closely related to wing function.

High Metabolic RateBirds have a system of branching air sacs that function with their lungs in respiration.

The system of air sacs supplies the high levels of oxygen needed to support a high rate of metabolism for the hard-working flight muscles.

This high metabolic activity also provides heat for endothermy.

Weight-ReductionThe air-sac system reduces the overall density of the birdBones have a honeycombed structure that makes them lightweight but strongBirds are missing some of the internal organs found in other vertebrates:– Females have one ovary instead of two – No teeth (a hard beak and a gizzard

with stones for grinding food)

Circulatory SystemBird circulatory systems efficiently deliver oxygen to cells.Like amphibians and reptiles, birds have two pathways of blood flow: one from the heart to the lungs and back, and the other from the heart to the rest of the body and back. Unlike amphibians and most reptiles, birds have a four-chambered heart with two separate ventricles that keep oxygen-rich blood from the lungs separate from the oxygen-depleted blood that passes through the heart.

The Origin of Birds

Most paleontologists agree that the common ancestor of all existing birds was a type of small, feathered dinosaur.

Recent fossil discoveries of feathered dinosaurs in China support this hypothesis.

CaudipteryxCaudipteryx zouizoui SinornithosaurusSinornithosaurus milleniimilleniiMicroraptorMicroraptor guigui

ArchaeopteryxRepresents an animal that lived about 150 million years ago.Unlike modern birds, Archaeopteryx had clawed forelimbs, teeth, and a long tail with vertebrae—all reptilian characteristics.In fact, if the feathers had not been preserved in the fossil, Archaeopteryx probably would have been identified as a small dinosaur.

Bird Biodiversity

There are about 9,000 species of birds living today, far outnumbering mammal species.Tremendous diversity of flying styles, beak and foot adaptations, and behavioral characteristics.

Flying Styles

Different styles of flying:– Cardinals and finches rapidly change

altitude as they fly, a behavior that helps them avoid predators.

– Falcons and albatrosses are hunting birds that soar, using their wings to gain altitude and then gliding on air currents without flapping.

– Hummingbirds flap their wings as many as 80 times per second, enabling them to hover.

– Penguins are flightless diving birds that use the same basic flight stroke to swim.

– Chickens belong to a group of birds that can fly short distances, but are incapable of long flights.

– Ostriches, emus, and kiwis are cannot fly at all.

Beak AdaptationsAlso called “bills”Made of keratinNostrils function in breathingVariety of beak shapes reflects a wide range of functionsBeaks are not used for chewing, though some are used to break food into smaller pieces

Foot Adaptations

Various birds use their feet for walking, perching on branches, wading, paddling through water, grasping food, in defense, and in some courtship ritualsMost bird feet are covered in scalesMost birds have four toes on each foot, though the arrangement varies from species to species

Behavioral Adaptations

Bird brains are relatively highly developed, with a brain to body size ratio second only to mammals.

Birds are capable of complex behavior, especially in elaborate courtship rituals, caring for their young, and communication.

A crow making a tool from an oak branch

A crow bending a wire to make a tool

A crow using traffic to crack a nut