CLASS AVES

Class Aves• 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.

Caudipteryx• Feathered

dinosaur• Flightless• Transitional fossil

– Dinosaur arms– Dinosaur teeth

• Only front of upper jaw

– Bird feathers

Archaeopteryx• Represents 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.

Evolution of Flight• Running Hypothesis

Evolution of Flight• Gliding Hypothesis

Ancestors• 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• Feathers• No teeth - beaks• Flexible long neck• Scales on legs• Bones with air spaces• Endothermic• Four chambered heart• Bipedal – walk on two legs• Large, yolked, hard-shelled,

amniotic eggs• The parent bird provides

extensive care of the young until it is grown

Adaptations for FlightSpecialized adaptations for flight:

• Honey combed bones– Air cavities– Less weight

• Reduce body weight– No teeth– No urinary bladder– No penis– Only one ovary

Adaptations for Flight• Feathers

– Light weight– Strong

• Enlarged sternum– Flight muscle

attachment

• Long neck– balance sternum

Wings• Most birds have airfoil-shaped wings.

• Wing shape is closely related to wing function.

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 cannot fly at all.

Beak• Also called “bills”

• Made of keratin

• Nostrils function in breathing

• Variety of beak shapes reflects a wide range of functions

• Beaks are not used for chewing, though some are used to break food into smaller pieces

Feet• Various birds use their feet for walking, perching

on branches, wading, paddling through water, grasping food, in defense, and in some courtship rituals

• Most bird feet are covered in scales• Most birds have four toes on each foot, though

the arrangement varies from species to species

Vision• Up to 8 times keener

than human vision• Each eye moves

independently

Metabolism• Birds 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.

Respiration• One of the most complex respiratory

systems 

• Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into posterior air sacs. – The other 25% of the air goes directly into the lungs.

• When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sacs is simultaneously forced into the lungs.

• A bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.

Circulation• Bird 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.

Digestion• Crop

– Storage• Proventriculus

– Enzymes• Gizzard

– Grind food (no teeth)• Cloaca

– Waste– Reproduction

Avian Reproduction• In the males of species without a phallus, sperm is

stored within the proctodeum compartment within the cloaca prior to copulation.

• During copulation, the female moves her tail to the side and the male either mounts the female from behind or moves very close to her.

• He moves the opening of his cloaca, or vent, close to hers, so that the sperm can enter the female's cloaca, in what is referred to as a cloacal kiss. This can happen very fast, sometimes in less than one second.

Avian Reproduction• The sperm is stored in the female's cloaca

for anywhere from a week to a year, depending on the species of bird.

• Then, one by one, eggs will descend from the female's ovaries and become fertilized by the male's sperm, before being subsequently laid by the female.

• The eggs will then continue their development in the nest.

Avian Reproduction• Many waterfowl and some other birds, such

as the ostrich and turkey, do possess a phallus.

• Except during copulation, it is hidden within the proctodeum compartment within the cloaca, just inside the vent.

• The avian phallus differs from the mammalian penis in several ways, most importantly in that it is purely a copulatory organ and is not used for dispelling urine.

Chicks• Altricial

– No feathers– Cannot walk or see– Cannot feed themselves

• Precocial– Down feathers– Can walk and see– Can feed themselves

http://www.youtube.com/watch?v=-Ah-gT0hTto