BirdsChapter 27

Class Aves – BirdsBirds (class Aves) are archosaurs but almost every

feature of their reptilian anatomy has undergone modification in their adaptation to flight.

Class Aves – BirdsBirds are found in most every habitat from forests to

deserts, even in caves. Some birds dive in the ocean to 45 m to catch prey.Birds have visited both the North & South poles.The bee hummingbird of Cuba weighs 1.8 g and is one of

the smallest vertebrate endotherms.

Derived Characters of BirdsA bird’s most obvious adaptations for flight are

its wings and feathers.Feathers are the feature that set birds apart

from other vertebrates.

Derived Characters of BirdsSome other theropod dinosaurs had feathers, but they

were not capable of supporting flight. Insulation – provides support for the idea that some

dinosaurs were endotherms.Bright colors may have been used to attract mates.

Characteristics of BirdsAll birds also have hindlimbs

adapted for walking, swimming, or perching.Foot structure in bird feet

shows considerable variation.

All have keratinized beaks.

All lay shelled amniotic eggs.

The Origin of BirdsBirds descended from theropods – a group of small,

carnivorous dinosaurs.

By 147 million years ago, feathered theropods had evolved into birds.

The Origin of BirdsArchaeopteryx

The oldest bird known.Skull similar to modern birds but with thecodont

teeth.Wings with feathers were present.

The Origin of BirdsMuch of the skeleton was

that of a theropod dinosaur.Long bony tailClawed fingersAbdominal ribsS-shaped, mobile neck

This fossil demonstrated the connection between theropods & birds.

The Origin of Birds

Archaeopteryx arose from the theropod lineage.

Closely related to Dromaeosaurs.More shared

derived characters.

Many had feathers used for insulation and/or social display.

Living BirdsThe ratites, superorder

Paleognathae, are all flightless.Primitive archosaur

palate.Ostriches, emus, rheas,

kiwis, tinamous.Flat sternum, poorly

developed pectoral muscles.

Living BirdsAll other birds are superorder

Neognathae, having a flexible palate.

The demands of flight have rendered the general body form of many flying birds similar to one another.

Flying birds have a keeled sternum with well developed pectoral muscles.

Living BirdsFlightlessness has

evolved in many groups of birds.Penguins (use wings

to swim through water).

Many fossil forms including flightless owls, pigeons, parrots, cranes, ducks, & auks.

Usually occurs on islands with few predators.

Form & Function – FeathersFeathers are lightweight,

yet tough, consisting of:A hollow quill emerges

from the skin.This becomes the shaft

which bears numerous barbs that form a flat, webbed surface, the vane.Each barb contains many

barbules.

Form & Function – FeathersContour feathers are vaned

feathers that cover and streamline a bird’s body. Called flight feathers if they

extend beyond the body.

Down feathers are soft and have no hooks on barbules.

Filoplume feathers are hair-like – function unknown.

Powder-down feathers disintegrate as they grow, releasing powder that aids in waterproofing.

Form & Function – FeathersFeathers are homologous to reptiles’ scales.

It develops from an epidermal elevation overlying a nourishing dermal core. In reptiles, this elevation flattens into a scale. In birds, it rolls into a cylinder and sinks into the follicle

from which it will grow.

Form & Function – FeathersAs a feather nears the

end of its growth, keratin is deposited to make some of the structures hard.

The protective sheath surrounding the new feather splits open, and the feather unfurls.

Form & Function – FeathersWhen fully grown, feathers are dead – like mammalian

hair.

Birds molt to replace worn out feathers.Usually feathers are discarded gradually to avoid bare

spots.Flight feathers & tail feathers are lost in pairs to maintain

balance.Many water birds lose all their primary feathers at once

and are grounded during the molt.

Form & Function – FeathersColors in birds may be pigmentary or structural.

Red, orange, & yellow are colored by pigments called lipochromes.

Black, brown, & gray are produced by the pigment melanin.

Blue is created structurally by the scattering of shorter wavelengths of light by particles within the feather.

Form & Function – SkeletonA light, yet still strong skeleton is a requirement for

flight.Bird bones are laced with air cavities.

Form & Function – SkeletonBirds are archosaurs, and had ancestors with diapsid

skulls.Bird skulls are highly specialized – mostly fused into one

piece.

Leg bones in birds are heavier – this helps lower the center of gravity giving aerodynamic stability.

Form & Function – SkeletonModern birds are toothless.

Instead they have a keratinized beak.

Most birds have kinetic skulls.They have a wide gape.Upper jaw is attached loosely increasing the gape.

Form & Function – SkeletonAll birds that can fly

have a large, thin keel on their sternum that provides area for the large flight muscles to attach.

Food & FeedingEarly birds were carnivorous, feeding mostly on

insects.Many birds are still insectivores.Other foods include nectar, seeds, berries, worms,

crustaceans, molluscs, fish, frogs, small birds & mammals.

Food & FeedingSome birds are generalists, feeding on a wide range

of food items.Perhaps more competition for food, but less danger of

something happening to the food source.

Others are specialists, only feeding on one type of food.Less competition, more danger of losing the food source.

Food & FeedingThe beaks of birds are strongly adapted to

specialized food habits.

DigestionAt the end of the esophagus of many birds is the crop.

Used for storage.

DigestionThe stomach has two compartments:

The first secretes gastric juices.The second, the gizzard, is lined with keratinized plates

that serve as millstones for grinding food.Birds swallow small stones to help this process.

DigestionOwls can’t digest the bones & fur or feathers of their

prey.These materials are bundled together and ejected

through the mouth.Owl pellets can be used to determine what the owls in a

particular area have been eating.

Circulatory SystemBirds have a four-chambered heart.

Separate systemic and respiratory circulations.

Fast heartbeat – faster in smaller birds.

Red blood cells are nucleated and biconvex.Mammals are enucleated and biconcave.

Respiratory SystemThe highly adapted respiratory system of birds is

adapted for the high metabolic demands of flight.The finest branches of the bronchi are developed as

tubelike parabronchi through which air can flow continuously – instead of ending in saclike alveoli as in mammals.

Respiratory SystemThere is an

extensive system of nine interconnecting air sacs that connect to the lungs.

Air flows to the posterior air sacs, to the lung, then to the anterior air sacs and out.

Respiratory SystemThe result is that there is an almost continuous stream

of oxygenated air passing through the highly vascularized parabronchi.

Excretory SystemUrine is formed in large, paired metanephric kidneys.

There is no urinary bladder.Nitrogenous wastes are secreted as uric acid rather than

urea.Bird kidneys can only concentrate solutes to 4-8 times

that of blood concentration.

Excretory SystemSome birds,

including marine birds, have a salt gland to help rid the body of excess salts.Salt solution is

excreted from the nostrils.

Nervous SystemBirds have well

developed cerebral hemispheres, cerebellum (important for coordinating movement & balance), and optic lobes.

SensesBirds usually have poor sense of smell & taste.

Some, carnivores, waterfowl, flightless birds have well developed sense of smell & taste.

Birds have the keenest eyesight in the animal kingdom and also very good hearing.A hawk can clearly see a crouching rabbit a mile away!

FlightTo fly, birds must generate lift forces greater than their

own mass and they must provide propulsion to move forward.

Bird wings are designed to provide lift.

Flight - Wings are Specialized for Particular Kinds of Flight

Elliptical wings are good for maneuvering in forests.

High speed wings are used by birds that feed during flight or that make long migrations.

Dynamic soaring wings are used by oceanic birds that exploit the reliable sea winds.

High lift wings are found in predators that carry heavy loads. Soaring over land with variable air currents.

MigrationMany species of birds

undergo long migrations using well established routes.Some species make

the trip quickly, others stop along the way to feed.

Often, they follow landmarks such as rivers and coastlines.

MigrationThe stimulus for

migration has to do with changing hormone levels brought about by a change in day length.

MigrationBirds navigate using a number of cues:

Visual cues – landmarks.Accurate sense of time.Some may use the Earth’s magnetic field.Celestial cues – sun by day, stars at night.

Social Behavior – Mating SystemsTwo types of mating

systems found in birds:

Monogamy where an individual has one mate.Rare in animals,

common in birds. Seasonal or lifelong

Social Behavior – Mating SystemsBirds have a high incidence of monogamy because

both parents are equally able to perform most aspects of parental care.Often success of the hatchlings requires care from two

parents.

Social Behavior – Mating SystemsPolygamy where an individual has more than one

mate during a breeding season.Polygyny – one male, many femalesPolyandry – one female, many males

Social Behavior – Mating SystemsThe most common

form of polygamy in birds is polygyny. In some species,

such as grouse, males gather in a display area or lek. Each male defends part of the lek and displays for the females.Only females care for

young. http://www.youtube.com/watch?v=s2_wdMmEupQ

Social Behavior – Mating SystemsAn example of polyandry occurs in spotted

sandpipers.Females defend territories and mate with several males.Each male incubates a nest of eggs in the female’s

territory and does most of the parental care.

This system may have evolved in response to high predation rates.

NestingMost birds build nests

in which to lay eggs.Often great care is

taken to hide the nest, or make it inaccessible to predators.

When the young hatch, they usually must be fed by one or both parents.

NestingPrecocial young, such as

ducks, water birds, fowl and quail are covered with down when they hatch and can run or swim as soon as their down dries.Most precocial young

must still be cared for by the parents for a time.

NestingAltricial young are naked and

unable to see or walk at hatching.They must remain in the nest

for a week or more.Parents must spend lots of

time & energy bringing food to hatchlings.

There is a continuum with the young of many species falling in between the two extremes.