birds: class aves chapter 27. class aves – birds birds (class aves) are archosaurs but almost...
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BIRDS: CLASS AVES
Chapter 27
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Class Aves – Birds
Birds (class Aves) are Archosaurs but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight.
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Class Aves – Birds
Birds 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.
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Class Aves – Birds
The bee hummingbird of Cuba weighs 1.8 g and is one of the smallest vertebrate endotherms.
The ostrich of Africa can weigh 350 lbs and grow 10 ft in height.
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Characteristics of Birds
A bird’s most obvious adaptations for flight are its wings and feathers.
Feathers are the feature that set birds apart from other vertebrates.
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Characteristics of Birds
All 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.
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The Origin of Birds
Birds descended from theropods – a group of small, carnivorous dinosaurs.
By 147 MYA, feathered theropods had evolved into birds.
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The Origin of Birds
Archaeopteryx The oldest bird known. Skull similar to modern birds but with thecodont
(reptile) teeth. Wings with feathers were present.
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The Origin of Birds
Much of the skeleton was that of a theropod dinosaur. Long bony tail Clawed fingers Abdominal ribs S-shaped, mobile neck
This fossil demonstrated the connection between theropods & birds.
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Living Birds
The Ratites are all flightless. Primitive nonflexible palate Ostriches, emus, rheas,
kiwis, tinamous. Flat sternum, poorly
developed pectoral muscles
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Living Birds
All other birds have 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.
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Living Birds
Flightlessness 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.
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Feathers
Feathers 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.
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Feathers
Contour 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.
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Feathers
When fully grown, feathers are dead – like mammalian hair.
Birds molt to replace worn out feathers. Usually 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.
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Skeleton
A light, yet still strong skeleton is a requirement for flight.Bird bones are laced with air cavities.
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Skeleton
Modern 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.
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Food & Feeding
Early 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.
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Food & Feeding
Some 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.
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Food & Feeding
The beaks of birds are strongly adapted to specialized food habits.
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Senses
Birds usually have poor 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!
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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.
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Migration
Many 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.
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Migration
The stimulus for migration has to do with changing hormone levels brought about by a change in day length.
Cues include: landmarks. Earth’s magnetic field. sun by day, stars
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Social Behavior – Mating Systems
Two types of mating systems found in birds:
Monogamy where an individual has one mate. Rare in animals,
common in birds. Seasonal or lifelong
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Social Behavior – Mating Systems
Polygamy where an individual has more than one mate during a breeding season.Polygyny – one male, many femalesPolyandry – one female, many males
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Nesting
Precocial 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.
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Nesting
Altricial 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.