1. Birds
Chapter 27

2. Class Aves – Birds
Birds (class Aves) are archosaurs but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight.

3. 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.
The bee hummingbird of Cuba weighs 1.8 g and is one of the smallest vertebrate endotherms.

4. Derived Characters 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.

5. Derived Characters of Birds
Some 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.

6. 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.

7. The Origin of Birds
Birds descended from theropods – a group of small, carnivorous dinosaurs.
By 147 million years ago, feathered theropods had evolved into birds.

8. The Origin of Birds Archaeopteryx The oldest bird known.
Skull similar to modern birds but with thecodont teeth.
Wings with feathers were present.

9. 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.

10. 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.

11. Living Birds The ratites, superorder Paleognathae, are all flightless.
Primitive archosaur palate.
Ostriches, emus, rheas, kiwis, tinamous.
Flat sternum, poorly developed pectoral muscles.

12. Living Birds
All 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.

13. 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.

14. Form & Function – 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.

15. Form & Function – 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.

16. Form & Function – Feathers
Feathers 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.

17. Form & Function – Feathers
As 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.

18. Form & Function – Feathers
When 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.

19. Form & Function – Feathers
Colors 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.

20. Form & Function – Skeleton
A light, yet still strong skeleton is a requirement for flight.
Bird bones are laced with air cavities.

21. Form & Function – Skeleton
Birds 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.

22. Form & Function – 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.

23. Form & Function – Skeleton
All birds that can fly have a large, thin keel on their sternum that provides area for the large flight muscles to attach.

24. 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.

25. 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.

26. Food & Feeding
The beaks of birds are strongly adapted to specialized food habits.

27. Digestion At the end of the esophagus of many birds is the crop.
Used for storage.

28. Digestion The 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.

29. Digestion Owls 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.

30. Circulatory System Birds 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.

31. Respiratory System
The 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.

32. Respiratory System
There 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.

33. Respiratory System
The result is that there is an almost continuous stream of oxygenated air passing through the highly vascularized parabronchi.

34. Excretory System Urine 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.

35. Excretory System
Some birds, including marine birds, have a salt gland to help rid the body of excess salts.
Salt solution is excreted from the nostrils.

36. Nervous System
Birds have well developed cerebral hemispheres, cerebellum (important for coordinating movement & balance), and optic lobes.

37. Senses Birds 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!

38. Flight
To 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.

39. 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.

40. 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.

41. Migration
The stimulus for migration has to do with changing hormone levels brought about by a change in day length.

42. Migration Birds 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.

43. 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

44. Social Behavior – Mating Systems
Birds 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.

45. Social Behavior – Mating Systems
Polygamy where an individual has more than one mate during a breeding season.
Polygyny – one male, many females
Polyandry – one female, many males

46. Social Behavior – Mating Systems
The 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.

47. Social Behavior – Mating Systems
An 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.

48. Nesting Most 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.

49. 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.

50. 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.
There is a continuum with the young of many species falling in between the two extremes.