1. Class Aves
evolved from either a dinosaurian or other reptilian group at least 150 mya
forelimbs modified for flight
scaled legs
epidermal feathers
“constant flow” respiratory system with parabronchi and air sacs
no bladder (semisolid urine of uric acid)
internal fertilization
offspring precocial or altricial

2. Origin and Relationships
Archaeopteryx

3. Origin and Relationships
Superorder Paleognathae
flat sternum
weak pectoral muscles
Superorder Neognathae
keeled sternum
powerful flight muscles

4. Flight

5. The Evolution of Flight
a.k.a. Two Wings and a Prayer
How did flight evolve?
1. Ground Up Scenario: From a bipedal cursorial (running) ancestor
2. Trees Down Scenario: From an arboreal ancestor
if that ancestor was a glider, then it must have co- opted its gliding structures into wings
if the ancestor was not a glider, then its wings must have developed from another function of the arms.
Why did flight evolve?
(1) To help escaping predators
(2) To help catching flying or speedy prey
(3) To help moving from place to place (leaping or gliding)
(4) To free the hindlegs for use as weapons.
(5) To gain access to new food sources or an unoccupied niche

6. Mechanics of Flight
The four forces

7. Source of Power
Flight Muscles

8. Source of Lift
Wing Shape
negative pressure 
lift

9. Adaptations for Flight
Feathers

10. Adaptations for Flight
Light weight skeleton
fused vertebrae
large open areas
bones with air cavities (pneumatized)

11. Pneumatized Bone
human
eagle

12. Perching Muscles & Tendons
Muscular System
Perching Muscles & Tendons

13. Digestive System Crop Liver Proventriculus Gizzard Intestine Kidney
Cloaca
Liver

14. Circulatory System
2 chamber
3 chamber
4 chamber

15. 4-chamber Heart
Circulatory System

16. Cross-current Exchange
Respiratory System
Cross-current Exchange

17. Respiratory System

18. Excretory System
Uric Acid
Salt Glands