Birds have a single occipital condyle, just like reptiles Features that unite birds and reptiles (archosaurs)
Birds have a single inner-ear bone, just like reptiles and amphibians. Mammals have 3.
The red blood cells of herps and birds are nucleated. Mammals are not.
Birds and reptiles have an amniotic, cleidoic eggs, with a clacified outer shell. Both reptiles and birds have egg teeth to break through the shell at hatching. Note, internal fertilizaiton is required.
2 important feather types in birds are down feathers (insulation and aerodynamic contour) and flight feathers. Flight feathers are assymetrical – an essential aspect if the Bernouli principle is to be employed.
No modern bird has teeth. Neither do turtles, but then turtles don’t often fly.
There is considerable fusion of elements in the bird skeleton. This insures that the flight engine is stable, and simplifies the task of controlling the moments of inertia associated with flight.
Long bones are pneumatic. This reduces total mass, and thus lessens the energy required for flight.
Forelimbs are adapted for flight. There is fusion of bony elements, reduction of number of bony elements, and specialization amongst the feathers.
Avian center of mass is at the center of the body (well duh), more importantly, the 2nd central moment is small.
Birds have vry high metabolic rates, with consequences. This makes flight at altitude possible, make presents a puzzle for long range migration.
Avian respiration is a 2-stage process, and involves lungs and air-sacks. The flow of air through the lungs is an example of a counter-current exchange system. Inhalation does not involve expansion of the thoracic cavity --- an important point because this would change the aerodymamics.
What are the hypotheses for the origins of birds? Theropod dinosaurs. Thecodont (early archosaurs) origins. Crocodylomorph origins.
Thecodont Hypothesis: Theropods share many (perhaps more) features with birds than with other groups. Theropods may also posess clavicles, which in birds become the furcula. However, not all Thecodonts have a clavicle, and it is unlikely to lose a structure and then re-evolve it. Thus, a more ancient ancestor may be more reasonable (namely the archosaurs).
Crocodylomorph Hypothesis: Crocodylomorphs include crocodiles. This hypothesis is more parsimonious than the thecodont hypothesis, because crocodiles are a monophyletic group. The synapomorphies (15-20) of birds and crocodiles have been tested, but they are general to the archosaurs.
The Theropod Hypothesis Cladistic analysis supports this hypothesis and shows that the sister group of theropods includes dromaesaurids such as Deinonychus. Synapomorphies of the skeleton and skull link birds and theropods. There are skeletal attributes in theropods that are avian-like. Basal theropod dinosaurs have lightly built bones and a foot reduced to 3 main toes. There is a reduction and loss of manual digits 4 & 5, and a reduction in the number of the tail vertebrae. In coelurosaurs, dromaeosaurids and others, the arms become longer and the first toe begins to rotate backwards behind the metatarsals. Fused clavicles (furcula) are basal and sternal plates occur in a variety of forms. The pubis and ischium show a greater disparity in length. Finally, in dromaeosaurids and Archaeopteryx, the pubis begins to point backwards instead of forwards, the anterior projection on the foot of the pubis is lost, the tail becomes even shorter and the hyperflexing wrist joint is present, which allows the action that is crucial to the flight stroke in birds.
Hypotheses for the origin of flight: Trees down – dinosaurs were not arboreal. Ground up – problems with selective advantage and with control of moments.