Zoology 321
AVIAN STRUCTURE
Ornithology
The avian skeletal system:
Major skeletal flight adaptations in flying birds (ratites, the
large flightless birds like ostriches, differ):
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fusion of elements
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reduction of elements
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hollow bones; air spaces in bones
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struts inside bones for support
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ribs, pelvic girdle, and pectoral girdle fused to form a box
Skeletal structures typical of or unique to birds:
Some General Features:
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pygostyle: fused caudal (tail) vertebrae; attachment for rectrices
(tailfeathers)
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ribs with uncinate processes so joined together (also occurs in
some reptiles)
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ribs jointed; chest expands, contracts in respiration
Hindlimbs: legs
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fusion of bony elements
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elongation of foot and ankle bones - tarsometatarsus (similar to
what is seen in some mammals, such as ungulates, but apparently independently
derived).
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usually 4 digits (may be reduced to 3 or 2)
usually digit 1 (hallux, "big toe") points back; digits 2-4
point forward (but we will see exceptions to this)
Head:
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no teeth; beak has bony frame and horny (protein) exterior)
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large eye socket
eye with sclerotic ring
Pectoral girdle: modified to prevent chest from collapsing in flight
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clavicles (collarbones) fused into furcula (wishbone)
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coracoids - elongate bone that goes from the sternum to the humerus
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scapula: thin, reduced from usual condition in other vertebrates
Carinate Sternum:
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carina (keel) to provide attachment of large mass of muscles that
raise and lower the wing
Forelimbs: modified as wings
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humerus short
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main flight feathers carpometacarpus (primaries) and ulna
(secondaries)
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carpometacarpus derived from fusion of wrist, hand bones
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pollux (digit 1;thumb) supports alula, important in flight
at most 3 fingers; fingers usually made of only 1, sometimes 2, bones.
Muscle and associated bone use in flight:
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pectoralis: downstroke. Origin on keel of sternum; inserts on underside
of humerus
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supracoradoideus: upstroke. Origin on keel of sternum deep to (under)
pectoralis. inserts via a tendon that passes through the pectoral girdle
to insert on the top side of the humerus.
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significance: aerodynamic. All main muscle mass in central, low, not adding
weight to wings.
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other muscles affect wing shape in flight
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bony elements at ends of wing automatically lock into place on downstroke
during level flight - other muscles needed on takeoff, landing, specialized
forms of flight
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complexity of wing bone structure related to attachments for tendons/ distal
wing muscles
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birds with more emphasis on specialized (not level) flight have more complexity