Lab 2 – Avian Anatomy
Things to know:
Skeleton
Skull/Face
 Cranium
 Occipital
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Parietal
Frontal
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Squamosal
Interorbital Septum
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Maxilla (upper mandible)
Dentary (lower mandible)
Foramen Magnum
Orbit
Sclerotic Ring
Hyoid Apparatus
Condyle
Culmen
Flight
Trunk and Limbs
 Vertebral ribs
 Sternal ribs
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Vertebral Column
 Synsacrum
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Pygostyle
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Pectoralis
Supracoracoideus
Tendon of the
Supracoracoideus
Uncinate processes
Sternum (with Keel)
Pectoral Girdle
 Scapula
 Coracoids
 Furcula (fused clavicles)
Wing
 Humerus
 Radius
 Ulna
 Carpometacarpus
 Pollex (1st digit)
Pelvic Girdle
 Femur
 Tibotarsus
 Fibula
 Tarsometatarsus
 Hallux
* I reserve the right to add to this list throughout the duration of class.
Digestive System
 Esophagus
 Crop
 Proventriculus
 Gizzard
 Intestine
 Cloaca
 Colic Caeca
Respiratory System
 Air Sacs
 Lungs
 Direction of Air
Flow
Skeletal System
Study the skeleton by observing skeletons, skulls, and individual disarticulated bones in
conjunction with the descriptions and diagrams below. Note especially the great lightness of the
bones, many of which are pneumatic; the tendency for the bones to be fused together for greater
strength; the modifications of the anterior limbs and girdle for flight; and the modifications of the
posterior limbs and girdle for bipedal locomotion. You will be responsible for knowing the
bones or anatomical features listed on page 1 (may appear on exam). Note other underlined
terms in the following descriptions.
SKULL
The lines of demarcation between individual bones of the skull are nearly impossible to
see in adult birds because of fusion that accompanies ageing. Consequently, you will be looking
at regions rather than separate bones in some cases. The skull can be divided into three groups
of bones: cranium, face, and tongue. Refer to Figure 1.
Cranium -- These bones enclose the brain and constitute the posterior part of the skull.
The occipital forms the base or rear portion of the cranium. The large opening in the base of the
bone is the foramen magnum. The spinal cord connects to the brain through this foramen. The
back and posterior part of the brain case are formed by a medially fused pair of squarish bones,
the parietals. Anterior to the parietals is another pair of bones, the frontals, which form the roof
of the skull and the orbit (eye socket). Lateral to the frontals and parietals on each side of the
head are the squamosals. They form the sides of the brain case and the posterior margins of the
orbits. The ear opening lies under the lower edge of the squamosal.
The floor of the brain case is formed by part of the sphenoid. Viewed ventrally, the
sphenoid is roughly triangular in shape with the base attached to the occipital and the apex
projecting anteriorly. This projection, the basisphenoidal rostrum, forms the central axis of the
base of the skull. The base of the sphenoid (basisphenoid) lies beneath a membranous bone, the
basitemporal plate. The basisphenoid also projects laterally to form the anterior border of the ear
opening and connects with the sphenoid to form part of the brain case and the lower posterior
and inner walls of the orbits up to and including the rear margin of the optic foramen, the
opening in the rear of the orbit for the optic nerve. Additional parts of the sphenoid extend
forward and upward from the optic foramen to form the central part of the interorbital septum - a
thin vertical plate separating the orbits. In some birds, the septum is incomplete, with one or
more openings between the orbits. Extending forward from the sphenoid to the nasal cavities is
the ethmoid, a perpendicular bone which completes the interorbital septum. It extends from the
frontal bones ventrally to the rostrum. Anteriorly, a pair of lateral plates of the ethmoid form a
septum separating the orbits from the nasal cavities. A small portion of the ethmoid extends
anteriorly beyond the septum to separate the two nasal cavities.
Face -- These bones compose, or are directly associated with the upper and lower
mandibles (the maxilla and dentary). The quadrate, a quadrangular bone with a central
constriction, connects the squamosal and sphenoid with the lower mandible, zygomatic bar, and
pterygoid. The quadrate is one of the kinetic (movable) bones of the skull; the articulation at
each end is free.
A slender zygomatic bar lies below each orbit. The quadratojugal makes up most of the
bar and is connected to the quadrate. The much smaller jugal lies at the anterior end of the bar
and is fused to the maxilla. The maxilla forms the posterior part and side of the upper mandible
and part of the palate. It connects with the jugal posteriorly, palatine ventrally. Since the right
and left halves do not connect medially, the palate is cleft. The two premaxillae fuse anteriorly
to form the tip of the upper mandible. Each premaxilla has three posterior projections. (1) One
helps to form the culmen (the upper ridge of the beak) by fusing with its equal on the other side;
it extends back to the frontal. (2) Another extends horizontally to the maxilla and forms half of
the tomium (cutting edge of the bill). (3) Part of the palate is formed by the third process which
extends along the roof of the mouth to meet the palatine.
Each of the nasals forms the posterior border of the external nares (nostril) by splitting
into two processes; a medial one projects forward to join a process of the maxilla. Posteriorly
the nasals fuse with the frontals and medially with each other; ventrally they rest on the ethmoid.
Another pair of bones dorsally visible from the anterior margin of the orbit; the lacrimals, are
small flattened bones extending from the frontals and nasals downward to the zygomatic bar.
In a ventral view of the skull, a pair of long, slender bones lying parallel to each other can
be seen extending from the premaxillary portions of the palate posteriorly to the basisphenoidal
rostrum. These are the palatines, which comprise most of the palate. Their shape varies among
birds. In some they are little more than slender rods; on others they are flattened at one or both
ends, sometimes in different planes. At their posterior tip the palatines articulate with pterygoids
- short thick bones extending obliquely between the quadrates and the palatines and the
basipterygoid process of the sphenoid. The pterygoid is another of the kinetic bones of the skull.
One other bone, the prevomer, is present in the palate of some groups, e.g., waterfowl. It
is a thin, vertically flattened bone extending forward from the rostrum between the palatines.
The V - shaped lower mandible completes the face. Each side is a strong fusion of five bones
which are derivatives of the reptilian jaw. The two sides are fused anteriorly. Note the cups and
processes at the posterior ends where the mandible articulates with the quadrate.
Tongue -- The tongue is supported by the hyoid apparatus or hyoid bone. Three bones
linearly arranged comprise the central element of the hyoid. The tongue is supported on the
anterior most element of this core, aided by a piece of cartilage projecting forward from that
element. Two horns project from the central axis. Three bones make up each horn.
To project the tongue, the bird slides the hyoid forward. Upon retraction, the horns,
encased in a muscular sheath, slide backward behind the skull. In woodpeckers, the horns even
curve forward over the top of the skull and down onto the forehead.
TRUNK AND LIMBS (Figure 2)
A. Vertebral column -- The vertebrae comprise five groups. (1) The cervicals extend from the
skull to the first vertebra with a complete rib reaching the sternum. There may be 13 - 25 in
birds. The high degree of flexibility is made possible by the heterocoelous condition of the
centrum of each vertebra. The anterior articular surface is convex dorsoventrally and concave
from side-to-side. The posterior surface is the reverse. Thus, the articulation between vertebrae
is similar to two saddles fitted together. Note the first two vertebrae, the specialized atlas and
axis. The atlas is ring-shaped and forms a ball-and-socket joint with the occipital condyle on the
occipital. It rotates on a process of the axis. (2) The five thoracic vertebrae bear complete ribs.
(3) The three lumbar vertebrae, (4) four sacral and (5) six of the twelve caudal vertebrae are
fused with the pelvis into the synsacrum. Note that the transverse processes of these vertebrae
can be seen within the synsacrum. The last six caudal vertebrae are unfused. The last one is a
fusion ancestrally of several vertebrae and is called the pygostyle. It bears the tail feathers.
B. Ribs -- Some of the first ribs (the first two in pigeons) do not attach to the sternum. Note that
the ribs are in two segments, the vertebral ribs and the sternal ribs. On the vertebral portion of
all but the first and last ribs is an uncinate process - a tab-like projection. These overlapping
processes provide some rigidity to the rib cage as well as serving for muscle attachment.
C. Sternum -- The large breastbone in most birds has a large keel (carina) to which the massive
pectoral muscles used in flight are attached. Flightless birds lack the keel (ratite condition). The
posterior margins of the sternum are used as a taxonomic character. There may be one or two
notches or holes (fenestrae) or nothing at all.
D. Pectoral Girdle and Wings -- The pectoral girdle consists of three pairs of bones: The
scapulas, the flat bones lying on the dorso-lateral surface of the ribs; the coracoids, the stout
bones which brace the shoulder against the sternum; and the clavicles, which fuse anterior to the
sternum to form the furcula (wishbone). At the shoulder the coracoid and scapula join to form
the glenoid cavity, the point of articulation for the humerus. All three bones unite to form the
foramen triosseum (Figure 3), the opening through which the tendon from the supracoracoideus
(Figure 3) muscle passes to insert on the humerus. This serves as a pulley causing the wing to be
raised as the muscle contracts.
The wing has several fused and missing elements from the basic vertebrate limb. The
humerus is the stout bone connected to the shoulder. Note the small opening at the proximal end
through which the cavity within the bone is connected to an air sac inside the body. At the distal
end the humerus widens to create two surfaces which articulate with the ulna and the radius. The
ulna is the stouter and more curved; the radius is the more slender. The rows of bumps on the
ulna are the points of attachment of the secondary flight feathers. The carpals are reduced by
fusion to two in birds: the radiale at the end of the radius, and the ulnare at the end of the ulna.
These two bones are fusions of carpals. Some carpals also are fused with some of the
metacarpals to form the carpometacarpus. The phalanges are reduced to three and are distributed
among the digits as follows: I-II, III, and IV. (There is some disagreement over which digits are
present). Digit I-II is referred to as the pollex and is an attachment for the allula.
E. Pelvic Girdle and Legs -- The pelvis is a fusion of the three pelvic bones shared by all
vertebrates: ilium, ischium, and pubis. On the lateral surface is a socket, the acetabulum, for the
attachment of the femur. All three bones meet at this point. The ilium is the dorsal part of the
pelvis. It is dorsally concave anteriorly and convex posteriorly. It is fused with the transverse
processes in the synsacrum. The ischium is the thin bone extending ventrally from the ilium and
forming the side of the pelvis. The opening in the side of the pelvis is along the line of fusion of
the ilium and ischium. The pubis is the thin rod-like bone.
The legs also exhibit fusion and reduction or loss of parts. The femur has a prominent
head which fits into the acetabulum. The groove on the distal end is a space for the patella and
the two knobs are points of attachment for the tibiotarsus and fibula. The fibula is much reduced
and exists only as a splinter bone partly fused to and lying parallel to the tibiotarsus. The
tibiotarsus is a fusion of the tibia at its distal end with some tarsal elements. The other tarsals are
fused with a fusion of the second, third, and fourth metatarsals to form the tarsometatarsus.
Remnants of the three metatarsals are visible at the distal end where they articulate with the toes.
Digits I - IV are present in birds. The number of phalanges in each is 2, 3, 4, and 5, respectively.
Digit I, the hallux, projects posteriorly. Digit II is the innermost toe, III the middle, etc.
Figure 1. Lateral view of the skull.
Diagram from:
Proctor, N., and Lynch, P. (1993). Manual
of ornithology: Avian structure and function.
New Haven: Yale University Press.
Figure 2. Lateral View of the Rock Pigeon
Skeleton. Diagram From:
Proctor, N., and Lynch, P. (1993). Manual
of ornithology: Avian structure and function.
New Haven: Yale University Press.
Muscles
1) Ventral concentration of mass (below the wings and the center of gravity = stable flight) –
small aircraft
2) Emphasis on pectoral girdle (huge breast muscles)
3) Complexity of the wing muscles (network of tendons and wire-thin muscles = aerodynamics)
4) Complexity of neck musculature = flexible – can’t move eyes in sockets, and can’t groom or
hold things with the upper limbs, so have to move neck
Only need to know 2 muscles (both attach to keel) and Supracoracoideus tendon
– Figure 3, also look at Figure 5-16 in Gill
- Supracoracoideus (“pulley”) – contraction pulls wings up
- Pectoralis major (direct from keel to humerus) – contraction pulls wings down
Figure 3.
Digestive System
The following structures are adaptations unique to birds:
 Crop (storage – large in herbivores)
 Proventriculus – secretes digestive juices
 Gizzard – “grinding” food
 Colic Caeca – harbors bacteria that breaks down cellulose – very large in herbivorous
birds
Respiratory System
Air sacs (1-2 cells thick – average of 9 sacs (6-12))
- throughout cavity and into legs and wings
- deliver huge quantities of oxygen to lungs (which are relatively small and dense in
birds)
- remove body heat from flight
- protect internal organs
Lungs
- made up of increasingly branching, smaller tubes.
- connected to system of air sacs
- air capillaries intertwine with blood capillaries.
Air flow
Inhale #1 = air to posterior air sacs
Exhale #1 = air from posterior air sacs to lungs
Inhale #2 = air from lungs to anterior air sacs
Exhale #2 = air from anterior air sacs to environment
Inhale = air leaves lungs; exhale = oxygenated air enters lungs