WEEK 8 HANDOUT FOR BIOLOGY 03051, VERTEBRATE ZOOLOGY: BIRDS 3/9/00
Text (7th ed.): Ch. 30 Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu
http:www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://theaviary.com/ (The Aviary--resources on both pet and wild birds)
possible HVCC audio/visual resource: VT 3381,"Eyewitness: Bird" (to be seen in lab)
Birds, class Aves (9000 species, 27 orders), have many characteristics in common:
feathers, epidermally derived
endothermy (maintenance of body temperature by internal metabolism, not surroundings)
wings, involving the pectoral appendages
lightened skeleton, with pneumatic (air) spaces
horny bill without teeth, epidermally derived
Dinosaurs"live on" today as birds. Archaeopteryx ("ancient wing") from the Jurassic Period (150 million years ago) has features which make it look like a"missing link" that shows the relationship between those reptiles Superorder Archosauria and the birds. The skeleton of Archaeopteryx is very similar to small, meat-eating dinosaurs (theropods), and its wings may have been for gliding or short flights. Recent additional fossil evidence highlights the anatomical"procession" from therapod to bird, e.g., pelvis, wrist, hand, clavicle, etc.4
Feathers are homologous to reptile scales, with a dermal pulp and epidermal, keratinized features. Different types of feathers:3
1) contour or flight (pluma) feathers start with a quill, then continue with a shaft (rachis) and a vane of barbs and barbules. Small hooklets on the barbules help to hold the vane together.
2) down (plumula) feathers for insulation, lacking shaft and hooklets.
3) pinfeathers (filoplume feathers), hair-like, scattered among the contour feathers for sensory purposes (the"hairs" on a plucked chicken).
Preening (cleaning, repairing, oiling by beak rubbing) and molting (shedding) help to keep the feathers in good shape. The (thin) bird skin produces a tail oil gland for preening (especially seen in aquatic birds-why?).
Bird skeletons are light and strong. The skull has a reduced number of bones and a beak instead of teeth. The rest of the skeleton is also modified for flight. Many bones are fused, e.g., the synsacrum is a fusion of much of the posterior vertebral column to allow the bird to maintain the proper position for landing, etc. The rear limbs are modified so that the bird"walks" on its toes. Usually, three toes are directed forward, and the first digit (big toe) is aimed backward. Perching tendons assist in holding the bird in place.2
The sternum is large, broad, and keeled, allowing for muscle attachment. Fused clavicles (our collarbones) are fused into a wishbone (furcula) that assists the wings to rebound.1 Most of the skeletal muscles of the bird are ventral on the sternum and rib cage, with a large pectoralis on the surface and a supracoracoideus internally (why you buy chicken breasts for meat). Briefly, the pectoralis muscle pulls the wings down and, with its tendon attaching to the upper side of the humerus, the supracoracoideus pulls the wings back up.
The front limbs of the bird are made of fused bones for feather attachment. After a humerus, radius, and ulna (which carries most of the secondary flight feathers), are highly modified carpals, metacarpals, and three digits, one carrying the alula, and the others carrying the primary flight feathers.
Wings work by forming an airfoil, in contrast to the balancing/steering tail feathers.
Elliptical wings are short and broad, with many wing slots. This wing form allows for high maneuverability and ease of quick take-off and landing, e.g., sparrows.
High-speed wings sweep back and have a slender tip, to avoid drag at high speeds. These wings are efficient at high speeds for such birds as swifts that feed on the wing.
Soaring wings are long and narrow, with no wing slots. While these wings are not maneuverable, they are useful for long periods of gliding, such as with albatrosses.
High-lift wings are broad, with wing slots, to allow predatory birds to carry prey.1,2
Birds are quite active and therefore eat a lot. Biologically,"eating like a bird" is incorrect, since birds need to maintain a high metabolic rate. Birds use beaks and tongues for eating, e.g., woodpeckers vs. hummingbirds. A crop or enlarged portion of the esophagus stores food, while the stomach secretes acid (proventriculus part) and has a gizzard portion in which grinding and crushing occur, often with the help of swallowed stones. Waste is expelled through a cloaca.
The bird heart is modified from the reptilian heart and now has four separate chambers, with no ventricular septum. The right atrium receives venous blood from the body and sends it to the right ventricle, where the blood is sent to the lungs. From the lungs, blood is collected by the left atrium, to be sent to the left ventricle and off to the body via a right aortic arch (vs. a left aortic arch in mammals).3
With the evolutionary selective pressure of high energy demands, birds have modifications of lungs as follows:2
a) air in via external nares and into the trachea
b) the syrinx (for songs and other sound-making) is at the site of the trachea branching into bronchi
c) lungs contain small parabronchi (tubes), vs. the alveoli of mammals
d) air also moves into air sacs in the thorax, abdomen, and even long bones
Overall, 75% of inspired (breathed-in) air bypasses the lungs, entering the air sacs. As the air is breathed out of the lungs, some of this fresh air sac air enters the lungs. Hence, a"two for one" passage of air in the bird respiratory system so that the lungs have fresh air in both inspiration and expiration. With no muscular diaphragm (a mammalian feature), birds use thoracic and abdominal muscles for moving the air in and out.
Birds have large eyes that with very good eyesight, and with many light-sensing rods and cones in the retina. Hearing is also acute, but typically not smell (except in birds like vultures).1
Birds, like reptiles, also secrete uric acid, from water-efficient metanephric kidneys.
All birds are egg layers (oviparous). Males have paired dorsal testes and sperm is transferred from male cloaca to female cloaca. In females, usually the left ovary is developed. Fertilization occurs high up in the oviduct and a shell gland puts on the shell afterward.2
Bird mating habits have been closely studied:
90% of birds are monogamous. After pairing, bird engage in nest formation. Eggs are incubated, and the hatchlings have an egg tooth to free themselves. Helpless altricial young need more parental attention than precocial young.
Migration is periodic round trips between breeding and nonbreeding areas. Photoperiod may be a useful clue for the time of year, and a sun compass based on UV light may be useful for navigation. Lastly, though many birds look alike (lifestyle constraints of flying), there are many species: e.g., the largest bird order is Passeriformes, the songbirds and perching birds (5000+ sp.).2
1 CL Harris, Concepts in Zoology, 2nd ed. (HarperCollins, New York, 1996), pp. 840-841.
2 CP Hickman Jr, et al., Biology of Animals, 7th ed., (WCB McGraw-Hill, Boston, 1998) pp. 647, 649, 651-653, 657, 662.
3 NM Jessop, Schaum's Outline of Theory and Problems of Zoology (McGraw-Hill, New York, 1988), pp. 330-331, 333.
4 K Padian, LM Chiappe,"The origin of birds and their flight," Scientific American 278 (2):38-47 (February 1998).
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