Phylum Chordata

Subphylum Vertebrata

Class Aves

Living birds are divided into two groups: (1) ratite - large flightless ostrich and ostrichlike birds which have a flat sternum with poorly developed pectoral muscles, and (2) carinate - the flying birds that have a keeled sternum on which the powerful flight muscles insert.

Nervous and sensory system:

A. Brain

  1. Cerebral hemispheres - left and right halves of the brain; make up the bulk of the brain; collectively called the cerebrum.
  2. Cerebellum - posterior to the cerebral hemispheres; controls equilibrium, posture, and movement.
  3. Midbrain tectum -posterior portion of the cerebrum which contains the optic lobes.
  4. Optic Lobes - form of a visual association apparatus; comparable to the visual cortex in mammals.
  5. Cerebral cortex - portion of the cerebrum which is the main coordination center in mammals, but is poorly developed in birds.
  6. Corpus striatum - the core of the cerebrum that is the principle integrative center of the brain, controlling eating, singing, flying, and reproductive behavior.

B. Hearing

1. External ear - sound-conducting canal extending to the eardrum.

2. Eardrum - tympanic membrane which receives sound waves.

3. Middle ear - internal region proximal to the eardrum; contains the rodlike columella which transmits vibrations to the inner ear.

4. Inner ear - inner-most region containing the cochlea which is the primary organ involved in hearing; transforms vibrations into impulses (chemical signals) which are transmitted to the brain by the auditory nerve.

C. Vision

1. Eye - Larger, less sspherical and almost immobile in contrast to those of other vertebrates. Turn heads rather than moving their eyes.

2. Retina - light sensitive, internal surface of the eye; contains rods (for dim night vision) and cones (for color vision).

3. Pecten - highly vascularized organ attached to the retina near the optic nerve and extending out into the vitreous humor - internal space of the eye.

The pecten is a distinctive feature of bird eyes; is thought to provide nutrients and oxygen to the eye.

4. Position of eyes

a. Vegetarian birds have eyes placed laterally to give a wide view to see predators.

b. Predaceous birds have eyes directed to the front of the head.

In the birds of prey, such as hawks and owls, the fovea (region of keenest vision is placed in a deep pit within the retina which makes it necessary for the birds to focus exactly on the source.

Food, Feeding, and Digestion -

A. Food and Feeding

1. Most birds are carnivorous, feeding mostly on insects. Many spp. feed upon worms, mollusks, crustaceans, fish, frogs, reptiles, mammals, and even other birds.

2. Many spp. (1/5) of all birds also feed on nectar.

3. Some spp. are omnivores = euryphagous ("wide-eating") - will eat whatever is seasonally abundant.

4. Other spp. are specialists called stenophagous= narrow-eating.

B. Digestion

1. Pharynx - the part of the digestive tract between the mouth (oral) cavity and the esophagus that, in vertebrates, is common to both the digestive and respiratory systems.

In birds, it is very short and leads directly into the esophagus.

2. Esophagus - the part of the digestive tract between the pharynx and the stomach; very long in birds (much longer than the pharynx).

Consists of elastic, muscular tissue which moves food through it by a repeating cycle of contraction and relaxation (peristaltic movement).

3. Stomach- primary digestive organ.

4. Crop- enlargement at the lower end of the esophagus that serves as a storage chamber.

In pigeons, doves and some parrots the crop also produces milk by the breakdown of epithelial cells lining the crop. This "bird milk" is regurgitated by both males and females into the mouth of the young birds.

"bird milk" has a much higher fat content than cow's milk.

5. Proventriculus - major portion of stomach which secretes gastric juice (acidic; very low pH- breaks down the macromolecules and polymers of proteins and carbohydrates by acid hydrolysis). Also contains some digestive enzymes such as peptidases (hydrolyze proteins) and some amylases (break down starch and glycogen). Most of the amylases are produced in the salivary glands of mammals, therefore, enzymatic break down of complex carbohydrates starts in the mouth in mammals but not birds.

6. Gizzard- muscular region of the stomach which is lined with sharp, tooth-like plates that serve as milling stones for grinding food. Birds also swallow small rocks and other coarse objects which lodge in the gizzard and assist the grinding process.

7. Intestine - further metabolism of small molecules, such as proteins, carbohydrates, lipids, and nucleic acids (oligopolymers); absorption of soluble nutrients; eventual passage of solid, wastes to cloaca.

8. Duodenum - first portion of the intestine where further metabolism occurs due to presence of additional digestive secretions:

a. bile from the gallbladder (produced in the liver and then stored in the gallbladder); two bile ducts transport bile from gallbladder to the duodenum; the bile contains no enzymes but has a high concentration of bicarbonate which neutralizes the gastric acids from the stomach; other bile salts help emulsify fats which are insoluble in water;

b. pancreatic enzymes from the pancreas; two or three pancreatic ducts empty into the duodenum; the additional digestive enzymes include:

peptidases (trypsin, chymotrypsin, carboxypeptidase)

lipases - break down lipids

pancreatic amylase - breaks down starch and glycogen

9. Lumen of intestine - highly-folded intestinal epithelium with high surface area for absorption of soluble nutrients.

10. Ceca - pair of organs located at the junction of the intestine and rectum; involved in reabsorption of water from the indigestible solid wastes before excretion.

11. Cloaca - terminal portion of digestive tract where solid and nitrogenous wastes are stored before excretion; also portion of genital system and is involved in internal fertilization.


A. Aerodynamics of wings

1. Airfoil (wing) - streamlined in cross section, with a slightly concave (cambered) lower surface with small, tight-fitting feathers where the leading edge meets the air; due to the shape of the wing, air moving over the top of the wing travels faster than the air beneath the wing, causing a vacuum on the upper side, thus providing lift.

2. Wing slot - gaps in between primary wings which direct more rapidly-moving air over the upper surface of the wing to provide more lift.

a. alula - group of small feathers on the "thumb" of the wing which provides a midwing slot.

b. slotting between primary feathers at the end of the wing - which provides a wing-tip slot

B. Types of bird wings

1. Elliptical - short, broad elliptical wings with large wing slots at tips and distinct alula at midwing; low aspect ratio (ratio of length to width); birds that must maneuver in forested habitats, such as flycatchers, sparrows, warblers, doves, woodpeckers, and magpies.

2. High-speed - long, slender wings with no wing slots and pointed tips; no alula; outer half sweeps back relatively sharply; birds that feed in flight, such as swallows and hummingbirds, or that make long migrations, such as sandpipers and gulls.

3. Soaring - long, narrow wings with no wing slots; only slightly sweeping back near outer third of the wing; high-aspect ratio; oceanic soaring birds birds that include albatrosses and frigate birds; suited for high speed, high lift, and dynamic soaring.

4. High-lift - relatively long, broad wings with large wing slots at tips and distinct alula at midwing, and pronounced camber, all of which provide high lift at low speed; many of these birds (vultures, hawks, eagles, and owls) are land soarers that require:

a. the ability to carry heavy loads (e.g. prey) provided by high-lift aspect

b. maneuverability over terrestrial habitats (also provided by the broad, slotted wings)

Migration and Navigation

A. Migration - the regular, extensive seasonal movements made between summer breeding and nesting regions, and wintering regions.

B. Routes - some birds travel relatively the same, established rout, round trip, while some spp. travel one route one way and a different route on the return trip.

C. Stimulus - increasing day length in late winter / early spring stimulates the pituitary gland to release the hormone gonadotropin which regulates various related activities, such as gonad growth and development, fat deposition, migration, courtship, mating behavior, and care of the young.

D. Navigation - use of the sun to maintain compass direction; called sun-azimuth orientation.


A. Mating Systems:

1. Monogamy - one individual mates with only one partner each breeding season; rare in most animal systems, but more than 90% of all bird spp. are monogamous.

2. Polygamy - one individual mates with two or more partners each breeding season.

B. Parental Care of Young (Parental Investments): In birds, both males and females provide relatively equal amounts of post-care resources for offspring, in contrast to other animal systems, especially mammals, where females, usually, provide the greatest amount of resources.

C. Nesting and Care of Young:

1. Precocial - e.g. quail and ducks are covered with down when hatched and can run or swim as soon as their down dries.

2. Altricial - e.g. meadow lark are naked and helpless at birth and remain in the nest for a week or more; require more care after hatching.