Describe the avian bones
Reduced number as some as fused together
Reduced density of bones
Lightweight and Strong
Some bones fused into single ossification e.g. pygostyle (tail bone)
Thinner cortex in the bone
Many long bones are hollow and filled with air spaces - pneumatic bones
What is the purpose of fused bones?
Makes the skeleton more rigid
Describe the sternum of birds
Laterally flattened to form an extended keel bone
What is the function of the keel?
Provides a large surface area for the attachment of flight muscles
(flightless birds don’t have a keel)
Uncinate processes add strength to the ribcage. Point backwards between each rib. Also give extra space for more attachment.
First few ribs are short and incomplete.
Describe the vertebrae of birds
Elongated neck means more cervical vertebrae. Makes neck flexible. Many have 13-25 cervical vertebrae
Approximately 12 caudal vertebrae. End of these fused together to form pygostyle
What is unique about the bird’s axial skeleton?
Only vertebrates to have a fused collar bone called ‘furcula’ or ‘wishbone’
What is the coracoid?
A large bone that lies between the keel and each shoulder joint to support the wing.
What do baby birds have?
Egg tooth. Used to help break out the shell and then falls out
Describe the skull of birds
Highly mobile kinetic skull
Wide gape with the elastic hinge
Cranium is very light with no suture lines.
Upper and lower jaw can move independently
No teeth- replaced by light beak (keratinised)
Sclerotic rings around orbits to support eye (mammals don’t have these)
Single occipital condyle attaches the skull to the atlas - allows bird to rotate head 180 degrees (mammals have 2)
Quadrate bone between mandible and skull allows a wide gape
Describe the pelvic girdle of birds
Extensive fusion = Synsacrum. Provides support for shock of take off and landing
3 paired bones
Lacks pelvic floor - allows passage of eggs
Describe adaptions in birds feet
Water birds have webbed feet
Psittacines are Zygodactyl (2 toes facing forward and 2 toes facing backwards) - for climbing
Passerines are Angiodactyl (3 toes facing forwards and 1 back) - for perching and grasping
What do medullary bones store?
Ca for egg production
What glands do birds have?
No sweat glands
Have preen gland on upper surface of tail
Describe the digestive system regarding teeth and the mouth
Beak replaces teeth and lips of mammals - Initiates mastication
Saliva of birds is more mucoid to lubricate food
Waterfowl have modified tongue to scoop up water and has bristles to filter it.
Why are there differences between the digestive systems of birds and mammals?
Modifications allow maximum energy absorption of food due to high metabolic rate. Digestive tract in birds is relatively short to keep it lightweight.
What is the difference between the oesophagus in mammals and birds?
More distensible in birds and lies on RHS
What lies between within the oesophagus? What is its function?
Crop - oesophageal diverticulum
Used for storage of food when stomach is full. Also used to moisten and soften food.
Allows the bird to only eat a few times a day instead of all day.
Pigeons have crop milk
Owls and penguins lack a crop
Describe the small intestine in birds?
Chemical digestion site
Short and highly convoluted
Thin-walled, narrow and lined with villi
Name the 2 parts of the stomach
- Glandular Proventriculus
2. Muscular Ventriculus (Gizzard)
What is the function of glandular proventriculus?
Production of gastric juices
Propulsion of food into the stomach
Indigestible material is regurgitated into pellets
What is the function of muscular ventriculus (gizzard)?
Allows grinding of food and mechanical breakdown
food not requiring grinding can bypass the gizzard
What species has a more well-developed pancreas. Why?
Pancreas is better developed in birds than mammals. Possibly due to compensation for the lack of saliva and chewing - It produces enzymes for digestion.
Describe the avian liver.
Has 2 distinct lobes each with its own duct leading into the small intestine
What is the function of the small intestine?
Describe the avian large intestine
Has 2 large caeca.
aids in digestion of cellulose and micro fermentation of a high fire diet. Reabsorption of water and electrocytes.
Not always present.
What is the cloaca?
Single body cavity that connects the digestive tract, the urinary tract and the genital tract.
What is the function of the cloaca?
Allows elimination of waste via one orifice
What are the sections of the cloaca?
The Copradeum, Urodeum and Proctodeum
Name the external opening of the cloaca?
What is the main difference between mammal and avian respiration?
Avian respiration is more efficient
Describe the respiratory anatomy of birds
Relatively small lungs and additional use of air sacs.
Nasal cavity - glottis - trachea - syrinx - 2 bronchi -mesobronchi
Birds have no diaphragm
Nasal cavity has chonchae to warm air
Describe the avian glottis
Not protected by an epiglottis
Regulates the passage of air via the dilator and constrictor muscles
Sits inside the choanal slit
How do birds vocalise?
Muscular air sacs.
What is the function of the air sacs?
Allows continuous gas flow as opposed to in and out motion.
It allows 10x more O2 to be absorbed
Describe the trachea?
Has complete cartilage rings.
Longer than in mammals due to increased number of cervical vertebrae.
Wider to decrease air resistance
What separates the lungs from the viscera?
Do birds breathe through their nose or mouth?
Can breathe through both
Describe the nares of the bird
At base of the beak surrounded by a thick cere (a waxy covering)
Describe where the choana can be found and its structure
Food on roof of mouth connecting the oropharynx with the nasal cavity. Has projections
What makes up the bulk of avian lung tissue?
Parabronchi. They are not dead-ended (like alveoli) so allow continuous gas exchange. This is where the majority of gas exchange takes place. Allows birds to fly at high altitude
Describe the lungs in birds
Smaller than mammals
Lungs are attached to the vertebrae
Very vascular and inelastic
How many air sacs do birds usually have?
What word describes the flow of air through the lungs in birds? Why is this?
Unidirectional. Maximises O2 extraction
What is the role of air sacs in thermoregulation?
Reduce the amount of heat produced during flight by evaporation - cooling.
What is another function of air sacs in water birds?
Describe the 1st inspiration in birds
Air taken in through the nasal cavity, travels through the larynx, trachea, syrinx…
And then most goes to the posterior caudal air sacs and a small amount goes to the parabronchi in the lungs.
Describe the 1st expiration in birds
Air moved from posterior air sacs to parabronchi in lungs and then onto the blood capillaries when CO2 and O2 are exchanged.
Describe the 2nd inspiration and expiration in birds
Inspiration - air moves into the cranial air sacs
Expiration - air moved out of the cranial air sacs via syrinx, trachea, larynx, nares.
What controls respiration in birds? Describe.
Internal and external intercostal and abdominal muscles. Chest muscles push the sternum out, creating negative pressure, drawing air in.
Expiration is not passive. Muscles contract to push air out the air sacs.
Describe the urinary system of birds
No bladder. No urethra. Urine goes from kidney to ureter to urodeum.
Describe reproduction in male birds
2 testes (left is bigger) (grow and shrink according to needs) 2 vas deferens Males have phallus - modified area of cloaca - engorged during mating.
Describe reproduction in female birds
ovarian tissue - infundibulum - oviduct - magnum - isthmus - uterus (shell gland) - vagina - cloaca.
Majority of albumen added at magnum. Largest part.
Isthmus - most of inner and outer shell membranes formed.
Shell gland - membranes become calcified.
Sphincter muscle before vagina controls the release of the egg. No pelvic floor to allow passing of egg.
Most only have 1 side - the left. The right side is vestigial.