LECTURE 22 - Aves Flashcards
What are reptilian features found in avian skulls?
- Sclerotic ring that support the eye
- Single occipital condyle
- Lower jaw articulate on quadrate
What is the Archaeopteryx?
- Earliest bird, Late Jurassic (145 Ma)
- Mosaic of BIRD and REPTILE characteristics
- bird: feathers, 3 digits forward and 1 back
- reptile: small teeth, unfused digits, no keel on sternum, long tail
Could Archaeopteryx fly?
CHARACTERISTICS OF FLIGHT IN MODERN BIRDS SHARED BY ARCHAEOPTERYX
1. Asymmetrical flight feathers on its wings and tail
2. Wing feather arrangement shared with modern birds
3. Flight muscles strong enough for ground-upward takeoff
4. Absence of keeled sternum, but robust furcula for anchoring of the pectoral muscles
5. Encephalisation, with dominance of the sense of vision
6. Proportion of the inner ear: expanded auditory and spatial perception
What is the cursorial hypothesis on the origin of flight?
- Feathers originated early in the Theropod radiation (hypothesized to be for insulation), while flight is a later innovation during the Mesozoic
- Protoavians thought to have hunted by running quickly and leaping after large flying insects
- This may have evolved into flight, but it would have been highly energetically costly (speeds > ~10 m/s to take off)
What is the arboreal hypothesis on the origin of flight?
- Made use of gravity and expanded body surface area to slow descent during falls and leaps
- But, transition from gliding to flapping would be difficult (feathers increase drag more than adding lift)
What is the K/T mass extinction?
- 65 million years ago
- ~70% of species disappeared
- Thought to have created ecological opportunity for many new species
- Followed by the radiation of modern birds
What is a bird?
- Size constraints
- Adapted for flying
- One-way breathing
- Efficient digestive system
- Excellent vision and hearing
- Oviparous (egg laying)
Explain how body size can affect a bird
- Body weight in birds spans about a 1:6,000 ratio, much less than in other animal taxa (e.g., mammals = 1:1,000,000)
- Small birds have problems obtaining enough food due to high surface to volume ratios causing rapid heat loss
- Large birds have problems flying due to high weight to wing ratios causing excessive wing loading
How are birds adapted for flight?
- Light and agile
- Weight reduction: hollow bones, large eye sockets take up much of the forepart of the skull, reduced jaw without teeth, reduced tail, spindly legs
- Weight concentrated at centre of gravity: large sternum provides a point of attachment for massive flight muscles, legs retracted during flight to reduce drag
What is aspect ratio in birds?
- Measure of wing shape
- AR = wing span^2 / wing surface area
- High AR indicates narrow wings, low AR indicates broad wings
What is wing loading in birds?
- Measure of the relative size of the wing
- WL = body weight / wing area
- Larger birds have disproportionately larger wings and wing area
What are the different types of flight?
- Elliptical wing
- Low speed
- Highly maneuverable
- AR low
- e.g., Woodland birds
- High-speed wing
- High speed
- AR high
- e.g., birds that feed during flight, migratory birds
- Gliding wing
- Dynamic soaring
- Less maneuverable
- AR very high
- e.g., seabirds
- High-lift wing
- Static soaring
- High maneuverable
- AR intermediate
- e.g., hawks and eagles
What are the important skeletal elements for wing shape in birds?
- Humerus
- Ulna
- Radius
- Metacarpus
- Basal phalanx
- Terminal phalanx
Describe the hindlimbs in birds.
- Optimized centre of gravity for flight, which makes balance difficult during other forms of locomotion using hindlimbs (e.g., walking) since the pelvic girdle is well behind the centre of gravity
- Birds have overcome this problem by having the femur project forward along the side of the body, bound to the body by muscles
- Lower end of this bone (or knee) acts as the new “hip” joint, to which the lower left is attached and well positioned with respect to the centre of gravity
What are some weight reduction adaptations in birds?
- Large, heavy jaws and teeth of reptilian ancestors have been lost and replaced with the gizzard, a muscular portion of their stomach
- Reduce the amount of water carried in waste products by obtaining most of their water from their food using the hind gut
- Reproductive systems keep weight low by greatly reducing size of sexual organs of most for most of the year except breeding season
