Class Aves (Birds)

Definition and General Classification: Birds are defined as feathered, bipedal, vertebrate animals belonging to the Class Aves.
Metabolic Status: Classified as Homeotherms (endotherms / warm-blooded), a characteristic that has allowed them to be highly successful in various environments.
Evolutionary Origin: Evolved from theropod dinosaurs during the Jurassic period.
Historical and Scientific Quotes:
* T.H. Huxley: Referred to birds as ‐Glorified Reptiles,‐ highlighting their evolutionary link.
* J.Z. Young: Described birds as ‐The Masters of air.‐
Ornithology: The scientific study of birds.
Biological Extremes:
* Largest Bird: Ostrich (Struthio camelus).
* Smallest Bird: Bee Hummingbird.

Consensus Hypothesis: Most paleontologists agree that the common ancestor of all existing birds was a type of small, feathered dinosaur.
Supporting Evidence: Recent fossil discoveries in China support the hypothesis of feathered dinosaurs as avian ancestors.
Key Transitional Fossil: Caudipteryx:
    * Described as a feathered dinosaur.
    * Mobility: Flightless.
    * Transitional Features: Possessed dinosaur-like arms and teeth (present only in the front of the upper jaw) but also had bird-like feathers.
Significant Fossil: Archaeopteryx:
* Age: Represented an animal that lived approximately $150 \text{ million years ago}$ .
* Dinosaurian/Reptilian Characteristics: Clawed forelimbs, teeth, and a long tail containing vertebrae. Researchers note that without fossilized feathers, it likely would have been identified as a small dinosaur.

Manus: Archaeopteryx had a three-fingered manus, which is partially fused in modern birds.
Wrist: Archaeopteryx possessed a lunate wristbone.
Tail: Archaeopteryx had a long tail; this is reduced to a pygostyle in modern birds.
Pelvis: Characterized by a saurian pelvis.
Jawbones: Archaeopteryx had toothed jawbones, whereas modern birds are toothless.
Furcula: Present in Archaeopteryx.
Sternum: The sternum in Archaeopteryx was smaller; it is larger and keeled in modern birds to facilitate flight.
Ribs: Archaeopteryx had abdominal floating ribs, which have been lost in modern birds.
Metatarsals: Partially fused in Archaeopteryx, but completely fused in modern birds.
Microraptor gui: A cited example in avian evolution (noted attribution: Gay Portia Slown).

Body Morphology: The body is divided into a head, long neck, trunk, and a short tail. The overall shape is boat-shaped and streamlined for aerodynamic efficiency.
Locomotion: Bipedal. Forelimbs are modified into wings, while hind limbs bear the full body weight.
Hind Limb Adaptations: Adapted for diverse functions including walking, running, swimming, and perching.
Integumentary System:
* Skin: Generally dry.
* Glands: Only one gland is present, the Preen or Uropygeal gland, located at the base of the tail.
Exoskeleton Components:
    * Epidermal feathers.
    * Scales on the legs.
    * Claws on the toes.
    * Rhamphotheca: The horny sheath covering the beak.
Hind Limb / Foot Types:
    * Perching bird: e.g., Cardinal.
    * Grasping bird: e.g., Woodpecker.
    * Raptor: e.g., Bald Eagle.
    * Swimming bird: e.g., Duck.

Main Axis: Includes the Shaft and the Quill.
Branching Structure: Barbs extend from the shaft.
Microscopic Anatomy: Barbules extend from the barbs and are held together by hooks, creating the flat surface of the feather.

Digestive and Excretory Tract: Includes the Crop, Liver, Stomach, Gizzard, Intestine, and Cloaca (which serves as a common opening for digestive, urinary, and reproductive tracts).
Respiratory System: Includes Lungs and specialized Air sacs.
Circulatory and Nervous Systems: Includes a Heart and Brain.

Pneumatic Bones: Bones are hollow and contain air cavities without bone marrow, making the skeleton lightweight for flight.
Skull: Monocondylic (possessing a single occipital condyle).
Vertebral Specializations:
    * Sacral Vertebrae: Two sacral vertebrae are present.
    * Synsacrum: Formed by the fusion of the last thoracic, lumbar, sacral, and a few caudal vertebrae with the pelvic girdle to provide support for the hind limbs.
    * Pygostyle: The posterior-most caudal vertebrae are fused into a pygostyle to support the tail feathers.

Attributes: Birds possess powerful flight muscles.
Pectoralis Major: A large, red muscle connected to the lower side of the head of the humerus and the keel of the sternum. It functions as the depressor muscle responsible for the downstroke.
Pectoralis Minor: Arises from the sternum and connects to the dorsal side of the humerus. It functions as the elevator muscle responsible for the upstroke.
Coracobrachialis: Connects the coracoid and the lower side of the head of the humerus; acts as a depressor.

Ocular Anatomy:
    * Size: Eyes are large and possess a nictitating membrane.
    * Structural Support: Sclerotic plates are present in the sclerotic layer to maintain the eye’s shape.
    * Pecten: A comb-shaped structure that projects into the vitreous humour; it likely serves to nourish the eye and remove waste products.
Auditory System: External ear openings are present. The middle ear contains a single ossicle called the columella auris. The internal ear contains a cochlea with the organ of Corti.
Olfaction: Generally poor, with the exception of the Kiwi.
Visual Acuity and Field:
    * Vision can be up to $8 \text{ times}$ keener than human vision.
    * Monocular Vision: Each eye moves independently; seen as the area viewed with one eye (left or right).
    * Binocular Vision: The area seen with both eyes simultaneously, providing depth perception.