BIRDS: CLASS AVES Chapter 27

Class Aves – Birds Birds (class Aves) are Archosaurs but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight.

Class Aves – Birds Birds are found in most every habitat from forests to deserts, even in caves. Some birds dive in the ocean to 45 m to catch prey. Birds have visited both the North & South poles.

Class Aves – Birds The bee hummingbird of Cuba weighs 1.8 g and is one of the smallest vertebrate endotherms. The ostrich of Africa can weigh 350 lbs and grow 10 ft in height.

Characteristics of Birds A bird’s most obvious adaptations for flight are its wings and feathers. Feathers are the feature that set birds apart from other vertebrates.

Characteristics of Birds All birds also have hindlimbs adapted for walking, swimming, or perching. Foot structure in bird feet shows considerable variation. All have keratinized beaks. All lay shelled amniotic eggs.

The Origin of Birds Birds descended from theropods – a group of small, carnivorous dinosaurs. By 147 MYA, feathered theropods had evolved into birds.

The Origin of Birds Archaeopteryx The oldest bird known. Skull similar to modern birds but with thecodont (reptile) teeth. Wings with feathers were present.

The Origin of Birds Much of the skeleton was that of a theropod dinosaur. Long bony tail Clawed fingers Abdominal ribs S-shaped, mobile neck This fossil demonstrated the connection between theropods & birds.

Living Birds The Ratites are all flightless. Primitive nonflexible palate Ostriches, emus, rheas, kiwis, tinamous. Flat sternum, poorly developed pectoral muscles

Living Birds All other birds have a flexible palate. The demands of flight have rendered the general body form of many flying birds similar to one another. Flying birds have a keeled sternum with well developed pectoral muscles.

Living Birds Flightlessness has evolved in many groups of birds. Penguins use wings to swim through water. Many fossil forms including flightless owls, pigeons, parrots, cranes, ducks, & auks. Usually occurs on islands with few predators.

Feathers Feathers are lightweight, yet tough, consisting of: A hollow quill emerges from the skin. This becomes the shaft which bears numerous barbs that form a flat, webbed surface, the vane. Each barb contains many barbules.

Feathers Contour feathers are vaned feathers that cover and streamline a bird’s body. Called flight feathers if they extend beyond the body. Down feathers are soft and have no hooks on barbules. Filoplume feathers are hair-like – function unknown. Powder-down feathers disintegrate as they grow, releasing powder that aids in waterproofing.

Feathers When fully grown, feathers are dead – like mammalian hair. Birds molt to replace worn out feathers. Usually discarded gradually to avoid bare spots. Flight feathers & tail feathers are lost in pairs to maintain balance. Many water birds lose all their primary feathers at once and are grounded during the molt.

Skeleton A light, yet still strong skeleton is a requirement for flight. Bird bones are laced with air cavities.

Skeleton Modern birds are toothless. Most birds have kinetic skulls. Instead they have a keratinized beak. Most birds have kinetic skulls. They have a wide gape. Upper jaw is attached loosely increasing the gape.

Food & Feeding Early birds were carnivorous, feeding mostly on insects. Many birds are still insectivores. Other foods include nectar, seeds, berries, worms, crustaceans, molluscs, fish, frogs, small birds & mammals.

Food & Feeding Some birds are generalists, feeding on a wide range of food items. Perhaps more competition for food, but less danger of something happening to the food source. Others are specialists, only feeding on one type of food. Less competition, more danger of losing the food source.

Food & Feeding The beaks of birds are strongly adapted to specialized food habits.

Senses Birds usually have poor sense of smell & taste. Birds have the keenest eyesight in the animal kingdom and also very good hearing. A hawk can clearly see a crouching rabbit a mile away!

Flight - Wings are Specialized for Particular Kinds of Flight Elliptical wings are good for maneuvering in forests. High speed wings are used by birds that feed during flight or that make long migrations. Dynamic soaring wings are used by oceanic birds that exploit the reliable sea winds. High lift wings are found in predators that carry heavy loads. Soaring over land with variable air currents.

Migration Many species of birds undergo long migrations using well established routes. Some species make the trip quickly, others stop along the way to feed. Often, they follow landmarks such as rivers and coastlines.

Migration The stimulus for migration has to do with changing hormone levels brought about by a change in day length. Cues include: landmarks. Earth’s magnetic field. sun by day, stars

Social Behavior – Mating Systems Two types of mating systems found in birds: Monogamy where an individual has one mate. Rare in animals, common in birds. Seasonal or lifelong

Social Behavior – Mating Systems Polygamy where an individual has more than one mate during a breeding season. Polygyny – one male, many females Polyandry – one female, many males

Nesting Precocial young, such as ducks, water birds, fowl and quail are covered with down when they hatch and can run or swim as soon as their down dries. Most precocial young must still be cared for by the parents for a time.

Nesting Altricial young are naked and unable to see or walk at hatching. They must remain in the nest for a week or more. Parents must spend lots of time & energy bringing food to hatchlings.