Biology 10: Topic 12d: Class Aves (Birds)

- about 4000 species of modern birds

- originated about 150 million years ago from small lizard-like animals called thecodonts

- characteristics shared with reptiles: clawed toes, scales on feet, internal fertilization, amniotic egg

- Archeopteryx is the earliest known bird fossil. It provides a link between modern birds and reptiles, as it has feathers and wings (like birds) and teeth, long tail with bones and clawed front toes (like reptiles). The clawed front toes happen to be on the wing, which is weird, but….

Archeopteryx (artist's rendition)

Archeopteryx fossil

BODY STRUCTURE
Feathers: unique to birds. They are modified scales (made from keratin) that function to insulate the animal and allows for flight. The word, plumage, refers to all the feathers on a bird’s body. There are different kinds of feathers: down (pehely) for insulation, contour feathers which are found on the body and give the animal’s shape and flight feathers which are found on the wings and designed for flying. The image below shows the general parts of a feather. The rachis of a feather is hollow (to reduce weight) and strong.

When birds take care of their feathers they are preening. They run their beak through their feathers to spread oil from the preen gland found at the base of their tail. This oil helps to waterproof the feathers. Most birds moult in late summer, usually only loosing a few feathers at a time so that they can continue to fly (ducks and geese loose most of their flight feathers at once, so have a period when they can’t fly).

Skeleton: birds have a thin, hollow, bony skeleton, which helps to reduce their weight (important for flying). The largest bone is the sternum (mellcsont), which has a protruding keel (taraj) to which the bird’s large flight muscles can attach.

Their hind legs are adapted to their habitat, but they all have 4 toes and scaly legs. Examples of different ways in which they use their hind legs include perching (eg. sparrows (veréb)), running (ostrich), swimming (ducks), walking (pheasants), catching prey (falcons)

Birds are endothermic or homeothermic, so they maintain a constant body temperature (40-42C) and therefore can live anywhere

Circulation: 4-chambered heart and double-circuit circulation

Nutrition: varies greatly. Their beak is usually adapted to the kind of food they eat. eg. long, dipping beak – hummingbird, strong, hooked beak – hawk, huge with a pouch – pelican, short and stout for eating nuts and seeds – goldfinch. They have efficient digestion, therefore they need to eat a lot and often.

Excretory system excretes uric acid to help reduce water loss. The cloaca is the opening where the oviduct opens to release eggs and the anus opens to release uric acid

Respiration: unique lungs allow for double ventilation. The lungs take up 20% of the bird’s body volume. It is a very efficient system, birds never gasp for air.

Sensory: smell – bad, hearing – good, eyes – very good

Reproduction: internal fertilization follows a period of complex mating rituals. The female lays a small number of amniotic eggs with hard calcified shells. The yolks are larger than in reptiles, allowing for longer development within the egg. Parents (one or both depending on the species) sit on the eggs to keep them warm and most birds also practice parental care, feeding the young and teaching them to fly. Some species are altricial, which means the young are born helpless, usually without feathers (common in songbirds), while other species are precocial, which means the young are born well-developed and do not need a long period of parental care (eg. chicken chicks).

CLASSIFICATION

27 orders, (200 permanent species in Hungary and 150 migratory), some examples:

Falconiformes (solyomalkatúak) – eagles, hawks, vultures and falcons

- 304 species

- daytime predators

- called “birds of prey” or raptors

- good at soaring, drab plumage (for blending in), forward-facing eyes, hooked beak (for tearing apart prey), powerful talons (feet)