are archosaurs but almost every feature of their reptilian anatomy has undergone modification in their adaptation to flight. 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. The bee hummingbird of Cuba weighs 1.8 g and is one of the smallest vertebrate endotherms.
A bird’s most obvious adaptations for flight are its wings and feathers. Feathers are the feature that set birds apart from other vertebrates.
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 oldest bird known. Skull similar to modern birds but with thecodont teeth. Wings with feathers were present. 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.
The ratites, superorder Paleognathae, are all flightless. Primitive archosaur palate. Ostriches, emus, rheas, kiwis, tinamous. Flat sternum, poorly developed pectoral muscles.
All other birds are superorder Neognathae, having 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.
Flightlessness has evolved in many groups of birds, that aren’t in this superorder. 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 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. 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 are homologous to reptiles’ scales. It develops from an epidermal elevation overlying a nourishing dermal core. In reptiles, this elevation flattens into a scale. In birds, it rolls into a cylinder and sinks into the follicle from which it will grow. As a feather nears the end of its growth, keratin is deposited to make some of the structures hard. The protective sheath surrounding the new feather splits open, and the feather unfurls. When fully grown, feathers are dead – like mammalian hair. Birds molt to replace worn out feathers. Usually feathers are 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.
Colors in birds may be pigmentary or structural. Red, orange, & yellow are colored by pigments called lipochromes. Black, brown, & gray are produced by the pigment melanin. Blue is created structurally by the scattering of shorter wavelengths of light by particles within the feather.
A light, yet still strong skeleton is a requirement for flight. Bird bones are laced with air cavities. Leg bones in birds are heavier – this helps lower the center of gravity giving aerodynamic stability. Modern birds are toothless. Instead they have a keratinized beak. Most birds have kinetic skulls. They have a wide gape. Upper jaw is attached loosely increasing the gape. Pass around bird skull
All birds that can fly have a large, thin keel on their sternum that provides area for the large flight muscles to attach.
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. 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. The beaks of birds are strongly adapted to specialized food habits.
At the end of the esophagus of many birds is the crop. Used for storage. The stomach has two compartments: The first secretes gastric juices. The second, the gizzard, is lined with keratinized plates that serve as millstones for grinding food. Birds swallow small stones to help this process. Owls can’t digest the bones & fur or feathers of their prey. These materials are bundled together and ejected through the mouth. Owl pellets can be used to determine what the owls in a particular area have been eating.
Birds have a four-chambered heart. Separate systemic and respiratory circulations. Fast heartbeat – faster in smaller birds. Red blood cells are nucleated and biconvex. Mammals are enucleated and biconcave.
The highly adapted respiratory system of birds is adapted for the high metabolic demands of flight. The finest branches of the bronchi are developed as tubelike parabronchi through which air can flow continuously – instead of ending in saclike alveoli as in mammals.
There is an extensive system of nine interconnecting air sacs that connect to the lungs. Air flows to the posterior air sacs, to the lung, then to the anterior air sacs and out. The result is that there is an almost continuous stream of oxygenated air passing through the highly vascularized parabronchi.
How different is bird poop from mammal poop? Urine is formed in large, paired metanephric kidneys. There is no urinary bladder. Nitrogenous wastes are secreted as uric acid rather than urea. Bird kidneys can only concentrate solutes to 4-8 times that of blood concentration.
Some birds, including marine birds, have a salt gland to help rid the body of excess salts. Salt solution is excreted from the nostrils.
Birds have well developed cerebral hemispheres, cerebellum (important for coordinating movement & balance), and optic lobes. Birds usually have poor sense of smell & taste. Some, carnivores, waterfowl, flightless birds have well developed 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!
To fly, birds must generate lift forces greater than their own mass and they must provide propulsion to move forward. Bird wings are designed to provide lift. 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.
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. The purple martin (here last half of march) male above, female below
The stimulus for migration has to do with changing hormone levels brought about by a change in day length. Birds navigate using a number of cues: Visual cues – landmarks. Accurate sense of time. Some may use the Earth’s magnetic field. Celestial cues – sun by day, stars at night.
Two types of mating systems found in birds: Monogamy where an individual has one mate. Rare in animals, common in birds. Seasonal or lifelong Birds have a high incidence of monogamy because both parents are equally able to perform most aspects of parental care. Often success of the hatchlings requires care from two parents.
Polygamy where an individual has more than one mate during a breeding season. Polygyny – one male, many females Polyandry – one female, many males
The most common form of polygamy in birds is polygyny. In some species, such as grouse, males gather in a display area or lek. Each male defends part of the lek and displays for the females. Only females care for young. An example of polyandry occurs in spotted sandpipers. Females defend territories and mate with several males. Each male incubates a nest of eggs in the female’s territory and does most of the parental care. This system may have evolved in response to high predation rates.
Most birds build nests in which to lay eggs. Often great care is taken to hide the nest, or make it inaccessible to predators. When the young hatch, they usually must be fed by one or both parents.
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.
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. There is a continuum with the young of many species falling in between the two extremes.