Birds 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. • The bee hummingbird of Cuba weighs 1.8 g and is one of the smallest vertebrate endotherms.
Derived Characters 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.
Derived Characters of Birds • Some other theropod dinosaurs had feathers, but they were not capable of supporting flight. • Insulation – provides support for the idea that some dinosaurs were endotherms. • Bright colors may have been used to attract mates.
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 million years ago, feathered theropods had evolved into birds.
The Origin of Birds • Archaeopteryx • The oldest bird known. • Skull similar to modern birds but with thecodont 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.
The Origin of Birds • Archaeopteryx arose from the theropod lineage. • Closely related to Dromaeosaurs. • More shared derived characters. • Many had feathers used for insulation and/or social display.
Living Birds • The ratites, superorder Paleognathae, are all flightless. • Primitive archosaur palate. • Ostriches, emus, rheas, kiwis, tinamous. • Flat sternum, poorly developed pectoral muscles.
Living Birds • 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.
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.
Form & Function – 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.
Form & Function – 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.
Form & Function – Feathers • 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.
Form & Function – Feathers • 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.
Form & Function – Feathers • 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.
Form & Function – Feathers • 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.
Form & Function – Skeleton • A light, yet still strong skeleton is a requirement for flight. • Bird bones are laced with air cavities.
Form & Function – Skeleton • Birds are archosaurs, and had ancestors with diapsid skulls. • Bird skulls are highly specialized – mostly fused into one piece. • Leg bones in birds are heavier – this helps lower the center of gravity giving aerodynamic stability.
Form & Function – Skeleton • 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.
Form & Function – Skeleton • All birds that can fly have a large, thin keel on their sternum that provides area for the large flight muscles to attach.
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.
Digestion • At the end of the esophagus of many birds is the crop. • Used for storage.
Digestion • 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.
Digestion • 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.
Circulatory System • 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.
Respiratory System • 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.
Respiratory System • 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.
Respiratory System • The result is that there is an almost continuous stream of oxygenated air passing through the highly vascularized parabronchi.
Excretory System • 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.
Excretory System • Some birds, including marine birds, have a salt gland to help rid the body of excess salts. • Salt solution is excreted from the nostrils.
Nervous System • Birds have well developed cerebral hemispheres, cerebellum (important for coordinating movement & balance), and optic lobes.
Senses • 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!
Flight • 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.
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 soaringwings 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.
Migration • 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.
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 • 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.
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
Social Behavior – Mating Systems • 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. http://www.youtube.com/watch?v=s2_wdMmEupQ
Social Behavior – Mating Systems • 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.
Nesting • 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.
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. • There is a continuum with the young of many species falling in between the two extremes.