Bird: the story on HearLore

Bird

Birds are not merely creatures of the sky but are the only surviving lineage of theropod dinosaurs, a fact that rewrites the history of life on Earth. For decades, the distinction between dinosaurs and birds was a rigid line drawn by the presence of teeth and claws, but the fossil record from the Liaoning Province in China has shattered that boundary. By the 2000s, discoveries of small theropod feathered dinosaurs like Anchiornis huxleyi and Microraptor revealed that the ancestor of all birds was likely arboreal, capable of gliding, and possibly even flying long before the first true bird took to the air. The Archaeopteryx, found in Germany and dating back to the Late Jurassic period about 155 million years ago, was the first fossil to display both traditional reptilian characteristics such as teeth and a long lizard-like tail, alongside wings with flight feathers similar to modern birds. While not a direct ancestor, it serves as a crucial transitional link, proving that the evolution of flight was a gradual process involving the loss of osteoderms and the accumulation of neotenic characteristics over 60 million years. The consensus view in contemporary palaeontology places flying theropods, or avialans, as the closest relatives of deinonychosaurs, forming a group called Paraves, which includes dromaeosaurids and troodontids. This evidence suggests that the first avialans were omnivores, unlike their meat-eating predecessors, and that the transition from ground-dwelling predators to aerial masters involved a continuous reduction in body size and the development of complex, pennaceous feathers. The oldest known paravian fossils come from the Tiaojishan Formation in China, dated to the late Jurassic period about 160 million years ago, indicating that the roots of modern birds stretch back deep into the Mesozoic era, long before the extinction event that wiped out the non-avian dinosaurs 66 million years ago.

The Flightless Giants

While the majority of birds are defined by their ability to fly, a significant number of species have lost this capability, evolving into giants or specialized swimmers in isolated environments. The only known groups to have completely lost their wings are the extinct moa and elephant birds, which once roamed the vast landscapes of New Zealand and Madagascar respectively. Flightlessness often arises in birds on isolated islands where the absence of mammalian land predators and limited resources allow for gigantism to evolve. Penguins, for instance, use similar musculature and movements to fly through water, while flightless birds like the kakapo and the extinct dodo have adapted to life on the ground. The evolution of flightlessness is almost exclusively correlated with gigantism due to an island's inherent condition of isolation, a phenomenon seen in the extinct moa which could grow to the size of an ostrich. In the Cretaceous period, the Enantiornithes, or opposite birds, were the dominant group of avialans, occupying a wide array of ecological niches from sand-probing shorebirds to tree-dwelling forms, yet they became extinct along with many other dinosaur groups at the end of the Mesozoic era. The Euornithes, meaning true birds, included the ancestors of modern birds, with some species like the Hesperornithiformes becoming so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. These early euornithians developed traits associated with modern birds, such as strongly keeled breastbones and toothless beaked portions of their jaws, though most non-avian euornithians retained teeth in other parts of the jaws. The loss of flight in birds like the penguins and ratites is a testament to the adaptability of the avian body plan, allowing them to thrive in environments where flight was no longer necessary or advantageous. The extinct Xenicibis was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail, demonstrating that even flightless birds developed specialized adaptations for survival.

Common questions

What is the evolutionary origin of birds according to the script text?

Birds are the only surviving lineage of theropod dinosaurs, a fact established by the fossil record from the Liaoning Province in China. Discoveries of small theropod feathered dinosaurs like Anchiornis huxleyi and Microraptor revealed that the ancestor of all birds was likely arboreal and capable of gliding. The Archaeopteryx found in Germany dates back to the Late Jurassic period about 155 million years ago and serves as a crucial transitional link.

Which extinct bird groups completely lost their wings and where did they live?

The only known groups to have completely lost their wings are the extinct moa and elephant birds. The moa once roamed the vast landscapes of New Zealand while the elephant birds lived in Madagascar. Flightlessness in these species arose in isolated environments where the absence of mammalian land predators allowed for gigantism to evolve.

How does the respiratory system of birds function to support flight?

Birds have a unique respiratory system where 75% of fresh air bypasses the lungs and flows directly into a posterior air sac upon inhalation. The other 25% of air goes directly into the lungs, and when the bird exhales, the stored fresh air from the posterior air sac is forced into the lungs. This ensures a constant supply of fresh air during both inhalation and exhalation to meet high oxygen demands.

What are the specific characteristics of bird feathers and how are they maintained?

Feathers are epidermal growths attached to the skin that facilitate flight, provide insulation, and are used in display, camouflage, and signalling. There are several types of feathers with distribution patterns called pterylae used in taxonomy, and the arrangement called plumage is regularly moulted. Birds preen or groom them daily spending an average of around 9% of their daily time on this maintenance.

What are the longest recorded migration distances for specific bird species?

Arctic terns were recorded travelling an average of 70,000 kilometers between their Arctic breeding grounds in Greenland and Iceland and their wintering grounds in Antarctica. Sooty shearwaters make annual round trips of 64,000 kilometers to their summer feeding grounds in the North Pacific off Japan, Alaska and California. The bar-tailed godwit is capable of non-stop flights of up to 11,000 kilometers.

How many bird species have become extinct due to human activity since the 17th century?

About 120 to 130 species have become extinct due to human activity since the 17th century. Human activity threatens about 1,200 bird species with extinction, and the loss of flightless birds like the moa and elephant birds was accelerated by human arrival. The discovery of the genome of the chicken and the zebra finch in 2010 has led to the sequencing of the genomes of 542 species of birds.

The Anatomy of Flight

The physiological adaptations required for flight are among the most complex in the animal kingdom, involving a lightweight skeleton, a high metabolic rate, and a unique respiratory system. Birds have one of the most complex respiratory systems of all animal groups, where upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones. The other 25% of the air goes directly into the lungs, and when the bird exhales, the used air flows out of the lungs while the stored fresh air from the posterior air sac is simultaneously forced into the lungs. This ensures a constant supply of fresh air during both inhalation and exhalation, allowing birds to meet the high oxygen demands of flight. The skeleton consists of very lightweight bones with large air-filled cavities called pneumatic cavities, and the skull bones in adults are fused and do not show cranial sutures. The forelimbs are modified into wings, which are more or less developed depending on the species, and the wings serve as an aerofoil. The pectoralis muscle accounts for 15% of the total mass of the bird, and the supracoracoideus muscle is also large, providing the power needed for flapping flight. The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac, which is generally larger than mammalian hearts when compared to body mass. This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight. Birds have a ten times greater surface area to gas exchange volume than mammals, and their red blood cells retain their nucleus, unlike in mammals. The nervous system is large relative to the bird's size, with the most developed part of the brain controlling flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. The avian visual system is usually highly developed, with birds being tetrachromatic, possessing ultraviolet sensitive cone cells in the eye as well as green, red and blue ones. Some species have dual fovea, and the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally, acting as a contact lens in many aquatic birds.

The Language of Feathers

Feathers are the defining characteristic of birds, serving as epidermal growths attached to the skin that facilitate flight, provide insulation, and are used in display, camouflage, and signalling. There are several types of feathers, each serving its own set of purposes, and the distribution pattern of these feather tracts, called pterylae, is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex, and is regularly moulted. Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. In passerines, flight feathers are replaced one at a time with the innermost being the first, and when the fifth or sixth primary is replaced, the outermost begin to drop. A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. The tail feathers are moulted and replaced starting with the innermost pair, and the general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from centre outward. Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly, and the skin there is well supplied with blood vessels and helps the bird in incubation. Feathers require maintenance, and birds preen or groom them daily, spending an average of around 9% of their daily time on this. The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland, which protect the feathers' flexibility and act as an antimicrobial agent. This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites. The scales of birds are composed of the same keratin as beaks, claws, and spurs, and are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers, and are thought to be homologous to those of reptiles and mammals.

The Migration Masters

Many bird species migrate to take advantage of global differences of seasonal temperatures, optimising availability of food sources and breeding habitat. These migrations vary among the different groups, with many landbirds, shorebirds, and waterbirds undertaking annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 1,000 kilometers, and shorebirds can fly up to 5,000 kilometers, although the bar-tailed godwit is capable of non-stop flights of up to 11,000 kilometers. Some seabirds undertake long migrations, with the longest annual migrations including those of Arctic terns, which were recorded travelling an average of 70,000 kilometers between their Arctic breeding grounds in Greenland and Iceland and their wintering grounds in Antarctica. Sooty shearwaters, which nest in New Zealand and Chile, make annual round trips of 64,000 kilometers to their summer feeding grounds in the North Pacific off Japan, Alaska and California. Other seabirds disperse after breeding, travelling widely but having no set migration route, and albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons. Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food, and irruptive species such as the boreal finches are one such group. Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics, and others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates. The ability of birds to return to precise locations across vast distances has been known for some time, and in an experiment conducted in the 1950s, a Manx shearwater released in Boston in the United States returned to its colony in Skomer, in Wales within 13 days, a distance of 3,000 kilometers. Birds navigate during migration using a variety of methods, including the sun, a stellar compass, and the ability to sense the Earth's geomagnetism through specialised photoreceptors.