Take Flight

Birds: A Summary of Evolution, Classification, and Diversity

Birds are warm-blooded vertebrates with feathers, toothless beaked jaws, the ability to lay hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.
There are over 10,000 living bird species, ranging in size from the 5.5 cm bee hummingbird to the 2.8 m common ostrich, and more than half of them are passerine or "perching" birds.
Birds have wings, which are modified forelimbs, allowing them to fly, and the digestive and respiratory systems of birds are uniquely adapted for flight.
Birds are social and communicate with visual signals, calls, and songs, and some species participate in cooperative breeding, hunting, flocking, and mobbing of predators.
Birds produce offspring by laying eggs, which are usually laid in a nest and incubated by the parents, and most birds have an extended period of parental care after hatching.
Birds are economically important as food for human consumption and raw material in manufacturing, and some species are popular as pets.
Birds figure throughout human culture, and recreational birdwatching is an important part of the ecotourism industry.
Birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians.
Birds are descendants of the primitive avialans, which first appeared during the Late Jurassic, and modern birds evolved in the Early to Late Cretaceous, diversifying dramatically around the time of the Cretaceous–Paleogene extinction event 66 mya.
The first large, diverse lineage of short-tailed avialans to evolve were the Enantiornithes, and the second major avialan lineage to diversify, the Euornithes, included the ancestors of modern birds.
There is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous or occurred later in the Cenozoic era, and this disagreement is in part caused by a divergence in the evidence.
The discovery of Vegavis from the Maastrichtian, the last stage of the Late Cretaceous, proved that the diversification of modern birds started before the Cenozoic era.
The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago, are still too controversial to provide fossil evidence of modern bird diversification.Birds: Classification, Genomics, Anatomy, Physiology, and Distribution
There is ongoing controversy over the molecular and fossil evidence of the origin and diversification of modern birds, but recent studies suggest that modern birds originated in Western Gondwana and expanded through two routes, including an Antarctic interchange and Paleocene land bridges between South American and North America.
All modern birds are classified as Aves, which has two subdivisions: Palaeognathae and Neognathae. Palaeognathae includes flightless ratites and weak-flying tinamous, while Neognathae contains all other birds. The number of known living bird species is around 10,906.
The classification of birds is a contentious issue, but recent studies using new fossil and molecular evidence are providing a clearer picture of the evolution of modern bird orders. As of 2022, the genomes of 542 species of birds have been completed, giving researchers new information about genes, DNA, and evolutionary history.
Birds live and breed in most terrestrial habitats and on all seven continents, with the highest bird diversity occurring in tropical regions. Many bird species migrate annually over great distances and across oceans. Some bird species have established breeding populations in areas to which they have been introduced by humans, either deliberately or accidentally.
Birds have many unusual adaptations to facilitate flight, including a skeletal system with lightweight bones, cervical, thoracic, lumbar, and caudal regions, fused skull bones, flattened ribs, and a keeled sternum. The respiratory system is complex, with air-filled cavities and posterior air sacs that fill the bones with air. The circulatory system is driven by a four-chambered heart, and the nervous system is large relative to the bird's size.
Birds are primarily uricotelic, and they do not have a urinary bladder or external urethral opening. Female birds have sperm storage tubules, and most have a single ovary and a single oviduct, both on the left side. The avian visual system is usually highly developed, with birds tetrachromatic and possessing ultraviolet-sensitive cone cells in the eye.Birds: Flight, Feathers, Behaviour, and More
Birds are capable of flight, which distinguishes them from most other vertebrates.
The avian eye lacks external pinnae and is lubricated by the nictitating membrane instead of eyelids.
Birds use a variety of defence mechanisms including spurs, bony knobs, and chemical defences.
Feathers are epidermal growths that facilitate flight, provide insulation, and are used for display, camouflage, and signalling.
Plumage is moulted regularly, and moulting patterns vary across species.
Most birds are diurnal, but some are nocturnal or crepuscular, and many coastal waders feed at appropriate tide times.
Bird diets are varied and include nectar, plants, carrion, and various small animals.
Water is needed by many birds, and some desert birds can obtain their water needs entirely from moisture in their food.
Feathers require maintenance, and birds preen or groom them daily.
Many bird species migrate to take advantage of global differences in seasonal temperatures.
Migration is highly demanding energetically, and birds need to cross deserts and oceans without refuelling.
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food.Bird Behavior: Migration, Communication, Flocking, Resting, Breeding, and Parental Care
Partial migration is common among birds in some regions, with 44% of non-passerine birds and 32% of passerines in Australia being partially migratory.
Altitudinal migration is a form of short-distance migration in which birds move to lower altitudes during suboptimal conditions.
Birds navigate during migration using the sun, a stellar compass, and their ability to sense the Earth's geomagnetism through specialized photoreceptors.
Birds communicate primarily using visual and auditory signals, with calls and songs being the major means of sound communication.
Flocking provides safety in numbers and increased foraging efficiency, but also has costs such as bullying and competition for resources.
Many birds use communal roosting to lower the loss of body heat and decrease the risks associated with predators.
Most bird species are socially monogamous, but extra-pair copulation is common in socially monogamous species.
Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds may build more open nests.
Incubation periods range from 10 days to over 80 days, and parental care varies widely among different orders and species.
The length and nature of parental care varies widely, with some species having extended periods of parental care and others having parental care that ends at hatching.
Alloparenting, where other members of the same species help with raising the young, is common in some bird species.
The point at which chicks fledge varies dramatically, with some chicks leaving the nest the night after they hatch and others taking up to six months to fledge.