Birds are a diverse class of vertebrates found worldwide. There are approximately 10,000 known living species of birds, making them the most speciose class of tetrapod vertebrates. Birds inhabit ecosystems across the globe, from the Arctic to the Antarctic. Over 60% of living bird species are found within just a few orders. Understanding the taxonomy and phylogeny of birds provides insight into their evolution and ecology. This article will examine the orders that comprise 60% of avian diversity.
Passeriformes (Perching Birds)
The order Passeriformes, also known as perching birds or passerines, is the most speciose order of birds, containing over 50% of all avian species. There are over 5,700 recognized species of passerines including larks, swallows, crows, finches, and sparrows. Many passerines are small songbirds, but the order also includes larger non-vocal species like ravens and birds-of-paradise. Passerines are found on every continent except Antarctica and occupy terrestrial habitats from the Arctic to the tropics. Most perching birds have feet adapted for perching with three toes pointed forward and one toe pointed backward. This foot arrangement helps passerines cling to branches. Passerines are vocal learners, meaning that songs are learned from parents rather than being entirely innate. This order contains some of the most intelligent and behaviorally complex birds. The large number of passerine species reflects their evolutionary success and adaptability. Perching birds likely originated during the Eocene around 50 million years ago and diversified rapidly. Their small body size, ability to exploit many ecological niches, and learned vocalizations have allowed passerines to become the dominant avian group.
Psittaciformes (Parrots)
Parrots are highly intelligent and social birds in the order Psittaciformes. There are over 390 species of parrots divided into 92 genera. Parrots have a cosmopolitan distribution, inhabiting tropical and subtropical regions across the world. The greatest diversity of parrot species is found in South America and Australasia. All parrots share several unifying features including a large, curved bill, zygodactyl feet with two toes facing forward and two backward, and a short, square tail. Parrots use their bills for cracking hard nuts and seeds. Their feet provide excellent grip for climbing and hanging upside down. Most parrots are herbivores, feeding on seeds, nuts, fruits, buds, and other plant material. Some larger species may eat insects and other small prey. Parrots are monogamous and mate for life. They nest in tree cavities and lay only a few eggs. Parrots have long developmental periods before fledging. As chicks, they depend on parental care and feeding for an extended period. Parrots are renowned for their intelligence and ability to imitate sounds and human speech. Their brains are relatively large compared to other birds, with developed areas associated with higher cognitive functions. The popularity of parrots as pets reflects their smart, engaging nature. Threats to wild parrot populations include habitat loss, capture for the pet trade, and competition with invasive species. Overall, Psittaciformes represent an evolutionarily distinct lineage of highly intelligent birds.
Apodiformes (Swifts and Hummingbirds)
Apodiformes is an order of specialized birds adapted for aerial life and flight. There are over 400 species of swifts, hummingbirds, and tree-swifts in the order. Apodiformes have a global distribution, occurring on every continent except Antarctica. They inhabit a wide range of habitats from forests to open scrubland. Apodiformes share several adaptations for rapid, agile flight including short legs with weak feet, long pointed wings, and short bills. Their feet are used mainly for perching rather than walking or climbing. The wings of Apodiformes are long, narrow, and curved to provide exceptional lift and propulsion. Many species have tiny feet with sharp claws for clinging to vertical surfaces. They have very short legs that they use primarily for perching, providing little leverage for walking or hopping on the ground. Aerodynamic streamlined bodies, lightweight skeletons, and small body size also improve aerial maneuverability. Most feed exclusively on aerial insects and nectar while in flight. Swifts spend nearly their entire lives airborne, even sleeping and mating during flight. Hummingbirds are the only birds able to hover and fly backwards. Their wings beat up to 200 times per second, making them capable of incredible agility. The specialized adaptations of Apodiformes allow them to thrive as nimble, aerial insectivores and nectarivores. Their mastery of flight has enabled swifts and hummingbirds to exploit food resources unavailable to other birds.
Gruiformes (Cranes, Rails, and Relatives)
The order Gruiformes encompasses a diverse group of medium to large birds including cranes, rails, coots, gallinules, finfoots, and trumpeters. There are approximately 210 living species distributed worldwide. Gruiformes occupy freshwater and marine wetland habitats. Most are associated with marshes, swamps, or the edges of ponds and streams. Gruiformes have long legs, toes, and necks adapted for wading and swimming. Their bills are frequently long, tapered, and pointed for probing into muddy bottoms in search of food. Omnivorous diets consist of insects, fish, amphibians, reptiles, plant material, and seeds. Rails tend to be secretive wetland birds that prefer dense vegetation, while cranes are more terrestrial and open habitat specialists. Cranes are among the largest flying birds and known for their elaborate courtship displays. The size and proportions of Gruiformes are variable based on foraging habits. Finfoots have powerful legs for swimming and lobes on their toes for diving. Trumpeters have chicken-like bills for rooting through loose soil. The common ancestry of Gruiformes is indicated by shared features of their skeleton, feathers, and egg structure. The wide range of aquatic ecological roles played by Gruiformes illustrates their evolutionary adaptation to wetland environments worldwide.
Charadriiformes (Shorebirds, Gulls, and Auks)
The diverse order Charadriiformes includes shorebirds, gulls, terns, auks, and other waterbirds. With over 350 species, Charadriiformes are found worldwide in proximity to water. Most occupy coastal marine habitats such as rocky shores, beaches, estuaries, and tidal marshes. Some species live around inland wetlands. Many undertake long annual migrations, breeding in the Northern Hemisphere and wintering in the Southern Hemisphere. Hallmarks of Charadriiformes include long legs, long wings, webbed feet, and bills of varied shapes specialized for different diets. Shorebirds such as plovers and sandpipers have slender bills to probe exposed sediments for invertebrates. Gulls are more omnivorous opportunists. Auks are pursuit divers that catch fish underwater using their wings for propulsion. Terns plunge-dive from the air to catch fish near the water’s surface. While variable in form, most Charadriiformes are linked by vocalization anatomy and a salt gland for excreting excess salt from their marine diet. They consume a wide range of aquatic animals including fish, mollusks, crustaceans, insects, and worms. Some also eat plant material, carrion, and refuse. Many species nest in large colonies for safety from predators. Charadriiformes are designed for aquatic habitats, endowed with adaptations for locomotion, foraging, and osmoregulation in marine and freshwater ecosystems.
Columbiformes (Pigeons and Doves)
Columbiformes encompasses pigeons, doves, and extinct dodos. With over 300 living species, Columbiformes are found worldwide in most habitats outside of polar regions and deep oceanic islands. Pigeons and doves comprise the most diverse family of birds after passerines. They primarily inhabit tropical and subtropical forests, savannas, grasslands, scrublands, and urban areas. Most columbiforms are stout-bodied with small heads, short legs, and thick bills with a fleshy cere. They are predominantly herbivorous, feeding on seeds, grains, fruits, and plants. Their flight is fast and direct using quick shallow wing beats interspersed with glides. Distinctive features of Columbiformes include unique head feathers, drink milk produced by their crop lining, and the ability to suck up water without lifting their head. They are monogamous and often display elaborate courtship rituals. Both sexes produce “crop milk” to feed hatchlings. Columbiformes likely originated 60-80 million years ago and diversified rapidly with the spread of grasslands and agriculture providing abundant food sources. Due to their near-global distribution, variety of ecological roles, and association with humans, Columbiformes represent one of the most successful avian orders.
Conclusion
In summary, over 60% of the world’s approximately 10,000 living bird species belong to just a few orders. Passeriformes alone account for over 50% of all species. The other most speciose orders are Psittaciformes, Apodiformes, Gruiformes, Charadriiformes, and Columbiformes. These major radiations occupy diverse ecological niches in terrestrial, wetland, and marine environments worldwide. Their distributions and distinct adaptations illustrate the evolutionary diversification of birds into a myriad of forms reflecting specialization for various lifestyles and habitats. Passerines in particular have achieved dominance through behavioral flexibility, vocal learning, and exploitation of small niches. From parrots to hummingbirds, shorebirds to pigeons, these orders encompass much of the avian diversity that has emerged over tens of millions of years since birds first took to the skies. Understanding the taxonomy and natural history of these groups provides deeper insight into the origin and ecology of birds, a uniquely successful vertebrate lineage.
Key Points
- Passeriformes (perching birds) account for over 50% of all bird species with around 5,700 species worldwide.
- Psittaciformes (parrots) have over 390 species and are known for intelligence, longevity, and ability to mimic speech.
- Apodiformes (swifts and hummingbirds) are specialized aerial insectivores and nectarivores with adaptations for agile flight.
- Gruiformes (cranes, rails) are wetland specialists with long legs and necks for wading and probing in marshes.
- Charadriiformes (shorebirds, gulls) have diverse bill shapes and adaptations for locomotion in coastal marine habitats.
- Columbiformes (pigeons and doves) are a widespread and successful order comprising over 300 primarily granivorous species.
Order | Common Names | Approximate Number of Species | Key Adaptations |
---|---|---|---|
Passeriformes | Perching birds, songbirds | Over 5,700 species | Adaptable, vocal learning |
Psittaciformes | Parrots | Over 390 species | Intelligence, mimicry |
Apodiformes | Swifts, hummingbirds | Over 400 species | Specialized flight, aerial agility |
Gruiformes | Cranes, rails | About 210 species | Wading in wetlands |
Charadriiformes | Shorebirds, gulls, auks | About 350 species | Coastal habitats, varied bills |
Columbiformes | Pigeons, doves | Over 300 species | Granivorous, globally distributed |
Ending Summary
To summarize, the majority of the world’s approximately 10,000 bird species belong to a handful of orders that display tremendous diversity reflecting adaptations to a wide array of lifestyles and habitats. Perching birds alone account for more than half of all avian diversity. Parrots, swifts, wetland species, shorebirds, and pigeons represent further radiations with hundreds of specialized species filling roles in ecosystems across the globe. The taxonomy and natural history of these diverse yet related groups provides insight into the evolution and ecology of class Aves. Understanding what orders encompass most living bird species is key to studying the origin, expansion, and adaptations of birds over time. These major avian radiations continue to thrive, inhabit nearly all environments and regions worldwide, and provide vital ecosystem services helping sustain ecological health.