When classifying animals, birds are a distinct group that have unique characteristics that separate them from other types of animals. However, there are some animals that may seem like birds at first glance but are actually considered different types of animals altogether. So which animals are commonly mistaken for birds but are not scientifically classified in the avian family?
Bats
One of the most common animals mistaken for a bird is the bat. Bats have wings and can fly just like birds, so it’s easy to see why people might group them together. However, bats are actually mammals, not birds. Here are some key differences:
Birds | Bats |
Have feathers | Have fur |
Wings made of arms/fingers | Wings made of stretched skin |
Lay eggs | Give live birth |
As mammals, bats have fur, give live birth to their young, and nurse their babies with milk. They are the only mammals capable of true flight. Their wings are formed by a stretched skin membrane between their long fingers, unlike bird wings which are made of feathers and adapted arms/fingers. While the flight of bats may resemble birds, they do not share the same anatomy.
Flying Squirrels
Like bats, flying squirrels can glide through the air but are not considered birds. Flying squirrels are rodents. They use a furry membrane called a patagium that extends between their front and back legs, allowing them to glide between trees. Unlike bats, they cannot flap their arms/fingers to gain lift and so can only glide for short distances. Their body anatomy is that of a regular squirrel, not a bird. While their gliding flight looks bird-like, flying squirrels do not technically fly and are classified as gliding mammals.
Insects
Many insects like flies, bees, moths, and beetles have wings and can actively fly through the air. However, insects have an entirely different anatomy and taxonomy from birds. They have six legs, exoskeletons, compound eyes, antennae, and other anatomical features that distinguish them from birds. Birds have two legs, feathers, and other avian characteristics. In terms of classification, birds are vertebrates while insects are invertebrates. So while their flight capabilities may seem similar, insects do not share enough biology with birds to be considered avian or closely related. Their wings and flight mechanics evolved separately.
Pterosaurs
Pterosaurs were flying reptiles that lived during the age of the dinosaurs. They had large leathery wings and could swoop and soar through the air. Although they looked rather bird-like, pterosaurs are not classified as birds or even dinosaurs, but rather as reptiles. Birds evolved much later after the extinction of the pterosaurs and dinosaurs. Unlike birds, pterosaur wings were formed by a membrane of skin stretched between an extremely long fourth finger and their body. Pterosaurs had scales, reproduced by laying eggs, and had other features of reptiles that birds lack. So despite appearances, pterosaurs are not considered primitive birds or even closely related to the avian family.
Bird Classification
Now that we’ve looked at some animals commonly mistaken for birds, let’s discuss the scientific classification of actual birds. Birds belong to the class Aves within the phylum Chordata. Some distinctive features of Aves include:
- Feathers
- Beaks without teeth
- Lightweight skeletons
- Two legs adapted for walking, perching, swimming etc.
- Wings adapted for flight (though some birds like ostriches cannot fly)
- Hard-shelled eggs
- High metabolisms and unique respiratory systems for flight
- Hollow bones
- Distinctive cardiovascular systems with four-chambered hearts
- Body temperatures around 105°F
- Lightweight bills instead of heavy teeth and jaws
- Excellent vision
- Lack of body hair
These anatomical traits define birds scientifically and distinguish them from bats, flying squirrels, insects, pterosaurs, and other animal groups that exhibit some aerial behaviors but lack other avian features. There are estimated to be around 10,000-18,000 species of birds living today. They are distributed worldwide and adapted to a diverse array of habitats and ecological roles. Scientists are still discovering new bird species as well, especially in remote tropical regions.
Major Bird Groups
Birds are divided into a hierarchy of different taxonomic levels:
- Class: Aves – all birds
- Infraclass: Neornithes – modern birds
- Superorders: Palaeognathae (ratites and tinamous) and Neognathae (all other neornithines)
- Orders: Struthioniformes (ostriches), Rheiformes (rheas), Tinamiformes (tinamous), Sphenisciformes (penguins), etc.
- Families: Anatidae (ducks, geese, swans), Laridae (gulls), Trochilidae (hummingbirds), and many more
- Genera and species: Specific bird species like Mallard duck (Anas platyrhynchos), Emperor Penguin (Aptenodytes forsteri), House Sparrow (Passer domesticus), and so on
This hierarchical taxonomy organizes the diversity of birds based on their evolutionary relationships and genetic similarities. Modern bird classification relies heavily on DNA evidence to sort out which species diverged from common ancestors earlier versus more recently. Scientists can look at the degree of differences in birds’ genetic codes to determine how closely or distantly they are related.
Examples of Birds
To further demonstrate proper bird classification, let’s go through some specific examples of common bird species and what groups they belong to:
Ostriches
- Order: Struthioniformes
- Family: Struthionidae
- Genus/species: Struthio camelus
Ostriches are large, flightless birds native to Africa. They belong to an ancient order of birds called Struthioniformes which includes emus, rheas, kiwis, and other ratites. Ratites are distinguished by their flat breastbones without keels. Ostriches cannot fly but are adapted runners.
Hummingbirds
- Order: Apodiformes
- Family: Trochilidae
- Example genus/species: Archilochus colubris (Ruby-throated Hummingbird)
Hummingbirds are small, agile birds specialized for hovering flight and nectar-feeding. There are over 300 described hummingbird species divided among dozens of genera. But they all belong to the exclusive family Trochilidae within the order Apodiformes along with swifts. Hummingbirds have very rapid wing beats enabling them to hover and fly backwards.
Bald Eagles
- Order: Accipitriformes
- Family: Accipitridae (hawks, eagles, kites, harriers)
- Genus/species: Haliaeetus leucocephalus
The bald eagle is a large raptor species native to North America. As a member of Accipitridae, it is related to other birds of prey like hawks, kites, and harriers. Its distinctive traits include a heavy straight bill, strong talons, keen eyesight, and large wings adapted for powerful flight. Bald eagles are a symbol of the United States.
Penguins
- Order: Sphenisciformes
- Family: Spheniscidae
- Example genus/species: Aptenodytes forsteri (Emperor penguin)
Penguins are flightless seabirds found exclusively in the southern hemisphere. They form their own distinct order Sphenisciformes. Penguins have evolved adaptations for swimming, diving, and underwater vision rather than flight. Their stiff, flipper-like wings provide propulsion in water. There are over a dozen penguin species adapted to various climates from Antarctica to the Galapagos Islands.
Pigeons and doves
- Order: Columbiformes
- Family: Columbidae
- Example genus/species: Columba livia (rock pigeon)
Pigeons and doves comprise the bird family Columbidae which holds over 300 species found worldwide. They are granivorous birds with small heads, compact bodies, small bills, and slender legs. Many species exhibit iridescent plumage and distinct cooing vocalizations. They are highly adapted for flight and have excellent navigation abilities. Both feral and domesticated pigeons are in the genus Columba.
Chickens
- Order: Galliformes
- Family: Phasianidae (pheasants, grouse, chickens)
- Genus/species: Gallus gallus domesticus (domestic chicken)
The chicken belongs to the diverse order Galliformes which includes turkeys, quail, partridges, pheasants, and other heavy-bodied ground birds. They are domesticated members of the genus Gallus, most likely from the Red Junglefowl species. Chickens are largely terrestrial birds that have lost the ability to fly for extended distances. Their feet, wings, beaks, and skeletons have been anatomically modified through selective breeding by humans over centuries for food production.
Conclusion
While many animals like bats, flying squirrels, insects, and extinct pterosaurs may appear at first glance to resemble birds when they take to the air, on closer examination none of these species technically qualify as avian or belonging to the class Aves. Scientifically, birds are defined by a distinct set of anatomical and physiological traits related to flight, from feathers, to bills, to metabolism, that are not found in any other animals. Within the evolutionarily related class Aves, ornithologists classify modern birds into orders, families, genera, and species based on morphological, genetic, and behavioral criteria. So only animals that meet the criteria of feathered, egg-laying, bipedal vertebrates within the class Aves are considered true birds. All other flying creatures may mimic some aspects of birds to become airborne, but remain biologically distinct and belong to other taxonomic groups like mammalia, insecta, or extinct reptilia. The next time you see an animal aloft, look for key indicators like feathers versus fur or scales to determine if it is really an avian species or just borrowing some tricks from birds to take to the skies. Proper classification continues to evolve with new research, but the distinctive features that define birds scientifically will remain.