Bat vs Bird: How Flying Mammals Differ From Birds

Key differences at a glance

Bats and birds are the only living vertebrates (animals with a backbone) capable of true, flapping flight. That shared skill makes them look alike at dusk, but almost everything else about them is different. A bat is a mammal, closer to a mouse, a dog or a human than to any bird; you can read more in are bats mammals?.

With more than 1,400 known species, bats make up roughly one in five of all mammal species, second only to rodents. Birds are a far larger group, with more than 11,000 living species, from hummingbirds to ostriches. Many birds cannot fly at all, but every bat species can.

Wings: skin membrane versus feathers

The biggest visible difference is the wing. A bat’s wing is built like a hand. Four very long finger bones fan out like the ribs of an umbrella, and a thin, elastic membrane of skin, called the patagium, stretches between them and down to the legs. The thumb stays free as a small hooked claw that bats use for climbing. The name of the bat order, Chiroptera, means “hand-wing.”

A bird’s wing works differently. The hand bones are reduced and fused, and the surface of the wing is formed by long flight feathers attached to the arm and hand. Feathers are made of keratin, the same protein as our hair and nails, and they are replaced in a regular molt.

Because a bat’s wing has many joints and muscles, bats can change its shape in flight with great precision. This makes many bats superb at slow, twisting flight among trees and around insects. Birds’ stiffer, feathered wings are better suited to long-distance travel, gliding and soaring. The famous fliers of the bird world, such as albatrosses and swifts, can stay airborne for weeks or months.

Fur, teeth and other body differences

Bats are covered in fur, like other mammals, and their wings are bare skin. Birds are the only living animals with feathers, which keep them warm, waterproof them and let them fly.

Bats have jaws with teeth suited to their diet: sharp cusps for crunching insects, broad teeth for fruit, or razor-edged incisors in the three species of vampire bats. Birds have no teeth. Instead, they have a lightweight beak and often a muscular gizzard that grinds food.

Inside, birds have many hollow, air-filled bones connected to a system of air sacs, which pumps air through the lungs in one direction. Bats have thin but not air-filled bones and lungs like other mammals. Both groups are warm-blooded, and both have large hearts and fast metabolisms to power flight.

Reproduction: live young and milk versus eggs

All bats give birth to live young. Most species have just one pup per year, which is unusual for such small mammals, and the mother nurses it with milk from her mammary glands. A newborn bat can be up to a quarter of its mother’s weight, and in many species the mothers gather in large nursery colonies.

Birds lay eggs with hard, chalky shells, which they incubate in a nest or, in some species, on bare ground or on their feet. Many birds raise large broods. A few, such as pigeons and flamingos, feed their chicks a nutritious “crop milk,” but this is a secretion from the throat, not true milk.

Because bats breed slowly, their populations take a long time to recover from losses. For their size, bats are also remarkably long-lived: a wild Brandt’s bat in Siberia was recorded at 41 years old.

Echolocation and senses

Most bats hunt at night and find their way using echolocation. They produce high-pitched calls, mostly too high for people to hear, and listen to the echoes to build a sound picture of their surroundings. Insect-eating bats can detect and catch tiny moths and mosquitoes in total darkness; see how many insects does a bat eat in a night?.

Contrary to the saying “blind as a bat,” no bat is blind, and the large fruit bats called flying foxes rely mainly on their big eyes and keen sense of smell. Most birds depend chiefly on eyesight, and many see colors that humans cannot. Only a handful of birds, such as the oilbird and some cave swiftlets, use simple clicking echolocation in dark caves.

Flight, roosting and daily life

Most birds are active by day and sleep at night, perched on branches, in cavities or on water. Most bats do the reverse. They spend the day roosting in caves, tree hollows, under bark or in buildings, and come out at dusk.

Bats rest hanging upside down. Special tendons in their feet lock the toes around a perch, so hanging costs little energy, and a bat can take off simply by dropping and opening its wings. Birds have a similar locking system that keeps them on a branch while they sleep, but they perch upright.

Speed varies widely in both groups. Brazilian free-tailed bats tracked from an aircraft in Texas reached ground speeds of about 160 km/h (99 mph), possibly helped by wind, which would be faster than any bird in level flight. In a dive, though, the peregrine falcon is the fastest animal on Earth; compare the records in our list of the fastest animals.

How bats and birds evolved flight separately

Bat wings and bird wings are a classic example of convergent evolution, in which unrelated animals evolve similar features to solve the same problem. Both are modified front limbs, but they were reshaped in different ways.

Birds descend from small, feathered theropod dinosaurs. Feathers appeared before flight, and birds such as Archaeopteryx were flying, or at least gliding, about 150 million years ago. Bats evolved from small, ground-living mammals much later. The oldest known bat fossils, from about 52 million years ago in Wyoming, already had fully formed wings, so the earliest stages of bat flight are still unknown.

Insects were the first animals to fly, and the extinct pterosaurs, reptiles with a skin wing stretched along one enormously long fourth finger, were the third vertebrate group to master powered flight.

Which is bigger and where they live

The largest bats are flying foxes of Asia and the Pacific, with wingspans of up to about 1.5–1.7 m (5–5.6 ft) and weights of around 1–1.6 kg (2.2–3.5 lb). The smallest is Kitti’s hog-nosed bat of Thailand and Myanmar, which weighs about 2 g (0.07 oz). Birds cover a much wider range: the bee hummingbird of Cuba weighs about 2 g, while the wandering albatross has a wingspan of more than 3 m (10 ft).

Bats live on every continent except Antarctica, and on many remote islands they were the only native mammals. Birds live on every continent, including Antarctica. Both groups play key roles in nature as insect hunters, pollinators and seed spreaders. In North America, bats face a serious threat from white-nose syndrome, a fungal disease that has killed millions of hibernating bats since 2006.