Bats and birds both possess the amazing ability to fly through the air with great agility and precision. At first glance, their wings and flight patterns even seem similar. This has led many observers throughout history to assume that the two types of animals must be closely related.

However, modern science tells a more nuanced story about the origins of flight in mammals versus birds and what they actually share in common from an evolutionary standpoint.

If you’re short on time, here’s a quick answer to your question: While bats and birds have evolved the ability to fly independently, they actually belong to completely different classes of vertebrates and are not closely related.

In this approximately 3000 word article, we’ll take an in-depth look at the evolutionary history and ancestry of bats versus birds. We’ll compare their wings, skeletons, and other adaptations for powered flight to see what they have in common and how they differ.

We’ll also overview what the fossil record tells us about when bats and birds each developed flight capabilities. By the end, you’ll have a detailed understanding of the evidence showing bats and birds occupied very distant branches of the vertebrate family tree.

Bats Are Mammals Whereas Birds Are Avian Dinosaurs

Bats belong to the mammalian class Mammalia

Bats are part of the mammalian class Mammalia, which includes creatures that have fur or hair and feed milk to their young. Mammals are warm-blooded, breathe air, and usually give birth to live young rather than laying eggs.

Bats share all these traits with other mammals like humans, dogs, whales, and mice.

Genetic evidence shows bats are most closely related to the Eulipotyphla order of mammals that includes hedgehogs, shrews, moles, and solenodons. Bats are the only mammals capable of true flight. Their wings are formed from webbed stretched skin rather than feathers.

Bat species comprise about 20% of all classified mammal species in the world.

Birds belong to class Aves, which descended from feathered dinosaurs

On the other hand, birds belong to the class Aves which descended from feathered theropod dinosaurs over 150 million years ago during the Jurassic period. They evolved from small, carnivorous dinos and still share many anatomical traits with them like feathers and hollow, air-filled bones.

Aves class creatures are warm-blooded, egg-laying vertebrates who have wings, beaks, and a unique respiratory system. Unlike bats, birds have feathers instead of fur, produce hard-shelled eggs, and do not nourish their young with milk. There are roughly 10,000 known living species of birds existing today.

Trait Bats Birds
Class Mammalia Aves
Body covering Fur Feathers
Reproduction Live birth Eggs
Nourishment of young Milk Regurgitation

As the table illustrates, bats and birds have very distinct features that set them far apart on the evolutionary tree. Bats are more closely related to other fur-bearing mammals whereas birds are literally the descendants of feathered dinosaurs.

While bats seem visually analogous to birds due to their flight capabilities and wings, their internal anatomy and reproductive traits clearly group them with mammals rather than modern avian species that evolved later.

So despite some superficial similarities, the two types of creatures are taxonomically very distant relatives!

Key Differences in the Wings and Flight Mechanics of Bats and Birds

Bats have skin flaps between elongated fingers for wings

Bats are the only mammals capable of true flight. Their wings are composed of stretchy skin known as patagium that extends between their elongated finger bones, attaches to the side of their bodies, and spans from forelimb to hindlimb (National Geographic).

In contrast to feathered bird wings, bat wings have no rigid support structure. While their thin wing membranes are vulnerable to tears and holes, they heal quickly unless infection sets in. Skin flaps enable bats to flex, twist, turn, and even clap their wings silently.

This gives them outstanding agility in navigating complex habitats.

Birds have feathered forelimbs modified from front legs

Birds are the only modern animals besides bats capable of powered flight. Their wings are virtually identical to bird arms and have the same basic bone structure as most mammals’ front legs. Rather than webbing, wings are covered with layers of stiff, aerodynamic feathers that produce enough lift and thrust for sustained flight (Audubon).

Avian wings angle backward to aid flight stroke patterns unlike bat wings that extend from fingers. Feathers also enable an alula or “bastard wing” that helps steady airflow over the wing at slow speeds. Their precise control supports versatile flight styles from hovering to soaring.

Wing shape and flight stroke patterns differ between the two

Bat and bird wings vary significantly in shape, size and orientation (Table 1). Most bird wings are fixed in size, while bat wings stretch as needed to alter airfoil properties. Long, narrow bird wings provide lift for sustained flight and migration efficiency.

Short, broad bat wings enable great maneuverability at the cost of endurance.

Attribute Bats Birds
Wing structure material Skin flaps between bones Feathered forearms
Wing shape Broad, short, flexible, reversible camber Long, narrow, relatively fixed
Stroke plane More horizontal More vertical

Bats pull their wings down and forward on the downstroke versus birds that push their wings down and back (Wu et al. 2012). These mechanics reflect their lifestyles: bats navigating cluttered habitats at night using echolocation; birds migrating vast distances in open air relying heavily on vision.

Fossil Evidence Shows Bats and Birds Evolved Flight Independently

Oldest bat fossils show flight adaptations 50-60 million years ago

The oldest known fossil bats come from the early Eocene epoch, around 50-60 million years ago. These ancient bats, such as Onychonycteris finneyi, already show clear signs of adaptations for flight – including elongated fingers to support wing membranes and light, hollow bones to reduce weight.

According to a 2021 analysis published in Proceedings of the Royal Society B, the wing proportions of these early bats were remarkably similar to modern bats. This suggests bat wings were already highly specialized for true, flapping flight by the early Cenozoic era.

Archaeopteryx and other feathered dinosaurs flew 150+ million years ago

Feathered theropod dinosaurs capable of aerial locomotion arose much earlier, during the late Jurassic period over 150 million years ago. The iconic Archaeopteryx, dating from around 150 million years ago, is considered a transitional form between feathered dinosaurs and modern birds.

While Archaeopteryx could perform gliding or flap-assisted flight, true flapping flight probably evolved in birds later, between 150 and 130 million years ago. Nevertheless, both feathered theropods and bats evolved the anatomical structures necessary for avian flight – feathers and wing membranes – separately from any recent common ancestor.

Skeletal Structures Confirm Bats Are Mammals, Not Closely Related to Birds

Hand bones and digits in bat wings

The wings of bats are actually elongated hand bones, skin membranes, and fingers – clear signs that bats are mammals, not birds. Bats have five fingers connected by flexible skin membranes that form wings capable of powered flight.

In contrast, bird wings have flight feathers attached to their forelimbs, rather than elongated fingers. So while the wings of bats and birds serve a similar function, their underlying structures are quite distinct.

The fact that bats have digits and hand bones in their wings points to bats sharing a common mammalian ancestor, rather than an avian one. In particular, the plagiopatagium part of bat wings corresponds to stretched skin and connective tissues between elongated bones in human hands and fingers.

Hollow avian bones

Birds have a skeletal system uniquely adapted to enable powered flight. Their bones are hollow, extremely lightweight and fused to withstand the stresses of lift and flight. Bats instead have small solid bones typical of mammals like mice.

Experts suggest the difference in density between bird and bat bones accounts for their differing flight styles. Birds flap quickly with shallow wing beats, while bats have slower, deeper wingbeats enabled by sturdier bone structures.

There are over 1,200 species of bats and their bone structures offer further evidence they diverged from a common mammalian ancestor rather than modern avian relatives.

Key differences in reproduction and metabolism

  • Bats are mammals so they give birth to live young and nurse them with milk, while birds lay eggs from which chicks hatch.
  • Mammals like bats maintain a constant body temperature, unlike birds which have variable metabolic rates that rise and fall significantly with ambient temperatures.
  • The average lifespan of a bat species is over 10 times longer than comparable bird species with similar size. Further evidence that birds and bats have distinct evolutionary lineages and reproductive strategies.
  • Metric Bats Birds
    Body Temperature Constant Variable
    Offspring Live Birth Eggs
    Average Lifespan 14 years 1-2 years
    The structural differences between bats and birds confirm they belong to mammalian versus avian lineages. While they both exhibit powered flight, bats clearly share common evolutionary traits and ancestors with other mammals according to comparative analysis of wings, bones, metabolism, lifespan, and reproduction.


    While bats and birds have both achieved powered flight, they did so completely independently through different evolutionary paths, adaptations, and timelines. Their wings utilize different anatomical structures, and their metabolisms, reproductive systems, and other features conform to their separate classifications as mammals versus avian dinosaurs.

    In summary, bats belong to the mammalian class Mammalia, whereas birds belong to the avian class Aves, descended from feathered theropod dinosaurs. Their wings and flight capabilities evolved from different foundations through convergent evolution.

    Bats and birds provide a fascinating example of how similar traits can emerge in quite unrelated species due to similar environmental pressures and advantages.

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