Hollow bones
Many bones are hollow with inner struts, light but strong. A frigatebird's skeleton can weigh less than its feathers.
The Zoo · New exhibits · Wings, keels, flightless birds
Birds that fly, birds that don't
A brown pelican glides inches over the waves on wings two meters wide. An emu cannot lift off at all. A wild turkey flies; a farm turkey cannot. Why?
Built to fly
Four adaptations
A keel
The breastbone has a deep ridge (keel) that anchors the big flight muscles. Most flightless birds have a flat breastbone.
Feathers
Flight feathers are stiff and lopsided, shaped like small wings; they overlap to make an airtight surface on the downstroke.
Air sacs
Birds breathe through lungs plus nine or so air sacs, so fresh air flows one way through the lungs: enough oxygen for flapping at altitude.
Birds that do not fly
Flightlessness evolved many times, usually where there were no ground predators (islands) or where running or swimming paid off better than flying.
Emperor penguin
Aptenodytes forsteri
NTNear threatened
Common ostrich
Struthio camelus
LCLeast concern
Emu
Dromaius novaehollandiae
LCLeast concern
Southern cassowary
Casuarius casuarius
LCLeast concern
North Island brown kiwi
Apteryx mantelli
VUVulnerable
Dodo
Raphus cucullatus
EXExtinct
Hops and heavy breasts
Chickens descend from the red junglefowl of Southeast Asia, which flies in short bursts up into trees to roost. Chickens keep that burst: a hop onto a fence or perch. Heavy meat breeds can barely get off the ground.
Wild turkeys fly well, fast and low for short distances, and sleep in trees. The broad-breasted white turkey on most farms was bred for breast meat until it became too heavy to fly, or even to mate naturally; farms use artificial insemination.
This is selective breeding: people chose which birds bred, and the birds changed. Darwin opened On the Origin of Species (1859) with domestic pigeons for the same reason. See Lamby: Feathers and eggs for how chickens and turkeys were domesticated.
Grades K–5
Why can't a penguin fly?
Penguins do fly, underwater! Their wings became stiff flippers for swimming fast after fish. An ostrich is too big to fly, so it runs instead, faster than a horse for a short way.
For classes
Grades 3-5
The Bird Aviary: Predict who lands wherePreview Grades 6-8
The Dinosaur Hall: Is a bird a dinosaur?Preview Grades 9-12
Evolution and Ecology: Natural selection on lavaPreview Drawer: Zoo: Birds that fly, birds that don't QR 3MBLJS
Quiz: Birds that fly, birds that don't quiz · 6 questions
Next door: Polar coast: penguins · Natural history: penguin room · Lamby: Feathers and eggs
Sources
- Cornell Lab of Ornithology: Bird Academy, How birds fly
- Cornell Lab: Wild Turkey
- Smithsonian National Museum of Natural History: the dodo
- Department of Conservation, New Zealand: Kiwi
- Mitchell, K.J. and others (2014) Ancient DNA and the evolution of flightless ratites. Science
Checked November 2026. Figures marked (est.) are estimates; statuses are flagged in the catalog for re-checking against the live IUCN Red List before acceptance.