Some birds lost their ability to fly gradually through evolution. Building wings and flying uses lots of energy and food, so natural selection weeds it out when it isn’t really needed.
This happened most often when birds settled on islands that had no land predators. In these conditions, birds that spent less energy maintaining wings and flying had an advantage, so they passed on their smaller wings and weaker flight muscles to their offspring, slowly producing a flightless species.
How many flightless birds are there?
There are about 40 species of flightless birds in the world including ostriches, emus, cassowaries, rheas, kiwis and penguins. A flightless bird has smaller wings, a smaller breastbone (which anchors the flight muscles) and more feathers than flying birds.
What's the smallest flightless bird?
The smallest flightless bird is the Inaccessible Island rail, which is just 12.5cm long and weighs 35g – that’s less than a small satsuma.
What's the largest flightless bird?
The largest is the ostrich, which stands 2.7m tall and weighs up to 156kg. Some species were far larger but, sadly, with no fear of predators, no defences, and no ability to fly away, humans hunted many species to extinction. Therefore the common ostrich is unsurprisingly the largest bird in the world
Susan Blackmore
Main image: Masai Ostrich, Struthio camelus, in the Masai Mara Game Reserve, Kenya, Africa © Getty Images