Birds are widely recognized for their ability to soar through the skies, a trait that helps them navigate diverse landscapes, escape predators, and find food. However, the common assumption that all birds can fly is not entirely accurate. While flight defines most avian species, a notable number have evolved to live without it.
Birds That Cannot Fly
Many bird species worldwide cannot fly. Ostriches, the largest living birds, stand up to 2.7 meters tall and weigh over 150 kilograms. These powerful runners use their strong legs to reach nearly 70 kilometers per hour to evade threats. Emus, native to Australia, are similarly large and swift, capable of dashing at almost 50 kilometers per hour.
New Zealand hosts several unique flightless birds, including the nocturnal kiwi, which has strong legs for digging, and the kakapo, the world’s heaviest parrot. Kakapos can climb tall trees and even “parachute” a few meters by spreading their wings. All 18 species of penguins are also flightless, adapting their wings for an aquatic existence.
Reasons for Flightlessness
The inability to fly in these birds is a result of evolutionary changes, often driven by specific environmental conditions. A primary factor is the absence of predators in certain habitats, particularly on isolated islands, which reduces the need for flight as an escape mechanism. When maintaining flight capabilities becomes unnecessary, natural selection can favor other traits. This leads to a reallocation of energy from developing strong flight muscles and lightweight skeletons to other functions, such as improved running or swimming.
Anatomical changes are evident in flightless birds. Unlike flying birds that have a prominent keel on their sternum (breastbone) for anchoring flight muscles, many flightless species, such as ostriches and kiwis, either lack or have a greatly reduced keel. Their wing bones are often smaller or even vestigial compared to their body size. Penguins, while flightless in air, have retained a keel that accommodates their powerful swimming muscles, transforming their forelimbs into paddle-like structures.
Birds with Unique Flight Abilities
Beyond those that cannot fly, some birds exhibit specialized or limited flight patterns that distinguish them from typical fliers. Hummingbirds, for instance, are masters of unique aerial acrobatics, capable of flying forward, backward, sideways, and hovering. Their wings can beat between 60 to 80 times per second, moving in a figure-eight pattern that generates lift on both the up and down strokes. This allows them exceptional maneuverability.
Penguins, while flightless in the air, demonstrate a form of “flight” underwater, using their wings as flippers to propel themselves with remarkable speed and agility. They can reach speeds of up to 22 miles per hour underwater, moving their wings in a manner similar to how birds fly through the air. Some ground-dwelling birds, such as certain species of ducks or domestic fowl, retain the physical capacity for short, infrequent flights, but primarily move by walking or running.