Biology
The Evolution of Flightlessness in Island Birds and Its Ecological Consequences
Quick fact
Over 1,000 bird species have lost the ability to fly, most of them island dwellers. Since 1500, nearly all the birds that have gone extinct (about 90%) were flightless island species, a vulnerability directly linked to their loss of flight.
Why this is interesting
You know the kiwi, the bird that can't fly and has whiskers like a cat? How did it and many others lose the ability to do the one thing that most defines a bird?
Read the full explanation
Understanding The Evolution of Flightlessness in Island Birds and Its Ecological Consequences
Islands are natural laboratories for evolution. When birds colonize a remote island, they often find a world without the ground predators (mammals, snakes) they evolved to escape. In this predator-free haven, the energy saved by not maintaining heavy flight muscles becomes valuable. Natural selection favors individuals that invest less in wings and more in body size, using that saved energy for reproduction or food gathering. Over generations, the descendants of the flying founders become larger, their wings shrink, and their breastbone keel (where flight muscles attach) flattens. They become flightless—but often more robust and successful in their new home.
A deeper explanation
The evolution of flightlessness on islands is driven by a classic trade-off in natural selection. Flight is energetically very expensive; a flying bird consumes up to 20 times more energy than a foraging flightless one. When predation pressure is low, the benefit of escape decreases, and the cost of flight becomes a liability. Selection therefore favors mutations that reduce the energy investment in wing structures. This often accompanies an increase in body size, which can improve thermoregulation, competitive ability, and the capacity to store energy for breeding. However, this adaptation has a fatal downside: flightless birds are extremely susceptible to new ground predators. When humans arrive on islands, they bring rats, cats, dogs, and other mammals. Flightless birds have no natural defenses, and because they often breed in low numbers and on the ground, they are easily wiped out. This explains the disproportionate number of island flightless species among recent extinctions.