Island Birds Are Losing the Ability to Fly

SCIENCE

New research shows island birds become flightless—quickly and repeatedly. (Nat Geo Phenomena blog)

Use our resources to learn more about birds.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

Southern cassowaries like this father and chick are endemic to the island of New Guinea and coastal areas of Australia’s Cape York Peninsula. Like most flightless island birds, cassowaries have powerful legs and feet. Photograph by Christian Ziegler, National Geographic

Southern cassowaries like this father and chick are endemic to the island of New Guinea and coastal areas of Australia’s Cape York Peninsula. Like most flightless island birds, cassowaries have powerful legs and feet. In fact, those legs can travel 50 kilometers per hour (31 miles per hour) and razor-sharp claws on those feet make the southern cassowary the “most dangerous bird on earth.”
Photograph by Christian Ziegler, National Geographic

Discussion Ideas

  • How did researchers determine that island birds are “edging toward flightlessness”?
    • They compared the anatomy of flightless and volant (flying) birds, as well as changes in the anatomy of island birds over long periods of time. Particularly:
      • flight muscles: Researchers evaluated two remarkable muscles: the pectoralis muscle, which draws wings down, and the supracoracoideus muscle, which draws wings up. Flightless and nearly flightless birds have less-developed flight muscles.
        • Makes sense—unused muscles aren’t well-developed.
      • breast bone: Both the pectoralis and supracoracoideus muscles are located on a bird’s breast bone, or sternum. Researchers found that volant birds generally have a much larger and stronger sternum.
        • Makes sense—a flying bird would need a stronger bone to support all that muscle weight.
      • legs and feet: Researchers found the legs and feet of flightless birds were generally longer and stronger than those of volant birds.
        • Makes sense—flightless birds would use those legs for movement much more than volant birds.

 

  • What factors are shared by island ecosystems that have a high number of flightless or less-volant birds?
    • Islands with few bird species overall. This would indicate less competition for resources among birds.
    • Islands with no mammalian predators. This would indicate fewer threats on the ground or low-lying shrubs.
    • Islands with fewer raptors, or birds of prey. This would indicate fewer threats from the air.

 

  • Why are island birds “edging toward flightlessness”? Read through the great Nat Geo blog post for some help.
    • “It’s easy to see why a diving bird like a cormorant or a ground-dwelling one like a kakapo might lose its ability to fly when predators are absent.”
    • Flight muscles require enormous amounts of energy to maintain. As volant birds such as hummingbirds and kingfishers reduce their flight time, they are “reallocating energy from forelimbs to hindlimbs, away from big flight muscles and towards longer legs.”
      • “Large flight muscles are especially useful when birds take off. That’s the most energetically demanding part of flying, and the bit that’s most important for escaping from ground predators. If such predators are absent, birds can take off at a more leisurely pace, and they can afford to have smaller (and cheaper) flight muscles. (This might also explain why they developed longer legs: they take off more by jumping than by flapping.)”

 

  • Research indicates less-volant island birds might be more vulnerable to introduced species. Can you think of any introduced species that might prey on flightless or less-volant island birds?
MM7817 Southern Cassowary male cassowary with chick (3-4 weeks) walking along beach

The biggest threats to the endangered southern cassowary are habitat loss, vehicle strikes—cars run into them a lot—and dog attacks.
Photograph by Christian Ziegler, National Geographic

TEACHERS’ TOOLKIT

Nat Geo: Birds on Islands Are Losing the Ability to Fly

Nat Geo: All Things Birds

BirdWatching: The amazing muscles and bones that make birds fly

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