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Birds on Islands Are Losing the Ability to Fly

Before the arrival of humans—and the rats, cats, and other predators that we brought—New Zealand was an idyllic haven for birds. Without ground-dwelling mammalian hunters to bother them, many of the local species lost the ability to fly. There’s the kakapo, a giant, booming parrot with an owl-like countenance; the takahe, weka, and other flightless relatives of coots and moorhens; a couple of flightless ducks; and, of course, the iconic kiwi.

Kakapo (Strigops habroptilus) at night, Codfish Island, New Zealand. Photograph by Stephen Belcher, Minden Pictures, Corbis
Kakapo (Strigops habroptilus) at night, Codfish Island, New Zealand. Photograph by Stephen Belcher, Minden Pictures, Corbis

These birds are part of a pattern that plays out across the world’s islands. Wherever predators are kept away by expanses of water, birds become flightless—quickly and repeatedly. This process has happened on more than a thousand independent occasions, producing the awkward dodo of Mauritius, the club-winged ibis of Jamaica, and the tatty-winged flightless cormorant of the Galapagos.

The call of the ground is a strong one, and it exists even when the skies are still an option. Natalie Wright from the University of Montana demonstrated this by collecting data on 868 species. She showed that even when island birds can still fly, they’re edging towards flightlessness. Compared to mainland relatives, their flight muscles (the ones we eat when we tuck into chicken breasts) are smaller and their legs are longer.

“Pretty much all island birds are experiencing these pressures to reduce flight, even if some can’t go to the extreme,” Wright says.

Her results show that flying isn’t a binary thing, with a clear boundary between taking to the air and staying on the ground. Instead, there’s a full spectrum of abilities between aeronautical swifts and shuffling kiwis, and island birds exist on all parts of that continuum. “None of the species I looked at were flightless or close to being truly flightless,” says Wright. “There’s no point where, all of a sudden, they have much smaller flight muscles.”

Her study began about 20 years ago, when her undergraduate advisor David Steadman started weighing the flight muscles of birds at the Florida Museum of Natural History. When Wright got her hands on the data set, she noticed that fruit doves had smaller flight muscles on islands that were further from the mainland. She then travelled to five natural history museums herself to examine more skeletons. For each one, she measured the long bones in the lower legs and the size of the breastbone—the latter revealed how heavy the bird’s flight muscles would have been in life.

Across nine major groups of birds, with a wide range of lifestyles, body shapes, and diets, Wright found the same trend. On smaller islands with fewer species, no mammalian predators, and fewer birds of prey, birds have repeatedly reallocated energy from forelimbs to hindlimbs, away from big flight muscles and towards longer legs.

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To her surprise, the trend even applied to hummingbirds, for whom flying is an inextricable part of life. Hummingbirds hover in front of flowers to drink nectar. A flightless hummingbird is a dead hummingbird. And yet, even though “islands hummingbirds look like hummingbirds when they fly, they were still reducing their flight muscles and evolving longer legs on islands without predators,” says Wright.

The same was true for kingfishers, flycatchers, tanagers, honeyeaters, and other groups that are extremely dependent on flight. Wright studied the Todiramphus kingfishers across 27 Pacific islands. “Members on islands with fewer than 20 species of birds, which don’t have any predators that can kill an adult kingfisher, have much smaller flight muscles and much longer legs than any members on larger and more populated islands,” she says. “They sit on perches and fly out to grab prey. Their foraging style requires flight, but they’re edging towards flightlessness.”

Why? 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. But why should a hummingbird or kingfisher, which flies all the time, sacrifice some of its aerial prowess?

Because flight muscles come with a cost. Even at rest, larger ones require more energy to maintain. So if birds can get away with smaller ones, evolution pushes them in that direction. 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.)

Wright’s results suggest that island birds might be more vulnerable to introduced predators than anyone appreciated. Even those that can fly aren’t as good at it as their mainland counterparts. They may also help to explain why island birds diversify into such wondrous forms. When they settle in a remote landmass, even the flying ones might quickly lose the power they need to cross oceans and find new homes.

Islands, it seems, create birds that stay on islands.

22 thoughts on “Birds on Islands Are Losing the Ability to Fly

  1. This article was fascinating and hit on tons of great topics – evolution, island biogeography, museum research. But the title is somewhat sensationalist and I think could easily be interpreted as an article about a current conservation concern. The freshman biology majors I teach often mistake natural selection for a conservation threat. I worry that the title could contribute to this kind of misconception.

  2. Does it in the favors of Ervin Zube ”No park is an island” . Habitat shrinking has caused the loss of genetic diversity worldwide. One day the lion will forget to Roar and Leopard will forget to run if our conservation policies implement according to human eyes analysis. This may be serious problem for biosphere.

  3. AFAIK, there’s also a bat in New Zealand that seemed to be evolving towards flightlessness. Probably reconsidering its decision these days.

  4. Buff-banded Rails are much more likely to run than fly,even on the Australian mainland, But they certainly can fly, as I have seen, or they would not suddenly appear at a newly-filled waterhole, or on offshore islands. On Heron Island,Queensland, the Rails were very tame, entering tourist cabins and the dining-room in search of hand-outs. When alarmed, they took off and reached about waist-height before gliding out of the way – they were certainly flightless for most purposes. But there is another advantage to this – it is assumed that the original Rails reached the island with the assistance of storms and high winds. Once there, sustained flight could expose them to the danger of being blown off the island again. Very few storm-blown birds reach a safe island to settle on – most must perish in the sea.

  5. Kiwis are an odd example: they’re ratites, so I assume their ancestors would have been flightless even when they lived in Australia. There’s no real scope for change towards flightlessness.

    Flightlessness needn’t be related to lack of predators, it could be related to lack of mammalian or reptilian competition. (Well, maybe not in the case of hummingbirds.) Did they correct for that?

    I wonder how these principles would apply immediately after a mass extinction, when all sorts of predator niches are empty.

    1. Tinamous are ratites, and they can fly after a fashion. I thought the idea that basal ratites could fly was pretty well accepted these days.

      1. OK, so maybe KIwis separately evolved flightlessness. But in that case emus did as well, despite not being on an island. I think it’s still a funny example.

    1. Thank you for making that simple observation.

      Simple is somehow too often overlooked for those trying to fit preconceived notions together. I find it almost hilarious the way that even the most learned of people, will connect the strangest of dots.

      SHAKING MY HEAD

  6. I wonder if a similar phenomenon happens in cities – where humans are there to keep the predators away, does the need to fly reduce? It certainly seems the pigeons of London are tending towards flightlessness.

  7. I think that research would be comprehensive if the author also provided information how the fact that birds are losing their ability to fly influences to the birds’ population.

  8. What an interesting article to learn about Natalie Wright’s findings across 868 species of birds.

    Although I appreciate the sentiments of the informative piece by Ed Yong, as an amateur birder and adjunct English instructor, I find the author’s choice of language in a couple of places to be a bit creative, as in birds who “have repeatedly reallocated energy from forelimbs to hind limbs” or birds for whom “the call of the ground is a strong one.” Such language may compel the undereducated to assume a Lamarckian-style change to birds, as if those birds that discover predator-free islands suddenly say to themselves, “Hey, let’s change our wings and legs” and then do it. The process of these changes is, as the author correctly states, a result of environmental pressures favoring one behavior over another and likely requires many generations, but the language choice in conveying this information could be a bit less metaphorical, even if it is a feature article. 🙂

    All-in-all, though, it was very interesting to read this concatonated version of Natalie Wright’s research and findings.

  9. “Islands, it seems, create birds that stay on islands.” and evolution says that slow evolvers could end up surviving when predators arrive…mistakes make things perfect

  10. what do you mean humans arrived. dont you think natives were humans. they lived comfortably with birds. it is the white anglosaxon animals you call humans . nothing has more destroyed this world than this white mans attitude

    1. The wording is, unfortunately, correct. Although it is certainly true that European colonialism led to many island extinctions, there is extensive evidence showing that earlier arrivals of humans on many oceanic islands (eg Madagascar, New Zealand, Hawaii, New Caledonia) were followed by extinctions. There is also evidence that birds such as the moss of New Zealand and the giant galling orm Sylviornis neocaledoniae of New Caledonia were hunted by humans well before European colonists arrived, and in fact became extinct before the arrival of the first Europesn ships.

    2. The Maori rapidly exterminated the flightless Moas, within a century or so of their arrival. Very few humans will refrain from catching and eating animals which can’t escape. We noticed in New Zealand that most seabird colonies were placed safely off shore on islands and stacks.

      Further to the topic of flightless animals on islands – Darwin commented on flightlessness in insects on oceanic islands – safer not to fly when winds might blow them out to sea.

  11. Our planet is on constant movement and exchanges, Once a supercontinent, and one day will be again. With the animals is the same.

  12. Fascinating! This is example of devolution.

    Evolution and devolution are depending on selection pressure.

    For human, the same rule applies. Depigmentation of human skin is likely devolutional due to sheltered life (low selection pressure). Increased brain volume in cold region is evolutional due to tough survival environment (high selection pressure).

    In civilized society, high taxation on the subjects in feudal society was high selection pressure for smart productive citizens. To survive, you have to produce enough to pay tax first and still have left over for your own family.

    1. I’m not sure I agree with this (and “devolution” is not a term much used ion zoology in my experience). Flightlessness is not merely a response to a relaxation of selection pressure on islands – it can be a specific adaptation to survival on islands, allowing the evolution of larger body size (and thereby increasing the range of foods available – though some flightless birds have remained small), conserving energy in development of unnecessary features such as large flight muscles, and possibly reducing the risk of being blown out to sea (the latter may be more important for flightessness in insects). In other words, loss of flight can be a positive evolutionary development (as it may have been in the evolution of large predatory dinosaurs, some of which may have had flying ancestors).

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