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For Unusual Birds, Does Distinction Raise the Risk of Extinction?

Using an innovative analysis of the "morphospace," scientists discovered that the world’s most unusual bird species are the ones most at risk of extinction. Their loss would leave a less diverse, interesting, and functional world.

a blue and black bird with a berry in its beak
Asian Fairy-Bluebird by Anonymous/Macaulay Library.

Birds with unusual bills, extreme size (big or small), and specialized survival strategies are most threatened by the global extinction crisis.

According to a study published in July in the journal Current Biology, the most superlative birds with the most extraordinary lifestyles—such as the Red-headed Vulture, Giant Ibis, Bengal Florican, and Seychelles Scops- Owl—face the greatest risks in this age of climate change and habitat loss. Their disappearance would lead to a “homogenization” of the world’s bird life, the study authors say.

According to lead author Emma Hughes, an ecologist at the University of Sheffield in England, the loss of the world’s weirdest birds would mean a loss for the world at large.

“Losing these diverse, unusual species is likely to have an impact on ecosystem functioning and services, as there are very few or no other species that have those extreme trait combinations,” she says.

Hughes grew up in the English countryside, where she was “fascinated with the diversity of life, and why there is so much variation in what species look like,” she says. “Birds in particular really captured my imagination as they are such a diverse group in terms of their appearance and behavior.”

Hughes and colleagues sought to characterize this diversity by measuring specimens of 8,455 bird species in natural history museum collections, primarily from the Natural History Museum at Tring and the Manchester Museum in England, as well as the Field Museum in Chicago.

“Natural history collections have been a hugely vital resource in the creation of large-scale trait datasets,” says Hughes.

The researchers compiled numerical data for eight characteristics, such as body size, beak dimension and shape, and leg and wing length, into a multi-dimensional “morphospace.” Imagine a multi-axis bell curve, with typical generalists— think backyard birds—clustered near the center, and the avian oddballs populating the lonely extremes.

The researchers then removed all the species that met the International Union for Conservation of Nature Red List classification of Critically Endangered (the highest risk). Next they removed those that were Endangered, and so on, through Vulnerable and Near Threatened.

As tranches of at-risk species fell away, the average distance of species from the center of the morphospace decreased. In other words, the birds at the extremes disappeared much faster than the generalists huddled at the center.

a composite illustration showing a red, yellow, and green bell curve with birds including sparrows, flamingos, ducks, hummingbirds, parrots, vultures, and owls superimposed on it
Endangered at the Edges of the Morphospace. University of Sheffield ecologist Emma Hughes plotted nearly 8,500 of the world’s bird species according to their morphology, or physical characteristics. The birds at the edges of the plot (with physical traits such as unusually long or short beaks, wings, or legs) tended to be species on the IUCN Red List. Macaulay Library photos (L to R): Seychelles Scops-Owl by Steve Agricole, Spoon-billed Sandpiper by Su Li, White-headed Duck by Daniel Pettersson, Velvet Scoter by Mehmet ertan Tiryaki, Bee Hummingbird by Robert Tizard, House Sparrow by Matthew Plante, Asian Fairy-bluebird by Vincent Wang, Blue-gray Tanager by Anonymous, Cinnamon Quail-thrush by Barry Deacon, European Starling by Matthew Plante, Polynesian Triller by Mike Greenfelder, James’s Flamingo by Evangelina Indelicato, Short-tailed Albatross by Po-Wei Chi, Andean Condor by Jorge Muñoz García CAQUETA BIRDING, Indigo Macaw by Bradley Hacker, and Giant Ibis by Matthew Kwan.

As the fascinating oddballs disappeared, the surviving birdlife became more typical, more alike—what the study called “homogenization across the avian class, with species becoming more alike in terms of their morphology.”

For example, Hughes points out that species like Buller’s Shearwater, Short-tailed Albatross, and White-necked Petrel are all listed as Vulnerable. The disappearance of these large seabirds, she says, would leave behind an assemblage of bird species on the oceans with much less diversity.

an albatross soars over a gray sea
Short-tailed Albatross by Po-Wei Chi/Macaulay Library.

It’s no surprise that unusual birds would be the first to disappear, says Eliot Miller, collection development manager at the Cornell Lab of Ornithology. But Miller, who was not involved in the study, says that putting numbers to this trend is an informative way to characterize the increasing sameness of surviving bird species.

“I like that about the paper—it makes it very tangible,” said Miller.

Hughes says that the most unusual birds are at the greatest risk because, by virtue of their weird proportions and shapes, they are specialized and occupy rather narrow niches. If their niches change, they can’t adapt to new conditions as readily as generalists.

And with the loss of odd shapes, proportions, and sizes among birds comes the loss of ecological services, says Hughes, “with important ramifications for humans as ecosystem services are lost.”

A striking example is the potential for losing four species of vultures in Asia. Vultures, as a group, lie far outside the avian average and also provide an indispensable service—disposing of carrion.

“Vultures provide vital ecosystem services by removing decaying carcasses, which could otherwise increase the direct transmission of infectious diseases and increase populations of opportunistic scavengers such as dogs and rats that spread rabies and bubonic plague,” says Hughes. “Losing diversity of morphological form means we are losing the most important elements of biodiversity. What happens if we lose ecosystem services that other species cannot fill, or lead to a decrease in the service provided?”

According to Hughes, the analyses in her study add to the urgency of global biodiversity conservation efforts. “Strategies that reduce human impact on the planet are desperately needed, and current strategies are not going far enough to stem the extinction crisis,” she says.

For Miller, a key takeaway from the study is the responsibility for keeping weirdness in the bird world.

“The world is a richer place with them here,” he says. “We owe it to ourselves to keep the world a beautiful, biodiverse place.” 

Greg Breining is a frequent contributor to Living Bird magazine. He writes about wildlife, the environment, health, and science.

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American Kestrel by Blair Dudeck / Macaulay Library

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