The most fascinating birds will be the first to become extinct

The giant ibis lives up to its name.

Adult specimens of the largest bird in the ibis family can reach 1.10 m in length, weigh more than 4.5 kg and have a 23 cm beak reminiscent of a Venetian plague doctor mask.

The species has also been on the brink of extinction since 1994, having been pushed to the brink of extinction by hunting, habitat disturbance, and deforestation. Today, fewer than 200 adult members of the species remain in its native range of Southeast Asia, according to the International Union for Conservation of Nature.

The giant ibis is more likely to be lost, along with other physically distinctive birds of extreme shapes and sizes, in the current biodiversity crisis, according to a study published Thursday in the journal Current Biology. This is because human activities have threatened or destroyed the limited landscapes in which they live.

The risk of extinction, the paper argues, is not random or evenly distributed across the birds’ tree of life. Instead, birds like the Sulu hornbill (with its huge and hollow onyx beak), the Chatham shag (penguin-like with a metallic sheen), the four-foot white-bellied heron or the seven-inch Seychelles scallop. Owls are more likely to be permanently erased from Earth.

“The global extinction crisis doesn’t mean we’re just losing species,” said Emma Hughes, an ecologist at the University of Sheffield in England and author of the study. It also doesn’t mean that we lose only the most attractive birds. “We’re going to have a huge loss in life strategies and functions,” she added, referring to the adaptations that have led to the unusual traits many birds have.

For the study, Dr. Hughes and her colleagues analyzed a range of physical traits—body size, beak size and shape, and leg and wing length—extracted from 8,455 bird species in natural history museum collections. They also looked at phylogenetic diversity, a measure that reflects evolutionary differences between species and can capture traits such as behaviors such as bird song, propensity to migrate, and food and eating styles.

They then sequentially eliminated species, starting with the most threatened before moving on to the least threatened, measuring the impact on anatomical and phylogenetic diversity. They found that as threatened species were removed, the remaining birds became more and more similar, resulting in ecological shrinkage in most biomes and half of all ecological regions, but especially in East Asia and the Himalayas.

The study sheds new light on scientific predictions about large bird losses, said Eliot Miller, a researcher and collections manager at the Cornell Lab of Ornithology who was not involved with the study. “What we’re talking about here is observable,” said Dr. Miller. “It happens. It’s not just about species being lost at random. There’s a predictability to it. It’s important, if a bit disappointing.”

The paper shows that the most threatened species are also the most genetically diverse, such as the giant ibis or the Bengal florican, the world’s rarest bustard, of which only a few hundred remain in a narrow band spanning Cambodia, India and Nepal extends. It also suggests that birds at both extremes of the size spectrum — from the shimmering, four-inch-long, turquoise puffleg hummingbird to the kakapo parrot, which rivals the size of a backpack — are at higher risk of extinction. “We’re losing the largest and smallest species,” said Dr. Hughes.

This loss of morphological diversity, she said, is closely linked to a loss of ecological roles each species plays in its habitat. After all, a bird’s appearance often depends on how it survives; Hummingbirds use long, thin beaks to gently slurp nectar, while a pelican’s pouch-like beak allows it to capture aquatic prey and swallow it whole.

And birds don’t just fly around in a vacuum. They pollinate plants, disperse seeds, control pests, regenerate forests, and carve, dig, or build dwellings for numerous other organisms. When a distinctive bird species disappears, the hole it leaves in its habitat may be unmissable, unattainable, or both. “The ecosystem is disintegrating,” said Dr. Miller.

For example, the new paper found that vultures are disproportionately endangered despite their distinctive ecological role. As scavengers, vultures help clean up decaying carcasses that would otherwise transmit infectious diseases, or feed smaller scavengers like rats and dogs, which in turn can transmit rabies and bubonic plague to humans.

“There are certain things that birds do in ecosystems that are important to us,” said Dr. Hughes. “We are potentially losing species that could benefit humanity.”

The study also shows that the planet’s feathered inhabitants are becoming increasingly homogeneous.

Already in The World of Birds, Dr. Miller: “Almost everything is really plain and brown and boring.” Not only will the extinction crisis cost us a certain number of species, but it will also impoverish the remaining biodiversity, he said, adding: “It shows that with our measures we can make the world a less rich place.”

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