The giant ibis earns its name.
Adults of the largest bird in the ibis family can grow to nearly 3.5 feet in length, weigh over nine pounds and boast nine-inch beaks reminiscent of a Venetian plague doctor’s mask.
The species has also been critically endangered since 1994, driven to the brink of extinction by hunting, habitat disturbance and deforestation. Today, according to the International Union for Conservation of Nature, there are fewer than 200 mature members of the species in its native range in Southeast Asia.
According to a study published Thursday in the journal Current Biology, the giant ibis, along with other physically distinctive birds of extreme shapes and sizes, is more likely to be lost in the current biodiversity crisis. That’s because human activities have threatened or destroyed the limited landscapes they evolved to live in.
The risk of extinction, the paper suggests, is not randomly or evenly distributed across the bird tree of life. Instead, birds like the Sulu hornbill (with its huge and hollow onyx bill), the Chatham shag (penguin-looking with a metallic sheen), the four-foot white-bellied heron or the seven-inch-long Seychelles scops owls have more chance of being 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 only lose birds that are the most attractive. “We will have a great loss of life strategies and functions,” she added, referring to the adaptations that many birds’ unusual traits have caused.
For the study, Dr. Hughes and her colleagues looked at a range of physical traits — body size, beak size and shape, and leg and wing length — recovered from 8,455 bird species in museum natural history collections. They also looked at phylogenetic diversity, a measure that reflects evolutionary differences between species and can record traits including behaviors such as bird song, migratory traits and foraging and eating styles.
They then sequentially eliminated species, starting with the most endangered before moving on to the least threatened, measuring the impact on anatomical and phylogenetic diversity along the way. They found that as they removed endangered species, the remaining birds became increasingly similar, leading to ecological shrinkage in most biomes and half of all ecological regions, but especially in East Asia and the Himalayas.
The study casts new light on scientific predictions of major bird losses, said Eliot Miller, a researcher and collection manager at the Cornell Lab of Ornithology who was not involved with the paper. “What we’re talking about here is observable,” said Dr. Miller. “It happens. It’s not just a species that gets randomly lost. There’s a predictability to it. It’s important, if a little disappointing.”
The article shows that the most endangered species are also the most genetically different, 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 that stretches across Cambodia, India and Nepal. It also suggests that birds at both ends of the size spectrum — from the glittering, four-inch-long, turquoise-throated Puffleg hummingbird to the Kakapo parrot, which is similar in size to a backpack — are at greater 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 the habitat it occupies. After all, how a bird looks is often related to how it survives; hummingbirds use long, thin beaks to delicately sip nectar, while a pelican’s pouch-like bill allows it to catch 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 cut, dig or build homes for numerous other organisms. When a distinctive bird species disappears, the hole it leaves in its habitat can be unignorable, unfillable, or both. “The ecosystem is unraveling,” said Dr. Miller.
For example, the new paper found that vultures are disproportionately endangered, despite their distinctive ecological role. As scavengers, vultures help clear up rotting carcasses that would otherwise transmit infectious diseases or feed smaller scavengers such as rats and dogs that can in turn spread rabies and bubonic plague to humans.
“There are certain things that birds do in ecosystems that are important to us,” said Dr. Hughes. “We may be losing species that could be beneficial to humanity.”
The study also shows that the planet’s feathered inhabitants are becoming more and more homogeneous.
Already in the world of birds, said Dr. Miller, “almost everything is very simple and brown and boring.” The extinction crisis will not only cost us a certain number of species, but also impoverish the biodiversity we have left, he said, adding: “It shows that we are making the world a species less rich place with our actions.”