Biologists identify traits correlating with all bird extinctions since 1500

Looking to inform the conservation of critically endangered bird species, University of Utah biologists have completed an analysis identifying traits that correlate with all 216 bird extinctions since 1500.

Species most likely to go extinct sooner were endemic to islands, lacked the ability to fly, had larger bodies and sharply angled wings, and occupied ecologically specific niches, according to research published this month. The work appears in the journal Avian Research.

While some of these findings mirror previous research on extinct birds, they are the first to correlate bird traits with the timing of extinctions, said lead author Kyle Kittelberger, a graduate student in the School of Biological Sciences.

“I’ve been very interested in extinctions and understanding the species that we’ve lost and trying to get a sense of how we can use the past to better inform the present and future,” said Kittelberger, who is completing his dissertation on how the bodies and wings of certain species of migratory songbirds have changed in response to climate change.

His team’s analysis tapped into BirdBase, a dataset of ecological traits for the world’s 11,600+ bird species compiled by University of Utah biology professor Çağan Şekercioğlu and the Biodiversity and Conservation Ecology Lab at the University of Utah. The team simultaneously analyzed a broad range of biogeographical, ecological and life history traits previously associated with extinction and extinction risk for bird species that have gone extinct as well as those that lack recent confirmed sightings and have therefore disappeared.

“Importantly, we examine biological correlates of bird extinctions through the lens of when birds went extinct, providing a novel extinction timing element that helps better inform why birds with certain traits disappeared when they did,” Kittelberger wrote in a series of posts on X.

By identifying traits that most predispose birds to extinction, the findings can help guide conservation efforts of hundreds of species that are at peril.

“One of the strengths of the approach that we use is that we compare all of these traits simultaneously against each other, whereas a lot of previous studies in the literature just look at traits in isolation.”

While only around 2% of the world’s bird species have gone extinct since 1500, the year Kittelberger’s analysis begins, even more had already disappeared by then. Before 1500, however, there is not as reliable a record of the birds that went extinct and data on their traits and characteristics.

This timing coincides with the rise of scientific observation, resulting in systematic documentation of plant and animal life. It is also the time when European exploration took off, leading to the disruption of ecosystems around the globe as a result of colonization and introduced species.

Today, 1,314 bird species are at risk of extinction, according to the IUCN Red List of Threatened Species, or about 12% of the total.

Many species, such as the ‘Akikiki (Oreomystis bairdi), endemic to the Hawaiian island of Kauai, are so rare that they are functionally extinct. Kittelberger photographed the pictured ‘Akikiki, also called Kauai’s creeper, in the Alaka’i Wilderness Preserve in 2022, when it was believed around 70 or so remained in the wild; today, only one individual remains.

As with many other Hawaiian bird species, the main threat to the ‘Akikiki comes in the form of introduced species, principally malaria-carrying mosquitoes and habitat-wrecking livestock, according to Hawaii’s Division of Forestry and Wildlife.

The single most critical trait associated with extinction is insularity, those species endemic to islands. Other traits that appear to increase extinction risk, such as body size, wing shape and ecological specialization, are also associated with island insularity.

The Pacific islands are home to a vast array of endemic species, many of which are now on the IUCN list. No archipelago has seen more loss of avian biodiversity than Hawaii, which accounts for 34 post-1500 extinctions, according to the study.

The bird family that experienced the most extinctions is Rallidae, or rails, which are globally distributed medium-sized semi-amphibious bird species. This family lost 26 members, while the entire line of the Mohoidae family, small nectar-sipping songbirds commonly known as Hawaiian honeyeaters, is completely gone (the only bird family to go fully extinct in modern times). The family’s last member, the Kauai O’o (Moho braccatus), was last observed in 1987.

The finding that did surprise Kittelberger’s team was the correlation of wing shape to extinction. They found that bird species with a larger hand-wing index, that is more pointed wings, were more likely to have vanished earlier. Since pointier wings indicate stronger flying and dispersal abilities, the team predicted such birds would be better equipped to respond to environmental pressures than those with rounder wings.

Yet birds with rounder wings, those with smaller hand-wing ratios, proved to be more resilient.

“It likely ties with the fact that a lot of these birds that went extinct were on islands. For these species or their ancestors to have arrived at these islands to begin with, they would’ve needed the ability to fly across large, open distances. So a lot of these birds on islands have, not necessarily longer, but more pointed wings,” Kittelberger said.

“Think of a swift or a swallow or a bird like an albatross that flies over the ocean. They have really pointy wings that allow them to fly really well, whereas birds in dense tropical rainforests are not flying these large distances, so they have rounded wings because they’re just moving in their local habitat patches.”

Ecological specialists, as opposed to generalists, face great challenges when their specific ecological niche is altered, so they tend to be less able to respond and adapt to disturbances, like invasive species or disruption of a food source.

In other findings, the study concluded more than 87% were endemic to islands; nearly two-thirds inhabited forests; 45% ate primarily insects and other invertebrates; and 20% were completely or partially flightless. Finally, species with big bodies were overly represented on the extinct list.

“Heavier birds have been more likely to be targeted for hunting, with several well-known examples of birds being hunted to extinction in part for food, including the dodo, great auk (Pinguinus impennis), and spectacled cormorant (Urile perspicillatus),” the study said.

“Body mass has also been linked with a species inhabitance of islands, as island birds tend to be larger than mainland species.” Of the 43 species whose average body weight exceeded 500 grams, or 1.1 pounds, three-quarters were endemic to an island.

The decade of the 1890s saw the highest number of extinctions, with 21 recorded. The 1980s was another bad decade for bird loss with 20 species, including the Kauai O’o.

This latter decade “is noteworthy since focused and targeted conservation efforts were already underway globally before and during this period,” the study said. “Though the rate of extinction declined in the subsequent decades, the number of globally threatened species has only increased.”