What traits make birds more susceptible to extinction?

In an effort to inform the conservation of critically endangered bird species, biologists at the University of Utah have conducted a comprehensive analysis identifying traits associated with all 216 bird extinctions that have occurred since the year 1500.

According to the research, species most likely to go extinct were those endemic to islands, lacking the ability to fly, possessing larger bodies with sharply angled wings, and occupying ecologically specific niches.

Bird traits and extinction timing

While some of these findings reflect previous research on extinct birds, this study is the first to correlate bird traits with the timing of extinctions, said lead author Kyle Kittelberger, a graduate student in biology at the University of Utah.

“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.

Comprehensive data analysis

The experts utilized BirdBase, a dataset of ecological traits for over 11,600 bird species compiled by University of Utah biology professor Çağan Şekercioğlu and the Biodiversity and Conservation Ecology Lab. 

The researchers simultaneously examined a wide range of biogeographical, ecological, and life history traits previously linked to extinction and extinction risk for bird species that have disappeared or lack recent confirmed sightings.

“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 said. 

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 we compared all of these traits simultaneously against each other, whereas a lot of previous studies in the literature just look at traits in isolation.”

Historical records of bird extinctions

Although only around 2% of the world’s bird species have gone extinct since 1500 – the starting point of Kittelberger’s analysis – many more had vanished before that time. 

However, records of birds that went extinct prior to 1500 are less reliable, and data on their traits and characteristics are often lacking.

This period coincides with the rise of scientific observation, leading to systematic documentation of plant and animal life. It also marks the onset of European exploration, which resulted in the global disruption of ecosystems due to colonization and the introduction of invasive species.

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

The case of the ‘Akikiki

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 ‘Akikiki, also known as Kauai’s creeper, in the Alaka‘i Wilderness Preserve in 2022, when approximately 70 individuals were believed to remain in the wild; today, only one individual remains.

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

Link between bird traits and extinction

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

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

The bird family which experienced the most extinctions is Rallidae (rails), which are globally distributed medium-sized semi-amphibious birds. 

This family lost 26 of its members, while the entire Mohoidae family – a type of small nectar-sipping songbirds commonly known as Hawaiian honeyeaters – is completely extinct, the only bird family to have gone fully extinct in modern times. The family’s last member, the Kauai O’o (Moho braccatus), was last observed in 1987.

Surprising role of wing shape in extinctions

A surprising finding for Kittelberger’s team was the correlation between wing shape and extinction. They discovered that bird species with a larger hand-wing index – meaning more pointed wings – were more likely to have disappeared earlier. 

Since pointier wings indicate stronger flying and dispersal abilities, the team initially predicted such birds would be better equipped to respond to environmental pressures than those with rounder wings.

However, 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.”

Specialists have a greater risk of extinction

Ecological specialists, as opposed to generalists, face significant challenges when their specific ecological niche is altered, making them less able to adapt to disturbances such as invasive species or the disruption of a food source.

Other findings of the study concluded that more than 87% of the extinct species were endemic to islands; nearly two-thirds inhabited forests; 45% ate primarily insects and other invertebrates; and 20% were completely or partially flightless. Additionally, species with larger bodies were overrepresented 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 authors wrote. 

“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 detrimental decade for bird loss with 20 species, including the Kauai O’o, becoming extinct. 

According to the researchers, this latter decade is noteworthy since focused and targeted conservation efforts were already underway globally before and during this period.

“Though the rate of extinction declined in the subsequent decades, the number of globally threatened species has only increased,” the experts concluded.

The study is published in the journal Avian Research.

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