A team led by researchers at University College London have published the results of a comprehensive review of the physical traits of the world's birds and how these characteristics relate to each species' risk of extinction.
Measurements, including bill shape and tail length, from museum specimens and wild birds were collected for each species reviewed and set against its status on the International Union for Conservation of Nature (IUCN) Red List.
Philippine Eagle is both one of the world's most unusual and most endangered bird species (Peter Simpson).
A total of 9,943 species, the vast majority of the world's birds, were analysed in this way, but kiwis were left out as 'extreme outliers with some traits that cannot be measured'. No other study has investigated the relationship between extinction risk and physical traits of birds at such a scale.
It was found that the species with the rarest and most unusual measurements tended to be at most risk of extinction. When the team ran a simulation in which all of the world's birds categorised as at least Near Threatened became extinct, the result was a world with a less physically diverse global avifauna.
As well as leaving the world a duller place, losing unique bird species would threaten the stability of ecosystems, due to the role that 'unusual' species play in functions such as predation and pollination. This potential loss of diversity has previously been shown in amphibians, mammals and marine species.
Among the world's Critically Endangered bird species are birds like Philippine Eagle, a monkey-eating bird of prey found only in the Philippines, and Waved Albatross, which nests only in the Galápagos Islands and has a 2.5 m wingspan.
Waved Albatross, restricted to the Galápagos, is one of 16 species of albatross and petrel categorised as Critically Endangered (Jon Mercer).
Jarome Ali, lead author of the paper, said: "Our study shows that extinctions will most likely prune a large proportion of unique species from the avian tree. Losing these unique species will mean a loss of the specialised roles that they play in ecosystems.
"If we do not take action to protect threatened species and avert extinctions, the functioning of ecosystems will be dramatically disrupted."
The paper could not prove what drives the link between uniqueness and danger of global extinction, but Ali suggested it could be "that highly specialised organisms are less able to adapt to a changing environment, in which case human impacts may directly threaten species with the most unusual ecological roles. More research is needed to delve deeper into the connection between unique traits and extinction risk."
Ali, J. R., Blonder, B. W., Pigot, A. L., & Tobias, J. A. (2022). Bird extinctions threaten to cause disproportionate reductions of functional diversity and uniqueness. Functional Ecology, 00, 1–14. https://doi.org/10.1111/1365-2435.14201