The biodiversity crisis is making birds more similar

The days of the most distinctive bird species could be numbered as the most unusual forms bear the brunt of global extinctions. 

A new study suggests that species with extreme traits, such as the largest and smallest birds, are among those most at risk of being lost. Groups such as vultures, which carry out vital services to support the health of ecosystems, are also vulnerable to extinction.

The loss of these traits may impact on the ability of ecosystems to recover from the unprecedented threats they face, such as climate change, habitat destruction and pollution.

Dr Emma Hughes, the paper's lead author who works alongside the Museum's bird group, says, 'As species go extinct, you expect the traits that they represent to also be lost. But we found that traits are being lost at a much greater rate than species loss alone could predict.'

'This demonstrates that the impacts of the global extinction crisis go beyond the loss of species. Species extinctions are going to lead to a major loss of ecological strategies and functions, which will have important ramifications for humans as ecosystem services are lost.'

'The loss of unique traits and evolutionary history could also risk the loss of benefits to humanity that are currently unknown.'

The findings of the study were published in the journal Current Biology. 

How diverse are birds?

With over 10,000 species of bird, they are among one of the most diverse groups of vertebrates around today. Their evolutionary history stretches back more than 150 million years, when they split from the rest of the dinosaurs into their own branch of life.

Following the extinction of their relatives, as well as other ancient reptiles such as the pterosaurs, birds adapted to fill the subsequent vacant ecosystem roles. This vastly increased their range of shapes and sizes, otherwise known as morphological diversity.

This included species such as the elephant bird, Vorombe titan, which lived on the island of Madagascar. Weighing an average of 650kg, and standing around three metres tall, these were the largest birds ever known.

However, the arrival of humans led to the extinction of this bird, leading to a significant loss of morphological diversity as the average size of birds shrank considerably. The impact of humans on birds continues to this day, with one in every eight species facing extinction. 

The most threatened groups of birds include cranes and albatross, where around 70% of species are at risk of being wiped out. With their large wingspans and unique body sizes, their extinction would also lead to significant loss of morphological diversity. 

While investigating trait evolution during her PhD at the Museum, Emma became interested in how morphological diversity might change in the future.  

'Morphological diversity is really important for capturing how species contribute to ecosystem services and ecosystem functioning,' Emma says. 'When I started thinking about global change, I wondered what would happen if we lost species threatened with extinction.' 

How are extinctions changing bird morphology?

The researchers compiled the measurements of almost 8,500 bird specimens held in collections around the world, including the Museum's collections. They measured characteristics such as beak shape, wing length and body size and linked them to the conservation status of the species.

They then gradually eliminated the measurements from the most threatened birds to the least to examine how this affected the overall distribution of features. The scientists discovered that the birds which are more threatened tend to have more unusual characteristics, while species less at risk have features closer to the overall average.

Many of these unusual features relate to their specialisation for a certain role. For instance, the small size and beak shape of hummingbirds allows these animals to drink nectar from plants, while the large bodies of vultures allow them to soar while they hunt for carcasses.

If these species become extinct, then the ecosystem services they provide will be at risk. Hummingbirds act as pollinators as they move between flowers, putting bird-pollinated flowers at an increased risk of extinction.

Meanwhile, vultures consume animal carcasses, which helps to prevent the spread of diseases and contributes to nutrient cycling in the environment.

Not all areas of the world are affected equally. The researchers found that the Himalayas and parts of southeast Asia would see the greatest loss of morphological diversity if unusual species became extinct, while islands, which often harbour unique species, would also experience significant declines.

'The Himalayan uplands and lowlands are particularly threatened because there are a lot of vulture species which are particularly endangered,' Emma explains. 'These birds are being heavily persecuted.

'As vultures have a really distinctive set of traits, such as their build and size, losing these species really shrinks the region's bird community, and makes it a lot more similar on average.'

The effect of extinctions in certain areas, such as the moist forests of Vietnam and Cambodia, goes beyond the loss of traits alone. These areas are home to some of the world's most evolutionarily distinct bird species, such as the giant ibis, which have few close relatives but are threatened by habitat clearance for plantations of crops such as palm oil and rubber.

Losing these species would result in the loss of millions of years of evolutionary history.

The scientists hope to explore how these losses are already affecting birds by examining how the impacts of the Anthropocene, such as the extinction of species like the moa, have affected the overall range of traits.

'Extinct species tend to be more phylogenetically and morphologically diverse than extant species, so birds may have already become significantly more similar because of our impacts.'

'This homogenisation could become even more pronounced in future as range expansions and invasive species influence this process. In particular, invasive species are more likely to be generalists and have traits closer to the average, which could mean we are underestimating the extent of morphological homogenisation.'