A new study finds that climate change may be causing birds to transform their physical shapes, with the resulting changes making them more likely to become prey for predators.
Researchers examined more than 1,500 nesting pairs of African wild parrots and red-winged blackbirds, confirming previous studies. The new work found that bird populations have increased in the region in recent decades, which apparently has been good for them. But their reproductive success has dropped off, and the so-called “organshield syndrome” that occurs when birds are so decimated by disease and trauma that they lose their familiar upper body shape – a torso with a more robust top half, for example – suggests a growing vulnerability to predators.
“It becomes harder for them to defend themselves and to feed themselves,” said Matt Bailey, the lead author of the study and a professor of biology at the University of Bristol in the United Kingdom.
The research was published Monday in the Proceedings of the National Academy of Sciences.
Bailey and his co-authors studied climate and body shape data from monitored populations of birds across the Sahelian Africa region and evaluated the prevalence of plumage modification. They compared the number of bird deaths with the frequency of changes in plumage. While their data set was large, it was limited – they only took advantage of the birds’ nesting season. And they had no data on how many birds are killed in the wild (often by hunting and eating poisoned crops).
Previous studies had found only a relative few bird-eating bacteria do not have a good meal on birds – meaning that the birds as a whole are more likely to be killed by predatory birds. But larger plums and many other plants grown in the area are now larger than they used to be. These changes, combined with the prevalence of bird-eating bacteria, give some birds more access to meat – which means more birds are vulnerable to predators.
“I’ve tried to be honest about the results,” Bailey said.
The focus on greenhouse gas emissions is misplaced
“They are not huge effects, but they are detrimental to survival,” he said.
Small changes in plumage can actually be important. Sperm bladders are important in artificial reproductive displays in which birds use their bodies to attract mates. The mammals that prey on birds include numerous species of leopards, leopard leeches and snakes.
“That plumage is being adapted because of predators,” Bailey said.
Risk for birds increases with warming climates, he said, because rising temperatures cause diseases to spread to bigger birds, while requiring more energy to fight the disease.
This is not the first work to demonstrate the dangers presented by climate change on birds. In a 2017 study, researchers looked at 19 species of migratory birds around the world. They found that bird species are on average 17 percent less active in warmer temperatures, which allows predators to more easily hunt them.
A recent study found that migrating birds around the world face a 25 percent increased risk of starvation from changes in climate, according to biologist Mark Osterberg, whose work was published in journal Conservation Letters in September.
Scientists have long known about the dangers that some migrating birds face on their return journey. These include birds that include the black-backed woodpecker and the bald eagle, which have a small “adopt-a-bird” program in order to provide habitats for particular species. Black-backed woodpeckers are particularly vulnerable to predation and mortality because they cannot fly as long, which makes them more difficult to spot.
But the study published Monday found bird changes are driving permanent changes in plums – specifically, the absence of a robust upper half.