Tropical birds are downsizing in the face of climate change

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Tropical birds are downsizing in the face of climate change

As they get smaller, the problem gets larger.

Cassidy Golden-crowned Spadebill (Platyrinchus coronatus)

The effects of human-driven climate change are no longer waiting for us beyond some distant horizon. They’re here and happening right now all around us. Most of those consequences, for better or worse, make a sort of intuitive sense. Global ice sheets are melting, average temperatures are rising, and forest fires are becoming more frequent and more violent. Then there are the consequences we don’t quite expect, bizarre yet unmistakable signs of the impact we’re having on the world and the non-human animals we share it with, even in those places which are mostly untouched by human hands.

A recent study carried out by Vitek Jirinec from the School of Renewable Natural Resources at Louisiana State University, and colleagues, outlines the impact of climate change on avian populations in pristine regions of the Amazonian rainforest. Their findings, published in the journal Science Advances, reveal an unusual circumstance unfolding inside the tropical canopy. The birds are shrinking.

The study builds on 40 years of population data gathered by tenacious scientists working to record the natural world. In the late ‘70s, scientists began capturing birds in the region and measuring their characteristics, not with any particular questions in mind, but as a standard practice.

“When you catch a bird, you measure the wing length, the mass, sex it, age it, all these things. So, we had the data. We started measuring these birds not because we wanted to write a paper in 2021, it’s just a benefit of these long-term data sets, and it’s important to maintain that,” Jirinec told SYFY WIRE.

Cassidy Variegated Antpitta (Grallaria varia)

We often think of the effects of climate change in terms of population loss, but the shrinking of body mass is a natural consequence of a warming world. There is a well-known relationship between latitude and overall body mass which maintains a strong correlation. Endothermic (warm-blooded) animals living in polar latitudes tend to have larger body masses as a way of maintaining internal heat, and it all comes down to the ratio between surface area and total mass.

Those animals who want to maintain internal body heat grow larger in order to have lower surface area in relation to their total mass. This minimizes the amount of heat lost to the atmosphere. The inverse is also true. Shrinking in size increases the ratio of surface area exposed to the atmosphere, making it easier to cool off. While increased temperature is likely one part of the equation pushing birds toward smaller body sizes, it isn’t the only factor. Food availability and precipitation patterns also likely play a role.

“When humans experience hardships or stress, they often lose weight. The same thing happens with wildlife,” Jirinec said. “It could also be that there’s less food available as an indirect consequence of climate change. Precipitation patterns were the best at explaining the variation in the body size metric. If you’re looking at endothermic regulation, water is extremely important.”

Because of the number of factors at play, it’s difficult to nail down exactly what is causing the loss of body mass. It’s probably true that several factors are contributing, though all of them are ultimately tied to changes in climate either directly by way of increased temperatures, or indirectly through secondary effects.

Overall, of the 77 species of birds observed, all of them showed a downward trend in overall body mass, equating to roughly 2% per decade since the late ‘70s. There was some noise in the numbers and some species didn’t have enough observations to make them statistically significant, but about half showed a clear trend with 95% confidence.

Cassidy Weighing a White-crowned Manakin (Pseudopipra pipra)

Strangely, researchers noticed another change in body structure which wasn’t as easily explained. In about a third of species, the size of the wings increased while the overall mass declined. In migratory birds this might be explained by flight economy. Lower mass with larger wings decreases wing loading, making it less energetic to fly long distances. But the Amazonian birds are non-migratory, spending most or all of their lives in a small part of the forest. It’s unclear precisely why these birds are growing larger wings, but scientists have a few ideas.

“Decreased wing loading means you don’t have to flap as much; you don’t generate as much metabolic body heat. You don’t have to spend as much energy on flying. It could also be something to do with changing air density. Warmer air is less dense, so you might need a bigger wing area to maintain the same lift,” Jirinec said.

As the world continues to warm, we might start to see similar changes in other animals. Before long, it could be that large portions of the world’s animal populations start shrinking down as a way to minimize the cost of living in an ever-warming world. It’s entirely possible that it’s already happening, and birds were only the first sign. They might be the proverbial canary down the climate change coal mine, so to speak.

“Birds are easier to study,” Jirinec said. “It’s easier to catch 30 birds and measure them than to do the same thing in mammals. There is some evidence mammals are shrinking as well. There is a paper about whales being smaller now than they were in the past. Birds are more exposed to environmental conditions, out there flying around. Mammals are nocturnal and hide in burrows, they might be able to buffer themselves to environmental changes.”

Some threats respond to making yourself larger and puffing out your chest, but climate change seems to answer only to hiding or getting smaller. If we want to make it through the next few centuries, we might want to get busy fixing the problem, or get busy shrinking.

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