As the tropics warm, some birds are shrinking – Science News for Students

Many tropical birds, such as this collared puffbird (Bucco capensis), are shrinking in size. The change may help these birds stay cool as their climate warms.
V. Jirinec

Researchers have spent decades catching and measuring birds in a remote corner of Brazil’s Amazon rainforest. They started coming here to study diversity in pristine forests. They wanted to compare species living there to those in areas where logging or roads have broken up the habitat. But a team has discovered a more subtle change: Birds are shrinking.
Over the past 40 years, dozens of tropical bird species have gotten smaller. Many species have lost about 1 to 2 percent of their average body weight per decade. What’s more, some species have grown longer wings. During the same time period, the climate has gotten hotter and more variable. Higher temps could put a premium on staying cool. Leaner, more efficient bodies may help birds do just that, the researchers say. They reported their findings November 12 in Science Advances.
“Climate change isn’t something of the future. It’s happening now,” says Ben Winger. “And,” he adds, it “has effects we haven’t thought of.” Winger is an ornithologist — someone who studies birds. He works at the University of Michigan in Ann Arbor. He wasn’t part of the new study. But he has witnessed a similar shrinkage in migratory birds. Seeing the same patterns in many different types of birds, including ones now that don’t migrate, suggests it may be “ a more universal phenomenon,” he says.
Many studies have linked body size and temperature. In colder climates, it pays to be big. A smaller surface area relative to one’s volume reduces heat loss through the skin. That keeps a body warmer. As the climate warms, “you’d expect shrinking body sizes to help organisms off-load heat better,” says Vitek Jirinec. He’s an ecologist at the Integral Ecology Research Center. It’s in Blue Lake, Calif. 
Many species of North American migratory birds are getting smaller. Winger and colleagues reported this last year in Ecology Letters. Climate change is the likely culprit, Winger says. But other factors could be at play. Migrators encounter a wide range of conditions on their travels. Damaged habitats, lack of food and other challenges can’t be ruled out.
Jirinec and his team wanted to see if birds that stay put also have been shrinking. They studied tropical species that don’t migrate. They focused on birds in an undisturbed part of the Amazon. (That helps rule out local human effects, such as logging.) They analyzed data from 1979 to 2019. It came from more than 11,000 individual birds. They represented 77 species. Measurements included body mass and wing length. The researchers also examined climate data for the region.
The study included birds with very different lifestyles. Some live high in the trees. Others lived lower, even on the ground. And all species got lighter over time, the researchers found. On average, species lost from about 0.1 percent to nearly 2 percent of their body weight each decade.
The region warmed over the same time period. The average temperature rose by 1 degree Celsius (1.8 degrees Fahrenheit) in the wet season. In the dry season, it rose 1.65 degrees Celsius. Temperature and precipitation also varied more during those years. Short-term changes, such as an especially hot or dry season, can have a big effect on species, the researchers say. They may affect body shrinkage even more than steady warming does.
“The dry season is really stressful for birds,” Jirinec says. Their mass dropped the most in the year or two after especially hot and dry spells. That further supports the idea that birds are getting smaller to deal with heat stress.
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The researchers can’t rule out all other factors. Scarcer food, for example, could also lead to smaller sizes. But birds with widely different diets all declined in mass, Jirinec notes. That points to a broader force — such as climate change — as the likely cause.
Wing length also grew in 61 species. The largest increase was about 1 percent per decade. Jirinec thinks longer wings make for more efficient, and thus cooler, fliers. He compares them to aircraft. A fighter jet, with its heavy body and compact wings, takes enormous power to maneuver. A light and long-winged glider can cruise along much more efficiently.
“Longer wings may be helping [birds] fly more efficiently” and generate less energy as heat, he says. That could be helpful in warmer conditions. “But that’s just a hypothesis,” Jirinec adds. He notes that wings grew most among birds that spend their time higher in a tree canopy. It’s hotter and drier up there than on the forest floor.
Birds could be evolving to adapt to climate change. Or their bodies might grow differently in response to warmer temperatures. Either way, the emerging changes point to the potential harm of human activity, Jirinec says.
“The Amazon rainforest is mysterious, remote and teeming with biodiversity,” he says. “This study suggests that even in places like this, far removed from civilization, you can see signatures of climate change.”
average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
birds: Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
canopy: (in botany) The top layer of a tree — or forest — where the tallest branches overlap.
climate: The weather conditions that typically exist in one area, in general, or over a long period.
climate change: Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague: Someone who works with another; a co-worker or team member.
diet: (n.) The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (v.) To adopt a specific food-intake plan. People may adopt a specific diet for religious or ethical reasons, to address food allergies, to control their body weight or to control a disease such as high blood pressure or diabetes.
diversity: A broad spectrum of similar items, ideas or people. In a social context, it may refer to a diversity of experiences and cultural backgrounds. (in biology) A range of different life forms.
ecology:  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
factor: Something that plays a role in a particular condition or event; a contributor.
force: Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
forest: An area of land covered mostly with trees and other woody plants.
glider: A vehicle (such as a plane in air or uninhabited submarine) that takes advantage of currents to travel long distances using little or no fuel. It also tends to move smoothly, creating few disruptions in the fluid or airstream through which it moves.
habitat: The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
maneuver: To put something in a desired or necessary position by using one or more skilled movements or procedures.
mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
migrate: To move long distances (often across many countries) in search of a new home. (in biology) To travel from one place to another at regular times of the year to find food or more hospitable conditions (such as better weather). Species that migrate each year are referred to as being migratory.
organism: Any living thing, from elephants and plants to bacteria and other types of single-celled life.
ornithologist: A scientist who studies birds, their behaviors and ecosystems.
precipitation: (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.
pristine: An adjective referring to something that is in original or near-original condition. It means something is somewhat old but in a seemingly “untouched” or unaltered condition.
rainforest: Dense forest rich in biodiversity found in tropical areas with consistent heavy rainfall.
range: The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which values can vary (such as the highest to lowest temperatures). Also, the distance within which something can be reached or perceived.
species: A group of similar organisms capable of producing offspring that can survive and reproduce.
stress: (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.
subtle: Adjective for something that may be important, but can be hard to see or describe. For instance, the first cellular changes that signal the start of a cancer may be only subtly different — as in small and hard to distinguish from nearby healthy tissues.
surface area: The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.
Journal: V. Jirinec et al. Morphological consequences of climate change for resident birds in intact Amazonian rainforest. Science Advances. Published online November 12, 2021. doi: 10.1126/sciadv.abk1743.
Journal: B.C. Weeks et al. Shared morphological consequences of global warming in North American migratory birds. Ecology Letters. Vol. 23, February 2020, p. 316. doi: 10.1111/ele.13434.
Jonathan Lambert is the staff writer for biological sciences at Science News, covering everything from the origin of species to microbial ecology. He has a master’s degree in evolutionary biology from Cornell University.
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