He was curious about the size variance, “although realistically it’s difficult to judge a difference by eye,” he says. Still, aware that climate change is causing many plants and animals to change to adapt to rising temperatures, he wondered whether it also was causing birds to shrink. Griffith, a scientist in the department of biological sciences at Sydney’s Macquarie University, decided to find out.
He launched several studies, and his results thus far suggest that his instincts were correct.
But it’s complicated.
After studying birds that don’t migrate in the winter — meaning they live through both hot and cold temperatures during the year — his research found that exposure to hot weather apparently affects body size, much more than cold. But this occurs only when birds are nestlings and still maturing. Developing during a hot summer, before leaving the nest, turns them into small adults — and they stay that way.
Griffith and colleagues have studied only house sparrows and zebra finches, but they suspect the phenomenon is widespread.
“Many species might get smaller,” Griffith says. “This might be an adaptive response and may make animals better able to cope with changing conditions. The key thing is to understand why it is happening and what the fitness consequences are likely to be for animals — that is, are they better off or worse off [evolving into smaller sizes] in a warming world?”
Doctoral student Samuel Andrew, who collaborated with Griffith, agrees. “If these changes in size are an important part of adapting to warmer climates, then birds that don’t change their size in response to temperature change could be more vulnerable,” Andrew says.
Their research adds to a growing body of evidence indicating that numerous plant and animal species are undergoing physical and behavioral changes in response to global warming. It’s affecting their breeding and migration patterns, which are shifting, for example, among monarch butterflies, songbirds and bats. And many species are moving to higher elevations, where it is cooler, to escape the heat.
This almost certainly is related to winters becoming shorter and warmer, prompting earlier-than-normal, or “false,’’ springs, and sporadic, hot summerlike days during winter months as well as extraordinary heat waves. These climate variations can throw ecosystems off balance, making it difficult for nature to adjust. Ecosystems are complex and connected; when one component changes, it has a ripple effect on the rest of the system.
Body size appears to be among the factors affected, and not just in birds. Last summer, researchers in Canada reported that many species of fish were shrinking — some by as much as 30 percent — because the warming ocean water makes it difficult for them to absorb oxygen — necessary for growth — through their gills.
“One of the big challenges for biodiversity generally is that the speed with which the climate is changing is faster than ever before,” Griffith says. “That means that the capacity of different species to respond is going to be key to determining the extent to which they can cope.”
The notion that temperatures influence body size isn’t new. Biologist Carl Bergmann first proposed in 1847 that cold climates give rise to bigger bodies and that animals that live in warm places typically are smaller. This makes sense because bigger bodies preserve heat more efficiently than smaller ones. But global warming has turned “Bergmann’s rule” into something not as simple as it was then.
“The overall pattern between temperature and body size has been seen in lots of organisms,” Griffith says. “What we are suggesting is that more people need to consider that it may be driven by problems caused by developing in very hot weather. We know that heat waves are increasing in severity and intensity, and therefore this could be driving the shrinking of animals.”
For the sparrows study, Andrew and his team captured and measured about 40 adult house sparrows at each of 30 locations across Australia and New Zealand. They found that maximum temperatures during the summer, when the birds breed, were a better predictor of adult body size at each location than winter minimum temperatures.
Because house sparrows are sedentary birds that live among humans, “this means you can compare the size of birds from different locations across a large distribution,” Andrew says.
Some sparrows live in the arid center of Australia, where it gets very hot during the summer but also cold during the winter, while others live where changes between the seasons are milder. “We expected that the hottest temperatures during the summer, or the coldest temperatures during the winter, would be most relevant to the survival of the birds,” Andrew says.
But that’s not what they found. “We found that house sparrows are more likely to be smaller in locations with hot summers and that there was no significant relationship between winter temperatures and size,” Andrew says.
The reduction in body size probably has to do with their ability to regulate development in response to environmental changes. The nestlings “could be reducing the amount of resources they use to grow larger to more effectively lose excess heat as adults,” Andrew says.
In a second study, the researchers found the same was true for zebra finches. They showed that in the wild, zebra finches that developed in hotter months were smaller, and then tested the idea experimentally by breeding zebra finch families either in cool rooms or hot rooms in a laboratory.
“Even though we used the same pairs to breed in the two different conditions — and therefore were able to compare full siblings — we showed that those grown in a hot room were smaller than those grown in a cool room,” Griffith says. “This was the first experimental test of the idea we demonstrated in the sparrow study.”
What the size question means for future species is unclear. “We just don’t know yet which species can and can’t adapt to a wide range of climates,” Andrew says. “If we can understand the traits that allow species to be successful, we can do a better job evaluating which ones will need the most help when it comes to climate change.”