A robin's nest holds a baby and two eggs.
A robin's nest holds a baby and two eggs.

How a Warming World Could Affect Bird Development

The blue robins’ eggs are nestled in straw, a pop of color against the brown cup-shaped nest. Next to the eggs lays a pink and featherless chick, only a few days old. On this early June day, University of Michigan School for Environment and Sustainability (SEAS) master’s student Isaac Smith (MS ’24), who had been keeping watch over the nest like a proud parent, was encouraged by the sight. Baby robins in other nests had been preyed upon when their parents left them alone—a disappointing outcome of Smith’s field research.

Smith, who focuses on avian conservation, spent two months from late spring to early summer at the U-M Biological Station conducting a nest-warming experiment that he hoped would offer additional clues about how birds respond to climate change. 

Placing specially designed heaters underneath the nests to simulate a warming planet, Smith wanted to see how heat would affect the size of birds during development. Would creating a warmer environment in the nest cause plasticity, resulting in the birds becoming larger or smaller, Smith wondered? Plasticity refers to how traits in birds may change in response to environmental factors, such as temperature or precipitation.

The research was conducted for Smith’s master’s thesis and ties into the larger research efforts of his advisor, SEAS Assistant Professor Brian Weeks, an evolutionary ecologist who studies how bird species and bird communities adapt to environmental change. 

Weeks looks at how rising temperatures impact the size and shape of migratory birds in North America. “We have found that increasing temperatures are making birds smaller, but we don’t know the mechanism underlying that relationship,” says Weeks. “It could be that when birds are warmed as they develop, they end up being smaller. So as the world warms, birds may get smaller just because of some fundamental aspect of their physiology.” 

Birds are an important indicator of the health of different ecosystems because they are responsive to environmental change, are easy to observe and have important ecological functions. “We know there are massive impacts on bird morphology,” Weeks says, “so if we can figure out the reason for that change, we can likely figure out what other species are changing in the same way.”

It could be that when birds are warmed as they develop, they end up being smaller. So as the world warms, birds may get smaller just because of some fundamental aspect of their physiology.”

Smith was authorized to find the nests of six different bird species, including sparrows, robins and warblers, but he only discovered robins’ nests. Birds hide their nests well, Smith notes, so it’s possible the other species’ nests were on high perches he couldn’t see, or they were on property that he couldn’t access. 

While previous studies have used nest boxes or incubators to warm the nests, Smith wanted to use a different approach for his study. “The only thing I wanted to change in the experiment was the temperature, so that’s why I warmed the nests that the birds had built,” he explains. “We thought it was important to do this experiment in the birds’ natural habitat because it’s most realistic to how Earth will be in 50 years.” 

Of the nests that Smith found, half were used as control nests that were attached to heating devices that weren’t turned on. The other half were connected to heaters that warmed the inside of the nests to a constant temperature of 2 degrees Celsius—the warming threshold that climate change experts predict the world will reach unless global emissions are brought to pre-industrial levels.

Smith visited the nests each day to change the heaters’ power supplies and keep an eye on the eggs until they hatched. He then carefully checked the size and weight of the chicks daily for 11 days—all birds were handled with appropriate training and permits and without harm—though sometimes the chicks didn’t survive because they had been attacked by other birds, most likely ravens or common grackles.

Issac Smith

“Something I learned about being up here is that field research comes with its own surprises,” says Smith, who was assisted in his fieldwork by U-M undergraduate student Mark Ziebell. “I didn’t think predation would be as much of an issue as it was, but it was definitely a challenge we faced and couldn’t control. 

”Preliminary findings from Smith’s research show two things: 1) The warming treatment worked—the warmed nests were about 2 degrees Celsius warmer than the control nests on average, and 2) A trend that is in line with his hypothesis shows that birds in warmed nests were smaller on average and developed at different rates than those in control nests. 

“It is likely that long-term changes in morphology are driven not only by plasticity, but also by natural selection,” says Weeks, who is continuing to delve deeper into the research statistics and data analysis with Smith. 

Smith’s work is complemented by ongoing research conducted by Tiffany Dias, a SEAS PhD candidate in Weeks’ lab who is exploring genetic changes through time. By sequencing whole genomes of bird specimens collected over the past 40 years, Dias will test whether there are specific genetic sequences associated with differences in size and then will quantify changes in the frequency of those sequences through time. Together, Smith and Dias’ work will give a picture of the relative contributions of plasticity and natural selection in driving climate-associated changes in morphology. 

Smith and Ziebell’s fieldwork at the U-M Biological Station was supported by the Nancy and Doug Schrank Summer Research Assistant Fund, as well as by the David and Lucile Packard Foundation.