Nest Egg Science

Researchers look at the connection between nest construction, climate and the survival and growth rates of Carolina Chickadees.

HIGHLANDS — The Great Smoky Mountains and the greater mountain chain of the southern Appalachians are a treasure trove of biodiversity. 

The forests and streams as well as the mountains and valleys are filled with all sorts of life—creatures both large and small. 

“We haven’t found any new mammals, our count is still about 66 mammals for the park, but we’re finding incredible finds of invertebrates and slime molds,” says Dana Soehn, a ranger with the National Park Service in Great Smoky Mountains National Park. “And all of these fit together to tell the story of the Smokies and what we are trying to protect.” 

Scientists have counted 20,000 species so far in Great Smoky Mountains National Park. But this is the story of one particular creature that is struggling to fill a niche in that web of life. 

Meet the Carolina Chickadee. 

“Most songbirds in North America, and in fact most of our breeding birds are in decline,” says Barbara Ballentine, assistant professor of biology at Western Carolina University. She explains why it’s important to learn about the Chickadee. “If we know more about them, it might enable us to be better able to protect and conserve them.” 

We know a few things about the Carolina Chickadee. It weighs about 0.4 ounces: that’s about the same as a half a slice of bread. The Carolina Chickadee is about five inches tall and its wingspan is about eight inches across. 

Like most songbirds, the female Carolina Chickadee builds the nest. Once the nest is ready, the female lays one egg per day. When the mother has laid all of the eggs for that season, she begins incubating the eggs. Female chickadees have a special layer of skin that helps her to cover all of the eggs to keep them warm. 

It’s a critical time, because Carolina Chickadees have one brood in a season. If it doesn’t work, they have to wait an entire year. To make it work, it’s critical for the female to keep the eggs within a certain temperature range for the eggs to develop properly. 

“Being on the eggs and keeping them warm means [the mothers] are not foraging and taking care of themselves,” adds Ballentine. “So they have to balance activity, keeping eggs warm and developing and taking care of themselves so they can reproduce. It’s a critically important time.” 

To understand more about how female Carolina Chickadees perform that balancing act, researchers needed to observe the nesting behavior. The trouble is, Carolina Chickadees usually build their nests in the cavities of trees. That’s not the easiest place to study nesting behavior. So researchers provided specially designed birdhouses, a kind of man made cavity, and watched for nest building to start. 

Nest boxes were placed at varying elevations, which meant different ambient air temperatures, and that translated into different times for nesting. Temperature sensors were placed outside and inside the boxes, as well as inside the nests for comparison. 

Once the nesting cycle was complete, and the hatchlings had left the nest, researchers opened the boxes and studied the nests. They found that the Carolina Chickadee didn’t build one standard type of nest— the construction depended on where the nest box was located. 

“This is the smaller version; it typically has a layer of moss on the bottom and then the cup of twigs is built with a little grass or fur that they find added around it,” says Traci Balance, a graduate student at Western Carolina University. She shows the short nest and narrow cup. “The second version of the nest is much taller, deeper and much more insulated." 

Different types of insulation, and different types of nest construction were used to adapt the nest to the climate. That allowed the female to be stronger and take better care of her nestlings. 

Scientists say it’s critical for Carolina Chickadee females to keep their eggs at 100 degrees Fahrenheit in the nest. If the eggs aren’t kept at the right temperature, they won’t develop correctly and then they won’t hatch at all. The alternative is a situation called asynchrony, in which the eggs develop at varying rates and don’t hatch at the same time. In that case, the hatchlings typically don’t do well. 

It makes sense, then, that nest construction is important. It turns out the varying climates require different types of insulation be used in nest building. 

“This is an investment they’re making that might help them when they’re incubating,” says Ballentine. “If the nest is well built, it might alleviate some of the pressure of being on the nest so often to keep the temperature up, because if there is a lot of insulation, the temperature might fall more slowly so she can be off the nest longer.” 

That means the female can forage longer and be stronger and healthier. Then there's another question to ask: what happens as the climate changes? 

“There are probably a lot of factors involved in the decline of songbirds in North America and the effects of climate change is one of the factors we’ll be looking at long term,” says Ballentine. “But the fact that these birds have one shot at reproduction in a season is probably going to be a big factor for chickadees.”

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