Published: Aug. 29, 2023 By

Five tiny chicks huddle together in a nest inside a wooden box. Their bright yellow beaks stand out against their barely-there grey-brown feathers. Photo by Vishva Nalamalapu.

The chicks were five days old. Their eyes were still closed, they hadn’t grown feathers, and they were a third of the weight they'll be when they leave their nest. Click to zoom.

Chickadee nestlings huddle together in a nest. They have bright yellow beaks, and thin closed feathers are starting to appear on their wings. Photo by Vishva Nalamalapu.

At nine days old, their eyes were open, and they were covered in closed feathers. Click to zoom.

Slightly older chicks huddle in a nest. They are visibly larger than a few weeks ago, and their feathers have started to open so that they look like they're covered in a soft coat.

At twelve days old, their legs were fully grown, so they were ready for banding! Click to zoom.

Out of its nest box for the first time, a young chickadee squints in the sun and stretches its wings.

Out of the box for the first time, they squint against the sun and stretch their wings. Click to zoom.

In a small wooden box strapped to a lodgepole pine sits a plump mountain chickadee. She lifts her small bill and white eyebrows and flies out, revealing five chicks that are a week old. Bony and featherless, they nestle together and breathe as one. 

This box is one of almost 400 that are scattered from the City of Boulder at 5,300 feet to the tree line above the University of Colorado Boulder’s Mountain Research Station at 11,000 feet. Researchers in the Taylor Lab set up them up to study interactions between higher-elevation dwelling mountain chickadees and the closely related lower-elevation dwelling black capped chickadees. Along with others in the lab, Kathryn Grabenstein, now a postdoctoral fellow at the Cornell Lab of Ornithology, recently authored a study in Global Change Biology. It investigates what prevents the two species from mating and what happens when they do.

The Taylor Lab researchers have found that mountain and black capped chickadees tend to mate in areas with more human disturbance, such as cities. This most recent study will help predict how human disturbance affects mating between species and how that affects the populations of both. In this way, Grabenstein says that chickadees are “really handy for studying these big questions in evolutionary biology, but they’re also charismatic and fun in their own right.” 

Other than those on the University of Colorado Boulder and the Mountain Research Station campuses, all the bird boxes are on private land. This comes with some challenges. Grabenstein says, “We have to manage dogs and kids and what gates to come through and what gates to lock. It’s a lot to keep track of, logistically.” But having most of the boxes on private land also helps the community learn about and get involved in the project. The landowners watch the researchers band the birds and collect blood samples, invite them inside to discuss birds over tea and biscuits, and even collect some data themselves. “One of the greatest returns is getting people who weren’t into birds excited about birds in their backyard,” says Grabenstein. 

Since 2019, the Taylor lab has been collecting data on which boxes the chickadees breed in and when, how many eggs they lay, and how many chicks hatch. They have also been drawing small blood samples from the adults and chicks, which they use to sequence their DNA. Combining and analyzing these datasets led to some surprising findings.

It turns out, mountain and black capped chickadees have a large overlap in where and when they breed. Despite that, only one of the 477 chickadees whose DNA they sequenced had one mountain and one black capped chickadee parent. Every other chickadee, however, had some DNA from the other species. These chickadees all looked like typical mountain or black capped chickadees, so sequencing their DNA was essential to this discovery. The one chickadee with a parent of each species looked like a blend of the two and did not lay eggs, but those with just some DNA from the other species seemed to reproduce normally. 

So, there is likely a large barrier to mountain and black capped chickadees mating. But when they do overcome that barrier, some of their offspring survive and reproduce. This study shows that even rare mating between two species can have far reaching impacts. 

Grabenstein is now turning to the question of what causes mountain and black capped chickadees to mate and when they began doing so. She is eager to learn more about these birds that people see every day but often know little about. For her, that is the heart of this work: “We only protect what we care about. We only care about what we know. So, part of my mission as a scientist is to help people know the things and love the things in their backyard.”