To Be or Not To Be (a Hybrid Chickadee)

By Olga Helmy August 10, 2022
Black-capped Chickadee

Photo by Bob Martinka

Chickadees are the epitome of a backyard bird. Of the seven species of chickadee that typically occur in North America, four of them call Montana home. You may know these birds well—they are relatively easy to tell apart. Two species, the Chestnut-backed Chickadee and the Mountain Chickadee, are distinctly western birds. The Boreal Chickadee ranges further east, but true to its name, it is mostly a northern species, only dipping into Montana at the state’s far northwestern corner. Black-capped chickadees, however, occur throughout the state. They range broadly, covering a wide swath of North America, from Alaska, south through the Midwest, to the east coast of Canada and the northeastern USA. Towards the southeastern US however, the Black-capped Chickadee’s range ends abruptly, bumping up against that of its morphological doppelganger, the Carolina Chickadee. Although these two species can be differentiated by an experienced (and keen-eyed!) birder, they look so much alike that, as UC Boulder researcher Dr. Scott Taylor says, “even they sometimes make mistakes”. In the narrow band where their ranges overlap, Black-capped and Carolina Chickadees commonly interbreed and produce hybrid offspring.

The question of what keeps the species separate also puzzled UM professor, Dr. Zac Cheviron. To investigate this question further, Dr. Cheviron teamed up with colleagues Scott Taylor at UC Boulder, Amber Rice at Lehigh University, and Matt Carling and Katie Wagner at the University of Wyoming. As  evolutionary biologists, these researchers are interested in learning how new species evolve, and then once the species have diverged, how they remain separate. What sort of barriers separate and maintain species that can naturally hybridize? The researchers hypothesized that reduced fitness in the hybrid chickadees could be due, in part, to parent species’ genetic incompatibilities, particularly in the regions of the genome that are involved with metabolism as it relates to cold-tolerance and aerobic performance. Dr. Cheviron describes surviving winter in cold climates as “an extreme endurance event” for tiny warm-blooded critters like chickadees. Truly an aerobic exercise, shivering muscles to create warmth requires a lot of energy! Genetic incompatibilities could disrupt the metabolic processes, which might explain why the hybrids are not as good as the parent species at withstanding colder temperatures. If the researchers are correct in their hypotheses, then they may expect to find between-species differences in these regions of the genome. And perhaps, when examined within-species, the location of these regions in the genome may also vary with latitude. These same regions of the genome might be important in allowing other animals to live in places with extreme cold, places like the Arctic (or Montana in winter!) for example.

The team has been funded by an NSF grant to study whether adaptations to local conditions, like cold temperatures, can lead to the evolution of reproductive barriers between species. Their project has three objectives. The first is to understand patterns that might exist in the genetic differences (genetic divergence) and similarities (genetic transfer, or gene flow) between populations of Black-capped and Carolina chickadees. The second objective is to measure differences in metabolism, body composition and gene expression in these two chickadee species and their hybrids. The third objective is to examine whether, and if so how, information gathered from the first two objectives relates to fitness and survival. The researchers will combine data from whole genome sequences, physiological performance measures, and over-winter feeding rates, to develop an integrated model to test whether local adaptation contributes to the evolution and maintenance of a new species. With so many moving parts, it’s lucky for the researchers that the chickadees have an enthusiastic and dedicated fan-base. Public participation is a critical part of the project. Citizen scientists are helping to collect over-winter feeding data by monitoring backyard bird feeders. The research team hopes that by combining feeding data with radio frequency identification (RFID) technology, they will be able to relate performance metrics and body condition to the over-wintering survival of individual chickadees, within and outside of the hybrid zone. The researchers have also partnered with the University of Colorado Science Discovery to create educational materials on evolution, physiology and genetics for K-12 classrooms.

Here in Montana, the research team is also exploring similar questions in some of our local chickadees. While not as well-studied as hybridization between Black-capped and Carolina Chickadees, hybridization also occurs between Black-capped and Mountain Chickadees. However, instead of latitudinal gradients, as seen in the East, in the West the latter two species overlap along elevational gradients. Similar to
those described above, disruptions to cold tolerance and aerobic performance may limit where, and how often, Black-capped and Mountain Chickadees hybridize, particularly in cold, high-elevation forests. The team is currently analyzing genetic samples from throughout the western US, including the mountains of western Montana, to better understand where hybridization occurs between Mountain and Black-capped Chickadees, and whether their hybrid offspring also suffer from fitness consequences.

Think you might be interested in getting involved with a citizen science project here in Montana? Below are some ways that you might be able to help!

  • If you have a confirmed sighting of both Mountain Chickadees and Black-capped Chickadees
    visiting your property, the research team may be interested in installing specialized RFID bird
    feeders in your yard. Send them a note!
  • Think you might have seen a Mountain x Black-capped hybrid while you were out birding? Try to
    capture a photo and send it to the lab.
  • Other excellent general citizen science efforts can be accessed through eBird, for example, the
    Great Backyard Bird Count. Your data may be a missing piece in the Chickadee puzzle!

Are you interested in learning more about on-going research in the Cheviron lab? Check out their web