Chickadee Settlement at Hawk Mountain

If you didn’t know by now, chickadees are kind of my thing.

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Me, with an angry Carolina Chickadee unamused by my banding skills….

I’ve been studying chickadees in the Curry Lab at Villanova University since the beginning of my sophomore year as an undergraduate student, where I first caught the “fever” for birds. (Dare I say… I caught the ‘bird flu’? ……… I’ll see myself out.)

For the last five years, I’ve been lucky enough to work in the heart of the hybrid zone (more on this below) between Black-capped (Poecile atricapillus) and Carolina (P. carolinensis) Chickadees, seizing all the wonderful opportunities these natural, outdoor laboratories provide researchers, like myself, to explore a wide array of questions about behavior and ecology.

One of the benefits of becoming so closely acquainted with a single system over a long period of time is the never ending stream of questions I have about the dynamics between these two charismatic, backyard species. Each time I learn something new about these birds, I’m filled with more wonder and curiosity about a different aspect of their ecology.  Coupled with the ability to develop new skills as a Master’s student in the lab (I’d like to say I’m on my way to becoming a GIS/stats wizard, but I shouldn’t get ahead of myself…), the possibilities of exploring more and more questions with the long term data collected by past cohorts of Curry Lab students becomes almost endless.

And that’s exactly where my poster for the American Ornithological Society (AOS) in Anchorage, Alaska comes in!
(You can view a PDF copy of the poster from the meeting here.)

Just in case you’re not a bird nerd, let me introduce to you the main characters in this story:

Black-capped Chickadees are a wide-ranging species of chickadee that stretch from coast-to-coast across much of the northern United States, and well into Canada and Alaska. Their iconic “hey sweetie/fee bee” song, striking black caps and bibs, and bold disposition make them a favorite of casual backyard bird appreciators and those people who sell winter-themed knickknacks. You can see a range map and learn more by visiting their All About Birds page (from the Cornell Lab of Ornithology) here.

Carolina Chickadees, on the other hand, have a range that covers the southeastern United States, use a four note song, and, though they appear quite similar in their plumage, have a slightly smaller size and somewhat duller appearance than their northern cousins. More on them here.

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Black-capped Chickadee (left) and Carolina Chickadee (right)

Both chickadees hang out in mixed-species flocks in the winter, where they travel around with other chickadees, nuthatches, titmice, etc., looking for food to help them survive the winter. If you’ve ever kept a bird feeder running during the winter months, you’ve likely seen these birds in a group, hustling for seeds so they don’t meet an untimely death at the hands of Jack Frost.

As the environment shifts with climate change, the Carolina Chickadees have taken advantage of the opportunity to shift their range northward into the territory of Black-capped Chickadees, while Black-capped Chickadees continue to retract a the south end of their range. All this action happens in a narrow band that stretches from Kansas to New Jersey, as one shifts north and the other retreats in a constant avian tango.

Most importantly, in these areas of overlap, the species hybridize (i.e. interbreed with one another) and create offspring that are not only able to live normal lives (well… mostly), but also breed to produce their own offspring… with either Black-capped, Carolina, or even other hybrid chickadees as mates!

So… how do we know all this?

Well… we basically stalk them.

As I said previously, the Curry Lab has collected data from our hybrid zone site, Hawk Mountain Sanctuary in Kempton, PA, for over 15 years. Chickadees are cavity nesting birds- rather spend tons of hours searching the woods for signs of chickadee-based-life, the site includes almost 200 artificial nest snags (think: super low-tech bird houses that mimic dead trees) that are monitored all breeding season. Each artificial snag has a unique ID, which allows us to record data on how its being used throughout the breeding season, year by year.

Once a pair of chickadees pick a place to start their nest, we go back intermittently to see how things are progressing. We watch them build their nest, count eggs, and eventually catch the parents and kids to band them for identification purposes. We also take a small blood sample for genetic analysis back in the lab, which is the only way to accurately determine whether they are Black-capped, Carolina, or hybrid chickadees.

For a while, I wondered how the spatial dynamics of the hybrid zone mapped out at the level of our field sites. Do Carolina Chickadees come in, presumably with their posse of other Carolina Chickadees in these winter flocks, wait for spring, and then settle in little “chickadee neighborhoods” to breed? Do the Black-capped Chickadees retreat into their own so-called neighborhoods? How do the hybrids fit in?

To investigate these questions, I analyzed all the data from 2003 – 2018 that included pairs of chickadees, where they built their nest, and whether they were Black-capped, Carolina, or hybrid. I then mapped this out for every year, and used a statistical test to determine if the chickadees clustered by their genotype (the result from the genetic analyses) into little “neighborhoods”, or if they clustered randomly within the site.

I’ll jump to the moral of the story: we didn’t find anything groundbreaking about the chickadee settlement at our hybrid zone site- the birds just settle randomly in the site. What is clear, though, is the change in the composition of the pairs, from only Black-capped pairs in 2003, to a mix of pair combinations, many involving hybrids, in 2018.

My next step is to compare the spatial location of chickadee nests of pairs that had extra-pair offspring. Chickadees, though cute and cuddly, are known to get a little bit spicy during the breeding season. Female chickadees, at times, will go off and mate with a neighboring male, resulting in one or a few kids in the nest that don’t belong to the male taking care of them. What’s unclear is whether female chickadees always go to their nearest nest neighbor, or if they’d like to ‘shop around’, especially in the hybrid zone where they have many different kinds of male chickadees (Carolina, Black-capped, hybrids) as an option for extra-pair mating.

The light blue dots (male/female genotype score 0 – 0.15) represent Black-capped Chickadees.

The medium blue dots (male/female genotype score 0.15 < 0.85) represent hybrid chickadees.

The dark blue dots (male/female genotype score 0.85 – 1.0) represent Carolina Chickadees.

If the dots appear all light blue/medium blue/dark blue, that represents a pair of individuals that have the same genotype.

Scroll to see how Hawk Mountain changed every year!

2003
2003
2004
2004
2005
2005
2006
2006
2007
2007
2008
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2009
2009
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2011
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2017
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2018

And if you made if this far: Thank you for reading!!!

Feel free to tweet me @TheTinyBirdGirl or email me thetinybirdgirl@gmail.com with questions (or further details) about this project!

References:

“Black-Capped Chickadee Overview, All About Birds, Cornell Lab of Ornithology.” https://www.allaboutbirds.org/guide/Black-capped_Chickadee/overview.
“Carolina Chickadee Overview, All About Birds, Cornell Lab of Ornithology.”  https://www.allaboutbirds.org/guide/Carolina_Chickadee/overview.
McQuillan, Michael A., and Amber M. Rice. “Differential Effects of Climate and Species Interactions on Range Limits at a Hybrid Zone: Potential Direct and Indirect Impacts of Climate Change.” Ecology and Evolution 5 (2015): 5120–37. https://doi.org/10.1002/ece3.1774.
Reudink, M.W., S.G. Mech, and R.L. Curry. “Extrapair Paternity and Mate Choice in a Chickadee Hybrid Zone.” Behavioral Ecology 17 (2006): 56–62.
Reudink, M.W., S.G. Mech, S.P. Mullen, and R.L. Curry. “Structure and Dynamics of the Hybrid Zone between Black-Capped Chickadees (Poecile Atricapillus) and Carolina Chickadees (Poecile Carolinensis) in Southeastern Pennsylvania.” Auk 124 (2007): 463–78.
Taylor, S.A., W. Hochachka, T.A. White, V. Ferretti, R.L. Curry, and I.J. Lovette. “Climate-Mediated Movement of an Avian Hybrid Zone.” Current Biology 24, no. 6 (2014): 671–76. https://doi.org/10.1016/j.cub.2014.01.069.
Taylor, Scott A., Erica L. Larson, and Richard G. Harrison. “Hybrid Zones: Windows on Climate Change.” Trends in Ecology & Evolution 30, no. 7 (2015): 398–406. https://doi.org/10.1016/j.tree.2015.04.010.