Molecular Natural History is a series of posts highlighting what population genetic data reveals about some of my favorite organisms. There’s no rhyme or reason to what species I’ll feature for this, beyond the fact that they’ve made me stop and look closer when I see them along a trail or in my neighborhood. If you’d like to write about the molecular natural history of a favorite taxon, why not pitch a guest post?
Step into the woods almost anywhere in North America and odds are good you’ll be met by a welcoming committee of tiny birds darting among the lower branches of the trees, shouting their name at you, chick-a-dee-dee-dee. Chickadees are delightful and accessible birds: proportioned like live, flying Peeps; curious and bold enough to risk visibility when you venture into their territory; happy to put on a show in your backyard for the low low price of unlimited sunflower seeds.
In many parts of the continent, they’re also an introduction to the challenges of bird identification. Starting with my mother’s old Peterson field guide, I followed the novice birder’s classic emotional arc from excitement at being able to name the songbirds and raptors and corvids that flew through our backyard — to the realization that some of those names were much trickier to apply than others. The first of these challenges I remember cracking was the distinction between the black-capped chickadee, Poecile atricapillus, and the Carolina chickadee, P. carolinensis, whose ranges abut in south-central Pennsylvania. Their size, shape, and shading of wings and underparts seemed relatively distinguishable, as did their songs —until I learned that they hybridize.
When I moved west for graduate school, I never left the range of P. atricapillus, but I discovered it had more, and more distinctive, congeners. The University of Idaho was within sight of Rocky Mountain foothills where mountain chickadees, P. gambeli, would sometimes venture; and it was almost as far from the Pacific coast as chestnut-backed chickadees, P. rufescens, care to settle. Mountain chickadees have striking white “eyebrows” that make them look perpetually irritated by your presence. Chestnut-backed chickadees have richly red-brown backs and flanks. I have a clear memory of pausing on a trail to count off all three species — black-capped, chestnut-backed, and mountain — chattering at me from a single tree.
I didn’t add the fifth North American chickadee to my life list until a couple years ago, on a lakeside trail in Denali National Park. I knew it immediately, though: that same round-songbird silhouette, filled in with soft brown in the back and cap, and reddish flanks below more standard gray wings. And of course, I was in boreal forest, and it was a boreal chickadee, P. hudsonicus.
The four chickadees west of the Rockies are only somewhat better behaved, as species, than the black capped/Carolina pair in the east. A very recent study of modern genome-scale DNA sequence data from range-wide samples of the black capped, chestnut backed, mountain, and boreal chickadees found evidence of ongoing low-frequency hybridization. The authors identified seven birds, out of 333 sequenced, that were descendants of recent hybridization events: between boreal and black capped, between chestnut-backed and black capped, between chestnut-backed and mountain, and between mountain and black capped. It’s not common enough that you need to worry about it when you’re identifying a bird in the field — but it’s not nothing, either.
The same study found signals consistent with even more hybridization in the four chickadee species’ past, including between pairs of species that didn’t present recent hybrids. The authors note that this is probably consistent with changes in the birds’ ranges with glacial cycles. At the last glacial maximum, some 20,000 years ago, much of North America — including almost everywhere we find boreal chickadees today — was covered by kilometers-thick ice sheets. The ancestors of North American chickadees would have found refuge to the south of their current range, and maybe in more northerly spots along the Pacific coast — which was considerably west of its current place, given that sea levels were up to 125m lower.
As global climate warmed to what we think of as present-day normal conditions, many species expanded their ranges northward into newly accessible habitat. That migration-mixing would have been concentrated in a corridor between the Pacific coast and the Rocky Mountains, and during this period the four chickadees would have had more overlap in their ranges, and more opportunities for intermixing in that corridor. Today, the contemporary populations that carry genetic indicators of past hybridization are all west of the Rockies.
The lesson of any extended exploration of biodiversity, from birds to blooms, is that the boundaries we might want to draw around species are an approximation, at best, of the evolutionary history behind the adorably cranky fluff-balls that visit your bird feeder or hector your hike. Learning the names of the species you find in your neighborhood and on extended adventures helps build a connection to those organisms — but nature is messier than any field guide, and the messiness is often where evolutionary history gets interesting.
References
Branch CL, JP Jahner, DY Kozlovsky, TL Parchman, and VV Pravosudov. 2017. Absence of population structure across elevational gradients despite large phenotypic variation in mountain chickadees (Poecile gambeli). Royal Society Open Science 4(3):170057. doi.org/10.1098/rsos.170057
Curry RL. 2005. Hybridization in chickadees: Much to learn from familiar birds. The Auk 122(3):747-758. doi.org/10.1093/auk/122.3.747
Lait LA, J Enciso-Romero, TTM Lekamlage, A Veale, DK Abeyrama, and TM Burg. 2024. RADseq data reveal widespread historical introgression in four familiar North American songbirds. Evolution. doi.org/10.1093/evolut/qpae014
Pavosudov VV, TC Roth, ML Forister, LD Ladage, R Kramer, F Schilkey, and AM Van Der Linden. 2013. Differential hippocampal gene expression is associated with climate-related natural variation in memory and the hippocampus in food-caching chickadees. Molecular Ecology 22:397-408. doi.org/10.1111/mec.12146
Spellman GM, B Riddle, and J Klicka. 2007. Phylogeography of the mountain chickadee (Poecile gambeli): diversification, introgression, and expansion in response to Quaternary climate change. Molecular Ecology 16: 1055-1068. doi.org/10.1111/j.1365-294X.2006.03199.x
Wagner DN, RL Curry, N Chen, IJ Lovette, and SA Taylor. 2020. Genomic regions underlying the metabolic and neuronal signaling pathways are temporally consistent in a moving hybrid zone. Evolution 74(7): 1498-1513. doi.org/10.1111/evo.13970
