Pleistocene Chickadees

It is difficult to discern the difference between a black-capped chickadee (Poecile atricapillus) and a Carolina chickadee (P. carolinensis).  The former has a longer tail and is a little heavier on average, but as the below photos show, they are hard to distinguish, even if examined side-by-side.  Moreover, the 2 species hybridize in regions where their ranges overlap.  The hybrid zone extends from New Jersey to Kansas with an outlying zone in the Appalachian Mountains.  In captivity male Carolina chickadees outcompete black-capped chickadees for mates, and scientists know female black-capped chickadees will choose male Carolina chickadees in the wild, often as “extra pair sires.”  Supposedly Carolina chickadees have a 4 note song, while black-capped chickadees have a 2 note song.  (See: https://www.allaboutbirds.org/search/?q=chickadee ) In the hybrid zone chickadees use both songs.  However, I live in Augusta, Georgia; far from the hybrid zone, and I discovered Carolina chickadees here use both the 2 note and the 4 note song.  I conclude the 2 species can’t be distinguished by which song they use.

bcchcach.gif (8294 bytes)

Partial range map for Carolina and Black-capped Chickadees.  

Black-capped chickadee

Black-capped chickadee.

Carolina Chickadee

Carolina chickadee.

 

The chickadees are tough Pleistocene survivors.  They are a year round resident of forest and woodland, capable of enduring harsh winters because they stash caches of seeds utilized during lean times.  Chickadees belong to the Paridae family that also includes tits and titmice.  Genetic evidence suggests the ancestors of all American Paridae originated in Asia and then colonized North America about 3.5 million years ago.  Cyclical climate change caused corresponding changes in the environment over time.  The founding population of these forest-dwelling birds became isolated into different populations by expanding desert grassland or in some cases by glaciers, resulting in the evolution of different species.

I was surprised to learn Carolina chickadees and black-capped chickadees are not sister species, even though they appear so similar and hybridize.  (Sister species are organisms that most recently evolved from the same common ancestor.  The official definition is “taxa derived from a common ancestral node.”)  Instead, the genetic evidence suggests the black-capped chickadee is a sister species of the mountain chickadee (P. gambeli), a bird that ranges throughout the Rocky Mountains, and the Carolina chickadee is a sister species of the Mexican chickadee (P. sclateri).  The chestnut-backed chickadee (P. rufescens) of the Pacific northwest, the boreal chickadee (P. hudsonicus) of Canada, and the gray-backed chickadee (P. cinctus) of Alaska and Scandinavia are sister species with each other.  Genetic evidence also shows black-capped chickadees and boreal chickadees have greatly expanded their ranges from single source populations, since the end of the last Ice Ages after the massive glaciers that covered their present day ranges melted.

Chickadees have likely been a common bird in southeastern North America for over 2 million years.  Yet, I’m aware of just a single specimen found in this region dating to the Pleistocene.  This specimen belongs to the University of Florida Museum of Natural History and was found at the Inglis fossil site in Florida.  It is early Pleistocene/late Pliocene in age (~1.9 million years old).  It was identified as a boreal chickadee, a species restricted to Canada today.  I can’t find the scientific journal within which this specimen was described.  Considering how hard it is to distinguish between species of chickadees, I’m uncertain how accurate this species diagnosis is.  However, boreal chickadees may have once been more widespread in North America before other chickadees, such as the Carolina, evolved and outcompeted them in various regions. The rarity in the fossil record of a species that was probably abundant for millions of years shows how fleeting evidence of an organism’s existence is.

References:

Gill, FB; B. Slikas and F.H. Shulde

“Phylogeny of titmice (Paridae): 11 Species Relationships Based on Sequences of the Mitochondrial Cytochrome Gene”

The Auk 122 Jan 2005

Reudink, Matthew; et. al.

“Structure and Dynamics of the Hybrid Zone between Black-Capped Chickadee (Poecile atricapullus) and Carolina Chickadee (P. carolinensis) in Southeastern Pennsylvania”

The Auk 124 (2) 2007

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2 Responses to “Pleistocene Chickadees”

  1. David Sibley Says:

    Hi, I can’t find a link to contact you privately so I am leaving a comment here as a friendly nudge. The illustration of the two chickadees is original, copyrighted work from my website http://www.sibleyguides.com. It should only be used with permission and with clear attribution to the copyright owner. Using it without permission is copyright infringement. I think you could claim “fair use” in this case (some info here https://en.wikipedia.org/wiki/Fair_use) but it is always safer to ask permission. Using the image without attribution is plagiarism. Also… the aspect ratio of the copied image is way off, and it looks bad. Please add a line to give credit to David Sibley and sibleyguides.com, and fix the appearance of the image. Thanks.

    • markgelbart Says:

      >95% of my readers understand most of the pictures I use for my blog are ripped off from google images.

      You should be taking issue with google…not me. Good luck with that.

      I don’t know how to fix the distortion on your image, so I found some other photos of chickadees online.

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