On Carolina Black-capped Chickadees
As the eBird reviewer for the state of North Carolina, I’m often in the position of making decisions as to the validity of difficult species. This is generally not as hard as it sounds, and the vast majority of identification issues can be addressed in a few e-mails back and forth to determine the skill of the birder and the robustness of the sighting. I’m not interested in judging birders or shaming their mistakes as much as getting the correct information in the database, and if that means invalidating a potential record based on simply not having enough information than I’d rather do that than allow a potentially shoddy record, and I try to be upfront about that. I’ve outlined my own strategy for dealing with this stuff in a previous post. The shorter version is that it’s not personal, it’s science.
For North Carolina the eBird review list is not terribly confusing, but the one species that I continually find to be the most difficult to ascertain its validity is one that is staggeringly common across much of its range. So common in fact, that most birders don’t even give it a second thought, which is actually part of the problem. It’s the Black-capped Chickadee (Poecile atricapillus), the little Parid admired for its tenacity and personality across most of the continent. Well, except for the southeast corner, where it’s replaced by the extremely similar Carolina Chickadee (Poecile carolinensis), and therein lies the problem and the bane of the North Carolina eBird reviewer’s existence. In fact, there’s little you can do that’s more likely to get an immediate response from me than report a Black-capped Chickadee in my state.
There’s a stretch of high mountains in the far western part of the state that historically has been home to an isolated population of Black-capped Chickadees. It’s notable not only for being the southern terminus of the species’ range in the eastern US, but also because it’s a full 200 miles farther south than the next population, a not unsubstantial distance for a bird that’s scarcely 6 inches long and hardly known as a dazzling aerialist. This isolation will be more notable later in this post, but it’s enough now to emphasize that the birds that live in far western NC and far eastern Tennessee have very little contact with, and are therefore generally genetically isolated from, the greater population of Black-capped Chickadees across the rest of the continent. This distinction is unique among the group of primarily boreal-nesting species, notably Red Crossbills, Saw-whet Owls and several nesting warblers, whose ranges extend south along the high ridges of the Appalachians where the ecosystem is significantly different than the rest of the state.
This is very much an island of atricapillus in a vast sea of carolinensis, and as such, there has always been a great deal of confusion as to how the two birds interact with each other. Confusion that often plays out on the eBird filters for the state. Adding to this, I’ve found that there’s very little information available online about the interesting story behind Black-capped Chickadees in North Carolina, so in the interest of dispersing some of that fog and perhaps making my job a little easier as a reviewer, here’s the scoop on this species and its history in the state. It may well be only relevant to a few, but it’s a cool story nonetheless.
In the 1940s, this unique population was well studied by James Tanner (yes, that James Tanner) which culminated in his 1952 article in the Auk called, appropriately, Black-capped and Carolina Chickadees in the Southern Appalachian Mountains (.pdf). Tanner spent several years in and around the recently dedicated Great Smoky Mountains National Park trying to figure out exactly where the line between Carolinas and BCs occurred and how the two species interacted with each other, something we’re still trying to get a handle on more than 50 years later, but I digress. As you might expect, he got some interesting results, not least of which is that these BCs average smaller in size than the birds in the next closest population in West Virginia*. They were actually closer in size to Carolinas, which you would expect in a largely isolated population so far south (so says Bergmann’s Rule). This was also one of the first published reports of the two species regularly copying each others’ vocalizations, and Tanner noted on several occasions individuals of both species switching back and forth. Even given their similar size and penchant for vocal impersonation, there was no acceptable amount of hybridization between the two species. It may be exceedingly difficult for birders to tell the two species apart in the field, but the birds seem to manage it just fine.
*Update: While Tanner compared the size of the NC BCs to birds from the West Virginia area, this includes Appalachian Virginia, where the closest population occurs.
Additionally, and perhaps most pertinent to the birder who wishes to find them, BCs in North Carolina nest exclusively at elevations above 400o feet, where they’re highly associated with remnant stands of Red Spruce, Balsam Fir, and, especially, Yellow Birch (Betula lutea). In fact, Tanner states that the BC nests that he found were exclusively in Yellow Birch cavities and the birds would even forgo nest boxes in areas where Yellow Birches were prevalent. On those mountains where BCs were present, Carolinas would not nest above that seemingly arbitrary 4000 feet barrier, despite the two species nesting as close as one mile of each other on occasion. However, and this is a crucial point, on mountains where Black-capped Chickadees are historically absent, Carolinas nest all the way up to 5000 feet. So there is clearly some sort of competition between the species, though altitude alone is not the only determining factor predicting which Chickadee you’re likely to find. It is interesting that Tanner notes that nearly all the high mountains where BCs are replaced at altitude by Carolinas had their native vegetation, including stands of Spruce and Birch, logged over in the 1920s. This includes Mount Mitchell, which at 6600 feet is the highest mountain in the eastern US, and while having nesting populations of other boreal species like Winter Wren and Hermit Thrush, hosts only Carolina Chickadees.
This misunderstanding of the altitude rule is the one that generally trips birders up. Many assume that every chickadee that they see, or hear, over a certain altitude is a BC. Add to that the birds’ propensity for rather accurate depictions of the others song, and you’ve got a recipe for poor or absent follow-up on high-elevation Chickadees. Too often I’m left with notes consisting of little more than “I was hiking above 4000 feet and I heard a Chickadee”, which of course, can’t be validated. It gets worse when you realize that Tanner’s work, though amazingly useful, is more than a little dated. Like all ecosystems, the Appalachians are dynamic and much has changed since his time.
In the 1950s a little invasive insect called the Balsam Woolly Adelgid was discovered in the mountains of North Carolina where, over the next two decades, it proceeded to wipe out 90 to 99% of the area’s Balsam and Fraser Firs, leaving “ghost forests” on the top of many of North Carolina’s highest mountains. As you might expect, the implications on the birds in these ecosystem was substantial, and a Purdue University study (.pdf) in 1990, 17 years after the adelgid arrived in Great Smoky Mountain NP showed significant declines in all boreal bird species, with the Black-capped Chickadee being particularly hard hit, to the point where at some historic strongholds, places where Tanner found populations of nesting BCs, the species was extirpated. Consequently, populations of undergrowth loving species, like Eastern Towhees, Chestnut-sided Warblers and yes, the more adaptable Carolina Chickadee, took off. The BCs were reduced to a handful of sites at best, and many in the NC birding community considered them to be completely extirpated from the state.
Now that 20 years have passed since the worst of the Adelgid breakout and some of the Fraser Fir forests have begun to recover, there’s evidence that Black-capped Chickadees are present in the state, but only definitively in four counties; Swain, Haywood, Jackson, and Transylvania, but whether they were remnants of populations that survived the Adelgid breakout or they’re re-colonizers from parts north is unknown. There have been reports outside of these counties, but they are extremely rare and accompanied by lots of evidence. It’s important to note that the two Chickadees can be fairly reliably distinguished given a good look. The white fringes on the secondaries of the BC is an very good field mark in all seasons, though late summer molty birds can be confusing. Black-capped Chickadee is an excellent bird in the state, but achievable in the right places, nearly exclusively in and around Great Smoky Mountains National Park.
Bottom line? Check your chickadees, folks. And be ready for an email if you report one to eBird.