For years I thought I could tell the difference between black-capped and Carolina chickadees by their songs and calls. After all, I had heard them one winter on the mall in Washington, D.C. and they certainly sounded different from the black-cappeds I was used to. So last winter, when I heard a calling chickadee that sounded different, I wondered if it could be a Carolina chickadee.
I located the bird perched in a cedar tree outside my window. While I studied the bird, I looked at my four field guides and learned that black-capped chickadees are larger than Carolina chickadees by a quarter to half an inch with longer tails and bigger heads. In addition, black-cappeds have more white on their wing edges and their white cheek patches are entirely white while those of Carolinas blend to pale gray at the rear.
But most of my guides agreed that their songs and calls should be my guide. Carolinas’ “chick-a-dee” calls are higher pitched and more rapid than black-cappeds and their whistled “fee-bee, fee-bay” is distinct from the single “fee-bee-ee” or “fee-bee” of black-cappeds.
As I examined my singer, I noticed its white wing edges and decided it was only a black-capped. Studying the most recent Carolina chickadee range maps in the Second Atlas of Breeding Birds in Pennsylvania, it was clear that none had been reported in Blair County where I live even though if I drew a straight line from east to west across the commonwealth, our home would be at the northern edge of Carolina chickadee territory.
But that’s not how it works in Pennsylvania. Since 1975, Carolinas have been expanding northward at about seven-tenths of a mile per year in southeastern and south western Pennsylvania, and they now live as far north as Lehigh and Schuylkill counties in the east and Beaver County in the west. However, they are mostly absent at higher elevations in the Allegheny Mountains and southern Appalachian physiographic sections of Pennsylvania according to Robert L. Curry’s account in the atlas.
Curry, who teaches biology at Villanova University, has been conducting field studies of the population ecology and behavior in hybridizing Carolina and black-capped chickadees in southeastern Pennsylvania since 1997. That’s when his students built 450 fake nests of short lengths of sewer pipe and made them look like trees with a cavity and top entrances in an attempt to attract cavity-nesting chickadees.
They erected their fake nests in Chester County to study Carolina chickadees, Nolde Forest near Reading in southern Berks County to study both Carolina chickadees and hybrid Carolina/black-cappeds, and Hawk Mountain Sanctuary on the easternmost ridge in the Valley and Ridge province of Pennsylvania to study black-capped chickadees. (Our ridge is the westernmost in the province, incidentally.)
Even though Carolina and black-capped chickadees have been distinct species for between one and 2.5 million years, from Kansas to New Jersey these two species meet, mate and give birth to hybrid species. But Curry and students have learned that male Carolinas, which are more aggressive than male black-cappeds, mate with female black-cappeds, but female Carolinas rarely mate with male black-cappeds. Furthermore, these hybrid birds lay a large number of infertile eggs, in contrast to a 92% hatching success of an average of five eggs laid by Carolinas in 55 nests in southeastern Pennsylvania with similar fertility rates in black-capped chickadees.
Curry and other researchers don’t agree that anyone can identify Carolina and black-cappeds by their unique calls and songs in hybrid areas because hybrids can sing Carolina, black-capped and intermediate songs. Carolinas can also learn and sing black-capped songs and black-cappeds can learn Carolinas’ songs. So researchers use “diagnostic protein, nuclear DNA, and mitochondrial DNA markers” to identify species according to the life history of Carolina chickadees in The Birds of North America.
Curry and students documented over the years a northward movement of Carolina chickadees, and by 2006 he added Tuscarora State Park in Schuylkill County to his study area for black-cappeds because almost half of Hawk Mountain’s chickadees were then hybrids while Nolde Forest had only Carolinas. Curry hypothesized that Carolinas are moving northward as a result of warming winter temperatures. Hybrids, he says, live in places with an average low winter temperature of between 15 and 19 degrees Fahrenheit, while black-cappeds prefer colder winter temperatures and Carolinas are primarily southern birds and one of only four birds John James Audubon discovered and named while in coastal South Carolina.
Today they range from south-central Kansas to the Atlantic coast and south from north and southeast Texas to the Gulf Coast and central Florida. Black-cappeds persist south along the highest mountains, but even on Mount Mitchell, the highest mountain in North Carolina, Carolinas replaced black-cappeds after logging and repeated burning, but after reforestation black-cappeds did not return.
Like black-cappeds, Carolinas are acrobatic, hanging upside down from branch tips as they search for insects that make up 80 to 90% of their spring, summer, and fall diets and half their winter diet. The other half is seeds and fruit, including bird feeder seeds, which they often cache for later use, especially in the winter.
They prefer to breed at the edges of deciduous forests with a healthy shrub, midstory, and overstory canopy, but they don’t mind a high human population in the area or fragmented forests as long as there are dead trees or nest boxes for nesting. In Pennsylvania, they breed in late April, which is a week sooner than black-cappeds.
Earlier, during flock formation in late summer or in winter flocks or even as late as early spring, they pair up, forming a bond that can last as long as three years. Males feed the females during courtship and guard them during egg-laying. The feisty Carolina males chase other males away from their mates and their breeding territory and may engage in feet-kicking, bill-jabbing and wing-fluttering in the air or on the ground.
By mid-March, the pairs are excavating and building nests in cavities, natural snags, fence posts or nest boxes. While both parents take turns excavating, the females construct nests with a base of moss and a thick lining of hair or plant fiber, especially the fur of eastern cottontails, white-tailed deer, eastern fox squirrels, opossums, raccoons, or even domestic cows or cats. This fur she fashions into a flap and the nest, with the flap down, probably is an attempt to keep house wrens from destroying the eggs. Other cavity nesters, such as eastern bluebirds, tufted titmice, and house sparrows are also known to take over Carolina chickadees’ nests even those with eggs or nestlings.
Carolina females lay three to 10 white eggs spotted with reddish brown. They also incubate the eggs for 12 to 15 days while the males feed them. In one case in southeastern Pennsylvania, a brooding female was killed on her nest, and Curry suspected a southern flying squirrel was the culprit. Other mammalian predators on eggs, nestlings, and young fledglings are raccoons, opossums, domestic cats, and rat snakes. Avian predators, even on adults, are sharp-shinned and Cooper’s hawks.
Once the eggs hatch, females continue to brood their nestlings until they are eight days old. Both parents feed them for 16 to 19 days, and then they fledge, usually in the morning. The parents continue to feed them for two to three weeks, and after that they are on their own.
Parents that remain paired hold the same territory in successive years, and they remain there where they are joined by young Carolinas to form a winter flock. In these flocks and generally males dominate females and larger males dominate smaller males. Although such dominance doesn’t guarantee winter survival, it does guarantee a breeding territory. Not all Carolinas stay in a flock. Instead, they are flock-switchers or winter floaters, either joining another flock or continuing to move from flock to flock.
To survive the winter, they spend their time along stream valleys in sheltered forest interiors when it is cold and the wind is blowing. They also decrease their movements and the distance they fly between foraging sites. They can enter a state of regulated night hypothermia by dropping their body temperatures to 84 degrees Fahrenheit and pairs will roost together in tree cavities, staying there as long as 15 hours during a 24 hour period to conserve heat. They sunbathe early in the morning by perching on branches sheltered from the wind and facing into the sun with their body feathers fluffed and their wings spread. To obtain water, they drink from bird baths and natural sources and eat snow.
From the first (1984-89) to the second atlas period (2004-09), Curry estimates a 90% increase in Carolinas’ population in Pennsylvania. During the second atlasing period researchers estimated there were 105,000 singing Carolina males. However, Curry concludes that atlas results do not support upslope movement into ridgetops in Franklin and Fulton counties, so it looks like I won’t see Carolinas or hybrids here anytime soon.