Little Winter Soldiers: Black-Capped Chickadees
Photo by Nick Park, “Black-Capped Chickadee,” 19 Dec 2022, Macaulay Library, https://macaulaylibrary.org/asset/514773551.
Many birds migrate to warmer areas during winter to escape frigid temperatures and a lack of readily available food, but some brave the cold and stay in their habitats year-round. What does it take to survive such an unforgiving time of year? The Black-Capped Chickadee (Poecile atricapillus) has a couple of tricks up its wing that make it a true little winter soldier!
Many people are familiar with the black-capped chickadee’s close relative, the Carolina Chickadee (Poecile carolinensis), which is a common sight at bird feeders across the Blue Ridge. With a more northern range, the black-capped chickadee is a less common sight in Virginia; however, these birds are found in and around high-elevation spruce-fir forests in the Appalachian Mountains. These nonmigratory birds take up year-round residence in tree cavities across much of the Northern United States and Canada, with the most southern populations residing in the Appalachian Mountains of the southeastern United States.
One of the most distinguishing characteristics of the black-capped chickadee is its namesake black “cap” of feathers that adorns its head. This cap is accompanied by a black bib around the throat and white patches on the cheeks. The belly is white with buff-colored sides, and the back, wings, and tail are all a light gray color. Black-capped chickadees are very small birds; on average, they are only four to five inches in length with a six to eight inch wingspan. These small songbirds are still noticeably larger than the Carolina chickadee, which is only three to four inches in length with a five to seven inch wingspan.
The black-capped chickadee and other chickadees are members of the family Paridae, the tit family. Like other members of Paridae, black-capped chickadees are primarily insectivores, feeding on a variety of insects during the summer months and switching to a diet of seed and berries during the winter months when insects are less active. During the summer, they will hop along tree branches looking for insects, particularly caterpillars, and pick them from the bark or leaves. They are also known to hover or hang upside down for brief periods to access otherwise inaccessible insects on the underside of the branches. During the winter, black-capped chickadees will switch over to a diet of primarily seeds and berries once the insects start disappearing to escape the cold. Given their beak shape is better suited to an insectivorous diet, these intelligent birds have to get creative to open the hard seed shells, generally opting to peck a small hole in the shell or smashing it repeatedly against a rock or branch.
With an incredibly fast metabolism, it is important for black-capped chickadees to have readily available food in order to survive the winter. To do this, black-capped chickadees will create numerous seed caches across their territory during the late summer and fall. These caches can be stored in leaf litter, behind loose bark, or in small knotholes in trees.
With so many caches to keep track of, the black-capped chickadee needs to have a great memory. Compared to other caching species within the Paridae family, chickadees have a much larger hippocampus, the section of the brain associated with memory. The black-capped chickadee has an even larger hippocampus than other species of chickadee! But that’s not the only adaptation in the hippocampus seen in this species.
During the summer and fall, a black-capped chickadee’s hippocampus grows in size–and over the course of the winter it shrinks back down. While the true reason behind the seasonal growth and shrinkage of the hippocampi has not been proven, it is believed to be an adaptation that allows this species to remember the exact locations of food caches so that they can easily be found. With each cache that is used up, the hippocampus shrinks in size. This correlates to observations of the black-capped chickadee's memory–most individuals are able to remember the location of their caches for up to 28 days, and some seem to remember the quantities of food items cached within the first 24 hours of creation.
The black-capped chickadee has another neat adaptation to survive winter: a combination of torpor and avian shivering. During the night when temperatures drop, it is hard for these birds to maintain their fast metabolism and high body temperature to fight the cold. To save energy that they cannot afford to waste, these birds will enter a state or torpor, or reduced activity that lowers the metabolic rate and body temperature of an organism.
Most animals that enter a state of torpor do it for extended periods of time (think: bears and raccoons). Instead, black-capped chickadees enter a state of torpor daily during the cold months to survive each night. Their normal body temperature of around 107 degrees Fahrenheit drops up to 20 degrees colder. Torpor is paired with a behavior known as avian shivering, using continuous flexing of the chest muscles to warm the body up. Avian shivering is achieved in small bouts of chest muscle flexing, with the time in between each bout decreasing as the night progresses.
In the time before they wake up in the morning, black-capped chickadees are continuously shivering to warm themselves back up to their normal body temperature so that they are ready to be active and search for food for the day ahead. This level of torpor is not common in most bird species, making these birds much more suited to cold environments than many other species.
These adaptations allow black-capped chickadees to brave the winter in the cold environments they inhabit without needing to migrate to more suitable conditions. So the next time that you happen to see one of these little birds, remember that they are fierce winter survivalists capable of overcoming whatever the cold throws at them!
