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Overwintering Strategies of the Black-Capped Chickadee By Tim Chin Slide 2 Overview of the Black-Capped Chickadee The Black-Capped Chickadee is one of the most studied and well known birds in North America. This is due to the fact that it is around all year long and readily comes to feeders. It is easily distinguished by its small size, black cap and bib, white cheeks, dark back and wings, and lighter breast area. It is a passerine, or perching bird, that is of the Paridae family. Its scientific classification is Poecile atricapillus. Slide 3 Population Distribution The population range of the Black- Capped Chickadee spans the whole of North America from coast to coast. They are predominantly found in the northern regions of the continent. Throughout the whole year, Chickadees remain as residents in this range. They are, for the most part, non-migratory birds. Slide 4 Winter Problems Chickadees are small birds. Due to their tiny size, they have a proportionally large surface area compared to the volume of their warm core. Because of this, there is more of an opportunity for heat to be lost. The cold air and wind of winter causes radiant heat loss, heat loss by convection, and evaporative heat loss. To counteract the loss of heat, Chickadees must find, eat, and metabolize enough food to keep from freezing. More so, unlike other birds that live in arctic climates, Chickadees do not have a crop (an internal food storage), and must forage for food each day in order to survive. Chickadees are primarily bark foragers. When trees are frozen over or covered by snow, it makes their quest for food more difficult. Black-Capped Chickadees only forage during the day. In winter, the days are short and the nights are long, leaving less hours to feed and more hours to expend energy in remaining warm. In the winter, the food supply is naturally lower. Their staple fare of insects becomes scarce and they must adapt in order to survive. Even seeds and other alternative foods may be difficult to find. Slide 5 Physical Adaptations Postbreeding molt Fluffing (control of contour feathers) Slide 6 Postbreeding Molt The vast majority of a Black-Capped Chickadees insulation comes from the contour feathers. Adults go through only one molt per year, with that being the postbreeding molt at the end of summer. By shedding their old and worn feathers and replacing them with new ones, Chickadees gain better protection from the cold. Slide 7 Fluffing One method that Chickadees use to keep warm is called fluffing. Black- Capped Chickadees have some control over the position of their contour feathers. By exerting control and puffing up their feathers, they can create a thinker insulating layer. Slide 8 Behavior Changes Food storage Winter feeding Roosting habits Activity in the cold Formation of winter flocks Irruption and vertical migration Slide 9 Food Storage Black-Capped Chickadee usually store food in the fall and early winter. They often create hundreds of food caches in any nooks and crannies they can find to make their winter survival easier. Chickadees can hoard hundreds to thousands of food items in a day. They store all varieties of food, such as seeds, insects, fruits, and berries. Before hiding their surplus, they often prepare it for consumption, such as stripping the shells off of seeds. Slide 10 Food Storage (continued) Black-Capped Chickadees have a very good memory. In order for their food storages to be of any use, it is only natural that they can remember where the caches are. It is suggested that Chickadees can remember where they store food through the process of physically traveling to the location. Even if other birds watch as the surplus is stored, they would not likely be able to find it with that knowledge alone. More amazingly, studies show that Black-Capped Chickadees even remember what types of food are in different locations, making it possible for chickadees to eat what they need when they need it. Slide 11 Winter Feeding In warmer weather, Black-Capped Chickadees usually feast primarily on insects, grubs, and larva, with supplements of plant materials like seeds and berries. The ratio of their diet is roughly 70% animal-based foods and 30% vegetable matter. In the winter, a Chickadees diet varies. They change their primary food source to seeds and berries. Some common foods a Chickadee will eat in the winter are conifers and seeds from ragweed, goldenrod, and staghorn sumac. Other common foods include the small waxy berries of poison ivy and bayberry plants. To a lesser extent they eat insects, such as spiders and caterpillars. Slide 12 Winter Feeding (continued) Black-Capped Chickadees still need to drink water in the winter. For this they have been known to eat snow if they cannot find liquid water. Chickadees also have a sweet tooth per say. When possible, they have been seen drinking from or eating icicles made of maple sap. In their foraging, Black-Capped Chickadees are sometimes met with the opportunity of highly rich food sources, such as the subcutaneous fat on animal carcasses. When the opportunity arises, flocks of Chickadees will gather together to gorge themselves on the high calorie food. Slide 13 Roosting Habits Black-Capped Chickadees roost in sheltered areas during the winter. These include holes in trees, dense vegetation, and any other nook or cranny they can find. It has been noted that in some cases, Chickadees will burrow into the snow to roost. By roosting in protected cavities, they reduce heat loss by convection to 0 percent and radiant heat loss by 60- 100 percent. Slide 14 Roosting Habits (continued) On very cold nights, Black-Capped Chickadees sometimes roost together in cavities. This social behavior benefits them in reducing the energy expenditure of any single bird. The majority of a Chickadees heat is lost from its head and face, where there is less insulating plumage. When roosting, Black-Capped Chickadees fluff themselves into a ball and turn their heads so that their beak and face are partially covered by their scapulars, or shoulder feathers. This aids in keeping them warm. Slide 15 Activity in the Cold On very cold and windy days, Chickadees try to forage in sheltered areas. They usually are less active on particularly bad days and fly as little as possible to minimize heat loss. However, despite this, they need more food on these colder days in order to keep their temperature up. Chickadees may also decide not to even leave their roost cavities if the conditions are too hostile. In order to do so though, they would need to have an ample storage of food in it. Slide 16 Winter Flock Formation In warmer weather, Black-Capped Chickadees hold territory as mating pairs. In the winter, they form flocks in order to survive. Winter flocks form throughout the fall. They are generally centered around one older and dominant pair of chickadees. From that starting point, other adult pairs, newly formed juvenile pairs, and other individuals join the group. Sometimes members of other bird species will even join too. Winter flocks, on an average, contain between 6 and 10 individual birds. The flock typically forages for food together and roosts close to each other. Winter flocks set up and defend feeding territories. These territories usually consist of an average of 36 acres. Slide 17 Winter Flock Hierarchy and Dominance The winter flocks carry with them a linear hierarchy, where the alpha male and female outrank the beta pair. The chain continues so that the beta male and female are dominant over the next highest ranking pair and so on. Older birds are usually dominant over younger birds, while males are often ranked higher than females. Dominant birds often supplant lower ranked individuals and take the best and safest foraging locations for themselves. However, unlike the lower ranked members of the flock, the high ranked chickadees will spend more energy in defending the flocks range and keeping an eye out for predators. Slide 18 Winter Flock Exceptions Winter wanderers are birds that almost always belong to a winter flock, but have an unusually large range. They are often older and more dominant individuals. It is suggested that they explore the borders of their flocks range in search of unusually rich food sources such as animal carcasses. Winter floaters, or flock switchers, are usually young, low-ranking, and unpaired birds that do not belong to any particular flock. In their case, they utilize the resources from the ranges of several different flocks. If an opportunity to join a winter flock arises, they may do just that. When a high ranking member of a flock dies, a winter floater may decide to and succeed at taking the dead birds place. Slide 19 Winter Flock Benefits Chickadees are inquisitive. When one member of the flock is successful, other individuals will try and mimic the successful action. This can lead to better foraging and survival. A flock of Black-Capped Chickadees is more efficient than a single Chickadee when it comes to searching for food. Being part of a flock ensures better chances at finding sustenance. Winter flocks are also useful in protecting against predators. There are more eyes to search and greater safety in numbers. Slide 20 Irruption and Vertical Migration Irruptions, or sporadic migrations, sometimes can occur if the population of Black-Capped Chickadees in an area is too high and there is a lack of food and territory. In an irruption, it would be typical for juveniles to head southwards in the fall. Some chickadees, mostly younger birds, will descend from mountainous regions in the winter, only to return in spring. This is most likely done in order to spend the winter in a more favorable location in terms of both climate and food supply. Slide 21 Physiological Adjustments Manipulation of fat deposition Shivering (control of metabolic rate) Thermoregulation Winter acclimatization Slide 22 Manipulation of Fat Deposition Chickadees can adjust their fat deposition by eating certain varieties of food. Since all sorts of food are often cached, they can easily get the type they need. Plant-based foods often provide more calories than those derived from animals. Black-Capped Chickadees can store enough fat to last them through a cold winter night. However, in the morning, their fat reserve will likely have been completely burned by morning. Slide 23 Shivering Black-Capped Chickadees produce a significant amount of their body heat through shivering. Most of the shivering is done in the large pectoral flight muscles. Chickadees use this shivering to adapt to the environmental conditions. When it is colder, they shiver more in order to maintain their internal temperature. Though it does produce a great deal of heat, it does so at the cost of their fat reserves. Slide 24 Thermoregulation As air temperature drops at night, it becomes more costly to maintain their body heat. In response to this, Black- Capped Chickadees have adapted and can forcibly lower their metabolic rate, which allows them to lower their energy expenditure. By decreasing their amount of shivering, Chickadees can enter a controlled hypothermia while roosting, which lowers their internal temperature by 10-12 degrees Celsius, which is essentially a 25% drop. This allows them to reduce their hourly energy costs by upwards of 23%. Slide 25 Winter Acclimatization Black-Capped Chickadees are adapted in that they are capable of storing twice the amount of fat in midwinter, where the days are the shortest, as opposed to early and late winter. Slide 26 Conclusion Whether it be through their perseverance or fascinating adaptations, Black-Capped Chickadees have become a widely known and popular bird among North Americans. Even in the dead of winter, they will never fail to show up at bird feeders. Through their physical adaptations, behavior modifications, and physiological adjustments, Chickadees can remain year-round residents in their population range. Overall, Black-Capped Chickadees are well adapted to survive the harsh conditions of winter. They truly are a hearty bird.