When autumn finally gives in to winter, Oklahomans have to say goodbye to such things as fall colors and warm-weather bird species, including purple martins, scissor-tailed flycatchers and other avian summer residents. But, winter is a time for greetings and with the brisk north winds comes wintering birds. Cedar waxwings, purple finches, pine siskins, bald eagles, and many different species of waterfowl and hawks can be seen across Oklahoma’s winter landscape.
And while we're fortunate that Oklahoma's climate and geographic location provide suitable conditions for a number of bird species, winter weather in this region can also be severe at times. Luckily, birds have special adaptations to help them survive winter’s harsh conditions.
Some birds migrate out of the state, some migrate into the state, and others call Oklahoma home year-round.
For many species, migration is dictated by food availability. The red-headed woodpecker, common in northwestern Oklahoma during the summer, will move a just few hundred miles to the oak forests of eastern Oklahoma to fulfill their diet of acorns. While other birds, such as scissor-tailed flycatchers, travel thousands of miles to wintering grounds in Central and South America.
Considering the tremendous amount of energy required to fly great distances, many songbirds, shorebirds and waterfowl travel only as far south as they have to. And so, while Oklahoma’s winter may be too harsh for some birds, others such as the Northern harrier, brown creeper and many sparrows take refuge in our state.
And then there are the year-round residents who never leave. Hawks, eagles and other predatory species for example, aren’t required to migrate because snow and ice cover rarely interfere with hunting activities.
Although birds are homeothermic (able to maintain constant body and brain temperatures), they are sensitive to sudden temperature fluctuations. However, birds have a unique physiological response that allows them to adjust their internal temperatures in response to external temperature variations. In fact, they are so well adapted to surviving fluctuating temperatures, they are more resistant to cold than mammals.
Black-capped chickadees weigh less than half an ounce and can maintain a body temperature of 100 degrees Fahrenheit - even when the air is 0 degrees!
Birds' temperature control centers (called the thalamus or hypothalamus) are located in their brains. They send out various thermoregulation instructions via nerve impulses or hormone secretions when a birds' inner body, hormones and skin-surface receptors detect temperature changes.
Unlike humans, birds can withstand greater fluctuations in body temperature without serious affects. A daily variation of about 10 degrees Fahrenheit has been recorded for song sparrows and American robins. That’s quite a lot when compared to humans who have fluctuations of less than two degrees Fahrenheit.
Even when body temperatures are low, birds are more "warm blooded" than mammals, exhibiting normal body temperatures ranging from 103 to 109 degrees Fahrenheit. High body temperatures characterize high metabolic rates, the speed at which food is used for body heat and moving muscles. Small animals typically have higher metabolic rates than large ones because they have a greater surface area relative to their heat producing body mass from which to lose heat.
To maintain this rapid energy and heat production, birds must constantly refuel, especially during winter. This is why birds eat energy-rich foods such as seeds, fruits, nectar, insects and other prey.
Bird feathers play an important role in heat retention, particularly for smaller species who have more feathers per unit of body weight than their larger counterparts.
Birds ruffle their feathers on cool days. Fluffing the plumage conserves body heat because it creates additional loft by trapping insulating layers of air.
Ornithologists speculate that even feather color is a cold-weather survival mechanism. Darker feathers absorb light and heat rays more efficiently.
Unlike mammalian ears and tails, birds have few fleshy appendages. For example, a bird's beak has no skin and is made of a material similar to human fingernails, and gives up little body heat. Avian feet contain no fleshy muscles but instead are made of tough tendons with limited nerves and blood circulation. By not having to spend energy keeping appendages warm, birds are able to reserve a great deal of energy and spend it where they need it the most.
Even resting birds are energy conscious, tucking a foot in belly feathers or tucking their heads under wings. Ducks, which don't necessarily migrate if open water and an adequate food supply remain, sit on the ice with their feet under them to reduce heat loss. Because arteries and veins in their feet and legs lie against each other, blood returning from extremities via veins is warmed by blood coming from the well-insulated heart and lung area.
Sleeping birds face a special challenge in fending off cold throughout the long, frosty nights. It's common to see a trembling chickadee huddled in a tree. Shivering, although a short-term solution, increases body temperatures in a resting bird by releasing heat as the result of the metabolism necessary to work muscles. On a cold night, some birds including bluebirds will roost together in groups of a dozen or more to share body heat and reduce heat loss.
Birds have to be particular about where they roost in winter, choosing places that protect them most from frigid winds. Pine grosbeaks as well as cardinals seek protection in thickly-needled evergreens when they sleep. Chickadees, woodpeckers and nuthatches find respite in tree cavities. Many bird species rest as near to the ground as possible where there is more shelter from wind.
Normally, it's also warmer near the ground because heat from the sun is absorbed by soil and slowly emitted during the night.
Meadowlarks may even burrow under an insulating layer of snow and stay there throughout the night when temperatures dip way below freezing.
Grosbeaks and finches survive wintry nights by storing seeds in their crops. These digest overnight providing additional nutrition so they can maintain their high metabolism and body heat.
Some species, including mourning doves and chickadees, which remain in-state year-round, exhibit a form of short-term hibernation. During a severe snow, ice or windstorm when foraging for food is all but impossible, they can decrease their body temperature and metabolic rate, and subsequently their food requirements.
However, this mock hibernation is only a temporary solution and, to survive, birds must replenish energy stores they've expended so they can maintain body temperatures and rebuild fat reserves.
Birds must work diligently and efficiently when weather is harshest. Winter days are short so birds have only a maximum of about 10 hours to feed. Snow or ice occasionally covers trees, shrubs and the ground, hindering those species foraging for seeds or berries.
To a large degree the success of Oklahoma's overwintering birds depends on feeding methods. Many factors come into play including whether birds feed alone or in flocks, how much energy they can afford to expend foraging, and the quality and quantity of food which determine whether a species can survive. For example, several sparrow species like the fox and song sparrows and spotted towhee are considered "energy conservers" because they are more stationary when feeding. They tend to feed in the concealment of tall, dense habitats where provisions are often sparse. Therefore sparrows are usually solitary feeders in winter because scant habitats support only a limited number of birds. If sparrows feed in high seed density areas they are likely to attract others which would allow them less feeding time due to interaction among birds. A larger group would also attract predators that could go unnoticed in the tall grasses.
On the other hand, birds such as the longspur are inclined to feed in open habitats with high seed densities. A rich food source attracts birds causing this species to feed in flocks . In this case, gregarious feeding provides "more eyes" to watch for predators. However, flocks force longspurs into high energy strategies because fields are quickly depleted, prompting them to search for additional food sources.
Survival depends on how birds eat as well as what they eat. While several species, primarily insectivores, migrate away from Oklahoma in winter to find food, some birds' diets change according to the season. Frugivores and granivores (fruit and seed eaters), summer insectivores such as flickers and chickadees, and most of our game birds change their eating habits to take advantage of seeds and berries that mature in late fall and remain attached into winter. Most Oklahoma songbirds feed on seeds and fruits of sumac, poison ivy, dogwood, holly, winterberry, hackberry, greenbrier, pecan and others. Wild turkey, quail, prairie-chickens and other game birds eat many of the same seeds and fruits consumed by songbirds. White oak acorns, seed pods of lespedeza, buttonweed, ragweed and other weed seeds are an important part of their winter diet.
A dietary staple of chickadees and other insectivores that overwinter in Oklahoma includes insects passing through winter in a dormant state as eggs, nymphs, larvae, pupae or adults. During warm spells, larger insects move closer to the soil surface and are also readily eaten by birds. After a hard freeze, domestic grains such as wheat, corn, rice and alfalfa may constitute a major portion of game bird, waterfowl and songbird diets.
Anyone interested in birds can help them cope with winter by supplementing food and water supplies. Sparrows, juncos, chickadees, titmice, cardinals, finches and jays are attracted to feeders stocked with seed. Popular seed choices include black oil-type sunflower, white proso millet and niger. Softened fruits such as apples, oranges, grapes and raisins provide high-energy nutrition for many species such as mockingbirds, robins, and cardinals. Insect-eating woodpeckers, nuthatches, chickadees, titmice and wrens prefer suet.
Birds are also attracted to yards that provide roosting shelter. Many species find thick tangled growths of trees, shrubs and vines ideal spots in which to rest and preen while out of the wind. Dead trees and snags offer protected cavities for bitter winter nights.
And remember, supplemental feeding and watering may lure birds to within easy viewing distance, but rest assured, when the mercury drops, our feathered friends can cope. Survival is serious business but birds are well prepared.