Winter weather brings many challenges to wildlife, with plunging temperatures, frost-covered ground, a shortage of naturally available summer fruits, plants and insect life, and shorter daylight hours for foraging.
Birds are no exception, and many struggle to survive the coldest months due to their fragile body size and high demands of energy expenditure to keep themselves warm.
Many species have developed particular adaptations to enable them to cope with subzero temperatures, and others rely heavily on extra food sources, supplied in gardens and backyards.
Birds’ feathers play a key role in insulating their bodies, enabling them to stay warm when the surrounding air temperature falls. Healthy feathers are vital, with many species undergoing a post-breeding molt that ensures their plumage is in tip-top condition when they need it most. Feathers are used to trap layers of warmth between each other, and ultimately around the bird’s body.
To ensure that these insulation techniques work effectively, birds need clean, dry and flexible feathers. Preening is necessary to keep their plumage in prime condition and birds adopt weatherproofing methods to protect their feathers, using oil produced by the preen gland near the base of their tails to coat their plumage which helps to preserve their own body heat.
Ahead of fall, many birds actively forage for high-energy foods, and put on layers of fat that they depend on for warmth in cold weather, but their plumper appearances are not always caused by carrying extra weight. By fluffing up their feathers, they are able to thicken the insulation around their bodies and preserve heat at their core.
An Annas Hummingbird - Birds’ feathers play a key role in insulating their bodies, enabling them to stay warm when the surrounding air temperature falls
Certain birds enter a state known as ‘torpor’ overnight, in which their heart rate is slowed, and their body temperature falls into an energy-saving mode similar to short-term hibernation. By lowering their own body temperature and breathing and heart rates, birds are able to significantly reduce their own energy expenditure, meaning that they do not use all their calories overnight just to stay alive.
Hummingbirds, Doves, Mousebirds and some Swifts are able to enter torpor for brief periods to maximize their chances of survival during winter.
Shivering is used by birds as well as humans as a method of maintaining body temperature when the air temperature drops. Shivering and quivering their feathers preserve their own body heat, but burn up vital energy stores and fat resources in the process, meaning that they need to feed as soon as daylight breaks.
Another interesting temperature-control technique present in certain birds is known as counter-current heat exchange, which utilizes the blood flow in the legs of birds to conserve warmth. The lack of feathers or other insulation on birds’ legs makes them a potential source of heat loss.
However, the flow of warm blood through the arteries warms the colder blood being carried to the heart by nearby veins, limiting the escape of body heat.
A Speckled Mousebird. Certain birds enter a state known as ‘torpor’ overnight, in which their heart rate is slowed and their body temperature falls into an energy-saving mode
Adaptable and resourceful, many bird species that have insect-based diets during the spring and summer months switch to seeds and berries from the fall, as these are more widely available throughout winter.
Winter berries, including holly, hawthorn, blackthorn, juniper and rowan, thrive in woodlands and hedgerows later in the year, and blackberries are a popular option in the fall, with high-energy sweet juices bringing multiple benefits.
Certain species, including Woodpeckers, Chickadees, Nuthatches and many Corvids, are cunning hoarders, stashing nuts and seeds in crevices in bark and notches in trees, and revisit these stores when food supplies start to dwindle.
Birds have much higher metabolic rates than humans and need to burn more energy to stay warm than we do. Every calorie counts, both in terms of intake and expenditure. By grabbing any chance to sit in the warm winter sunshine, and feeding whenever they can, moving as little as possible, they can preserve energy to help them survive overnight until it’s time to find food again.
An Acorn Woodpecker - Woodpeckers, Chickadees, Nuthatches and many Corvids, are cunning hoarders, stashing nuts and seeds in crevices in bark and notches in trees
Overnight roosting spots can make a huge difference in the nightly battle for survival, with insulated sheltered perches highly sought after.
As well as camouflage against being spotted and targeted by predators, protection against adverse weather, cold winds and heavy rain is vital. Roosting sites tucked up against tree trunks and deep inside dense vegetation offer effective shelter and can significantly boost a bird’s chances of survival against the elements.
While some birds are solitary and roost alone each night, others form communal roosts and flock together as night falls. Roosting alongside each other is an effective way of collectively raising body temperature with shared warmth.
Smaller species frequently gather in vast groups and huddle close together, providing heat and protection. Crag Martins are one example, with thousands heading to sheltered caves each winter evening and packing themselves tightly together until day breaks.
Birds do generally not use nests to sleep in outside of the breeding season. However, some cavity nesters may construct additional chambers that they later use as sheltered roosting spots once their young have flown the nests, for example, Flickers, Woodpeckers, Tawny Owls and European Robins.
Some birds will use empty nest boxes through the winter, frequently sharing the space with others of the same species for warmth and security.
A Tawny Owl - As well as camouflage against being spotted and targeted by predators, protection against adverse weather, cold winds and heavy rain is vital
Not all birds stick around in their breeding territories to experience the changing and often challenging winter conditions. While most winter migrations are triggered by the need to move to a new location in search of more available food sources, some northern landscapes become inhospitable, and movement to warmer landscapes is unavoidable.
Some migrations are relatively short-distance, with birds that breed at higher mountain altitudes moving downslope post-breeding to terrain with more abundant insect life or fruit and seeds. Other species cover epic distances twice a year, to and from breeding grounds, in order to reach the optimum environments that give them the best chance of survival.
Long-distance migrants include Arctic Terns that breed close to the North Pole and winter in the Antarctic Ocean, neither of which offer particularly warm climates. Swifts travel between sub-Saharan Africa and Europe twice each year, driven by the winter rainy season across Africa and the widespread boom in local insect populations.
Long-distance migrants include Arctic Terns that breed close to the North Pole and winter in the Antarctic Ocean
Many birds that are solitary and territorial become more gregarious in winter months, joining larger flocks to forage, roost or migrate together.
Flocks regularly consist of mixed species, with larger and smaller birds associating together in search of food and for protection against predators. Safety in numbers is a key factor behind flock formation, as groups naturally have a higher level of vigilance against potential intruders, as they are able to monitor threats from several angles.
Foraging as part of a flock improves a bird’s chances of locating food and safely eating it without being preyed on. Winter flocks of ground-feeding birds are a common sight in backyards and large open parks and gardens, with Dark-eyed Juncos, American Tree Sparrows, White-throated Sparrows, and White-crowned Sparrows joining forces. Other species that regularly buddy up in winter include Chickadees, Tufted Titmice and Downy Woodpeckers.
Bird feeders offer a true lifeline to many birds during cold weather, and it’s not an exaggeration to suggest that their survival through winter is mainly enabled by the provision of high-energy foods at bird tables and backyard feeders.
Species that do not visit yards during the rest of the year will show up eagerly as temperatures drop, and nuts, suet and fat balls offer vital sources of the fats and carbohydrates they need to survive.
Keeping a bird bath or drinking bowl topped up with fresh water is also vital. If it freezes over, warm water can be used to thaw the ice. Most birds need access to drinking water all year round, and if usual sources such as lakes and puddles become frozen, they can be a lifesaver.
Planting winter-fruiting berry bushes, hedgerows with dense foliage for shelter and covered feeding areas will support birds that visit your yard all year round, and offer protected spots for roosting as well as foraging.
It’s already becoming apparent that seasonal weather patterns are no longer as predictable as they once were, with some milder winters interspersed with colder than usual conditions, harsh unexpected blizzards and devastatingly hot heatwaves.
All of these factors impact the availability of food and shelter in the natural habitats of wild birds, and can change the timings or routes of migrations, the availability of sufficient food resources and the ability for birds to survive in the wild.
An American Robin - Keeping a bird bath or drinking bowl topped up with fresh water is also vital
High-energy food sources will be welcomed by birds that visit your backyard, in particular black sunflower seeds, suet, fat balls, peanuts and sweet fruits that are not naturally available, including oranges, grapes, cherries, apples and pears.
Keep your feeders frost- or snow-free and clear areas on the ground that are not covered by accumulated snow, so that food can be readily accessed.
Not all birds will readily use an artificial shelter in winter, but for those that might, it’s worth cleaning out a nest box once breeding has finished or hanging some purpose-built roosting pockets around your yard, that birds can take advantage of if they are in need of somewhere safe and secure to spend the winter nights.
As well as food and shelter, it’s crucial for birds to have access to fresh, unfrozen water for drinking and bathing during winter. Heated bird baths are available to buy, or alternatively, you can check daily to ensure that the surface of any drinking bowls remains ice-free.
A Red-breasted Woodpecker feeding on suet on a snowy winters day
Migration is driven by food resources rather than temperature. If there is sufficient food available all year round, species are less likely to be migratory and remain in their home territories throughout much of the year.
Migration is generally instinctive, and shifts towards lower ground or southwards may simply follow a pattern that is pre-programmed into the make-up of a bird. However, if enough insects, seeds, berries or mammals, depending on usual dietary preferences, are available, then migration is not always necessary as survival becomes less of a challenge when energy levels can be maintained.
Hummingbirds enter a nightly state of ‘torpor’ during winter, with their bodies entering a low-energy conservation mode similar to hibernation. They are able to slow their heart rate to around 50 beats per minute from over 1200 and can lower their body temperature to 18°C (65°F) from 40°C (104°F).
Despite these physical adaptations, hummingbirds still lose around 10 percent of their body weight each night and need to feed constantly throughout the day to meet their energy requirements just to stay alive.
High-energy food sources are ideal during winter months, as these offer birds a quick fix that will help raise their body temperatures and boost their chances of survival in cold weather. Peanuts, fat balls, suet, black oil sunflower seeds
The sheer resilience of tiny birds to survive night after night in subzero temperatures seems almost miraculous, and it’s almost incomprehensible that their fragile bodies do not succumb to the harsh conditions.
With the series of adaptations that have developed to help them cope with the seasonal challenges they face as summer ends, many birds can and do survive, simply by maximizing their calorie intake and minimizing their calorie expenditure.
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