Songbirds:
Songbird mortality can reach 30% to 50% during harsh winters, driven by freezing temperatures and food scarcity. Small birds, such as chickadees, must consume immense calories daily to survive, often dying from starvation or exhaustion within 6–24 hours of failing to find food. Key threats include, rapidly shifting food access due to snow cover and high metabolic demands for shivering to stay warm.
Causes of Winter Bird Mortality
- Energy Deficit: Birds must consume nearly half their body weight daily to survive, burning it off (shivering thermogenesis) to maintain body heat and compensate for body heat loss through radiation, conduction, and convection (wind). Their body temperatures vary between 102 to 109 degrees Fahrenheit.
- Food Scarcity & Snow: Snow cover acts as a barrier, preventing access to food sources, with the first major snowfall often being the most fatal.
- Thermal Stress: At -10∘𝐶 (14∘𝐹), small birds experience a 2–3 fold increase in metabolic rate.
- Vulnerability: Young birds from the current year and older individuals are most at risk due to lower fat reserves.
- Days are Short: Most birds are active only during the day (diurnal). They do not have many hours of daylight to find what little food there is, and winter nights are extra long and extra cold.
- Wet Conditions: Wet feathers lose insulating value, making birds highly susceptible to hypothermia.
Survival Strategies & Behaviors
- Roosting & Huddling: Birds seek shelter in tree cavities, thick evergreens, or birdhouses to escape wind and snow. Some birds will cluster together each night, huddling to keep warm.
- Hypothermia/Torpor: Some species, such as chickadees, can enter a state of regulated hypothermia (called torpor), sort of like short-term hibernation, to save energy and survive the night in cold places, though this reduces reaction time to predators.
- Food Hoarding: Species like nuthatches, tits, or chickadees store food (called caching) to manage scarcity. Cached food can provide 50% or more of a bird’s diet in the depths of winter. Black-capped chickadees increase their brain size by up to 30% in the fall to store, remember, and locate thousands of food caches, such as seeds, during winter.
How to Assist Bird Survival
- High-Fat Food: Provide suet, sunflower seeds, and peanuts, which offer necessary calories. High-energy food like suet and nut mixes help them maintain the 100°F+ core temperature needed to survive.
- Water Sources: Offer heated bird baths to prevent birds from having to expend energy melting snow for hydration.
- Shelter: Maintain dense vegetation or nesting boxes for protection from the elements.
Birds rarely get frostbite due to a countercurrent heat exchange system in their legs that warms blood returning to the body, keeping feet just above freezing. Their legs have little moisture, muscle, or nerve tissue, reducing freezing risk. They also conserve heat by tucking feet into feathers or sitting on them. They also "fluff" their feathers to increase insulation.
Birds of Prey:
Mortality rates for birds of prey, particularly juveniles, are exceptionally high during harsh winters, with estimates often ranging from 50% to over 60% for young, inexperienced birds in their first season. While adults have higher survival rates (around 95% in some studies), intense cold, snow, and limited prey reduce survival for all ages.
Key Winter Mortality Factors for Raptors:
- Age and Inexperience: Up to 60-65% of juvenile Red-tailed Hawks may not survive their first winter due to poor hunting skills.
- Starvation: Scarcity of prey (rodents, rabbits) due to deep snow and frozen conditions is the primary cause of death.
- Energy Deficit: Extreme cold increases the energy needed to maintain body heat, while reduced hunting success prevents sufficient caloric intake.
- Physical Hazards: Severe weather, such as biting winds and deep snow, makes hunting difficult and increases caloric expenditure.
- Species Differences: Specific studies have shown winter mortality rates of 62% for Barn Owls in certain environments.
Survival Rates and Context:
- Adult vs. Juvenile: In one study, immatures had a 68% winter survival rate compared to 95% for adults. Juvenile raptors often compensate for inexperience by hunting smaller prey or scavenging, which increases their vulnerability to starvation or accidents.
Key Adaptations to Avoid Frostbite:
- Countercurrent Heat Exchange: Arteries carrying warm blood to the feet run directly next to veins returning to the body, transferring heat back to the core to prevent the feet from freezing.
- Feathered Protection: Species like the Rough-legged Hawk have feathers extending to their feet (tarsi) for extra warmth.
- Specialized Feet: Raptors have specialized, heavily scaled skin on their feet to reduce heat loss.
- Behavioral Adaptations: Raptors often tuck one leg into their feathers while perched to conserve heat.
Winter Survival Strategies:
- Increased Metabolism: Raptors increase their metabolic rate in winter to generate extra body heat
- Shelter: Smaller raptors like screech owls use tree cavities to escape cold, wind, and frost.
- Preening: Preening maintains the waterproofing and insulating qualities of their feathers.
While rare, frostbite can occur in extreme, prolonged, or unusual conditions, particularly if the bird is already compromised.
Keeping your bird feeders full in winter, particularly during episodes of severe weather like we are having now, can make or break the survival of not just songbirds, but also birds of prey, like Cooper’s hawk, that feed on them.