|Offering high-energy suet can help birds like this acorn woodpecker survive the cold. Photo by Susan Brown Matsumoto|
Do you know how birds survive in winter?
In some ways it’s a lot like humans: they put on a down coat, hang with a group of friends, eat a lot and find a warm place to hunker down.
Because birds are exposed to the elements, they rely on a variety of physiological and behavioral adaptations to stay warm. For example, certain species will grow additional downy feathers before winter to increase their insulation.
“When goldfinches complete a full body molt in the fall, their feathers become more dense to help keep them warm,” said John Schaust, chief naturalist for Wild Birds Unlimited in Carmel, Ind. “Juncos are known to increase their feathers by as much as 30 percent in winter.”
Other avian physiological responses to the cold include the use of muscle contractions to generate body heat. While similar to shivering, it doesn’t involve the same trembling motion seen when humans are cold.
Chickadees, including the mountain chickadees that live in Southern California, use a mechanism called “regulated hypothermia,” Schaust explained. While they normally maintain a temperature of about 107 degrees Fahrenheit, they can lower this to about 86 degrees when their environment drops below freezing in order to survive the night. Hummingbirds can enter a state of torpor, in which their respiration and other body processes are slowed to conserve energy until their next meal.
Birds also change their behavior when the weather is nippy. They start by fluffing their feathers to trap a layer of warm air against their body. Generally, they roost earlier in the winter because temperatures can drop quickly after sunset. Titmice, chickadees and bluebirds will seek shelter from the wind in old woodpecker nesting cavities and man-made nesting boxes when available. And bluebirds and bushtits often huddle together at night to conserve heat.
When temperatures drop, all of our fine-feathered friends need to eat more to generate heat. “We promote high-fat, high-calorie foods,” Schaust said. These foods include black oil sunflower seeds, suet and mealworms, which help support birds’ high metabolic rate during winter.
Studies have shown that bird feeders make a difference, he said. In severe cold, the activity at feeders increases because the birds need more calories to stay warm under these conditions.
“An interesting fact is that pine siskins can pack enough seeds into their crops before roosting to survive five hours at -4 degrees Fahrenheit,” Schaust said.
Trying to keep warm takes energy. And birds can expend calories, which are stored in the form of fat, very quickly.
“This fat must be replaced every day,” he said. “As birds eat throughout the day, they’ll convert that food to fat. And they can store up to 10 percent of their body weight in fat. But they’re going to burn up all that fat overnight. Then they have to start that process over the next day.”
In addition to keeping bird feeders full, it’s also important to replenish fresh water in fountains and birdbaths for drinking and bathing, he said. Though a cold shower may not sound appealing to us, birds bathe in winter because feathers have to be in good shape for insulation against the cold.
While winter is a good time for tree trimming, consider leaving places for birds to take cover. Roosting in thick foliage is definitely warmer than being out on a limb.