For the Birds Radio Program: How Birds Keep Warm

Original Air Date: Jan. 18, 1988

How many seeds are in a 50-pound bag of sunflower seeds? Laura’s family figured this out just so she could figure out how birds keep warm in winter.

Duration: 4′00″


(recording of a Black-capped Chickadee)

How many seeds are in a 50-pound bag of sunflower seeds? My six-year-old son Joey asked me that question last week, and so we conducted a family experiment to see. First we made estimates, ranging from daddy’s guess of half-a-million to Joey’s more optimistic one billion. Then we counted five ounces of black-oil sunflower seeds one by one–it was just about exactly 3,000. That amounted to 9,600 seeds per pound, which makes it 480,000 seeds per fifty-pound bag. That’s pretty close to half-a-million. When it comes to estimating seeds, in our family father knows best. Grocery-store mixed seed is much tinier than sunflower seed–a 10 pound bag holds 580,000–meaning 50 pounds amounts to almost 3 million seeds.

What this is all leading to is how birds survive winter. Those tiny grocery-store seeds are poor food for many birds partly because it’s inefficient to open them one by one to extract the nutrition. Sunflower seeds are both big enough and have enough protein to fuel many kinds of birds through the coldest days. In areas without feeders, birds obviously survive, but they have to spend a lot of energy searching for natural food. That’s why there are more winter birds per acre near cities and towns than there are in wilderness. Small birds are vulnerable during our long northern nights, since they can’t search for food in the dark. Some, like redpolls and Evening Grosbeaks, have over-sized crops. They stuff themselves before nightfall so they can stoke their metabolic fires even as they sleep.

Although winter survival of birds depends first and foremost on whether they have enough food, they have other strategies for minimizing heat loss. For one thing, they wear more feathers in winter. House Sparrows have 11.5% more feathers in winter months than in July. Birds appear fatter on cold winter days because they puff out their down the way we fluff up a down sleeping bag to improve its insulation.

Birds also avoid exposure to wind as much as is humanly– or should I say birdly?–possible. The colder the temperature or the higher the wind speed, the lower chickadees stay in trees. When the weather is extremely harsh, they feed on the ground. They also use windbreaks, perching on the leeward side of thicker branches as temperature or wind become harsher, and cut down on their flying activity since their own speed increases exposure to the wind.

Some birds, like Snow Buntings and grouse, ride out the worst days and nights under a literal blanket of snow. The air temperature may drop to 25 below, but under a deep powdery snow the temperature may be all the way up at 25 degrees above zero.

Chickadees, nuthatches, and other species roost in tree cavities, where exposure to wind is minimal. Sometimes they pack themselves into cavities the way 1920’s college kids packed into telephone booths–as many as 50 titmice have been counted huddled in a ball-like mass on cold nights. In one laboratory study, 50% of House Sparrows roosted in contact with each other when the temperature was 14 degrees, while only 10% snuggled when the temperature rose to 32. A lot of human October babies are produced under much the same circumstances.

(Recording of a Black-capped Chickadee)

This is Laura Erickson, and this program has been “For the Birds.”