Why it’s harder to keep a small bird warm

The Short Answer:  Imagine if you put a thimbleful of water and a gallon jug of water out on a cold night. Which one would freeze first? Intuitively, you’d say, “the thimble.” And you’d be correct. But is that just because there’s more water in the gallon jug than in the thimble? Actually, no. Imagine if I took that same gallon of water and poured it out on a large flat pan so that it was only a ¼ inch (65 mm) deep. Now which one would freeze first? Probably the gallon, not the thimble. So it’s not just the amount of water, it’s the amount of water that is exposed to the cold air, and that has to do with the surface area of the container. Heat transfer occurs relative to the amount of surface area, and a ¼ inch of water on a flat pan has much more surface area than the gallon jug. And it has more surface area per amount of water than the thimble. So it would freeze first.

But you were right about the thimble freezing before the gallon jug because although the surface area of the gallon is bigger than the surface area of the thimble, the basic mathematics of volumes and areas tells us that the surface area of the gallon it isn’t as much bigger than the surface area of the thimble as you might think. As long as the shape stays roughly the same, the amount of surface area certainly increases as the object gets bigger, but it doesn’t increase as fast as the volume does. That’s why a thimble of water will freeze before a gallon.

Now let’s think about two birds, a very small chickadee and a fairly small starling. If you put these two birds out on a cold night, which one has to work harder to keep from freezing? Remember, if you’re trying to keep from freezing, surface area is your enemy. And remember that the smaller the object, the more surface area it has relative to its volume.

Let’s assume a chickadee has a volume that can be approximated by a one inch (2.5 cm) diameter sphere and a starling by a two inch (5 cm) diameter sphere. If you did the math, you’d find that the starling has about eight times the volume of the chickadee. So it’s reasonable to assume that its body generates about eight times as much heat. And if it had about eight times as much surface area, then we might assume that the two birds would have a fairly equal struggle to stay warm on a cold night. But remember, the volume increases faster than the surface area. The starling doesn’t have eight times as much surface area, it only has four times as much surface area … and therefore doesn’t lose heat nearly as fast as the chickadee. Assuming their feathered insulation is roughly the same, the chickadee will have to burn about twice as much energy for its body size as the starling to stay at normal temperature.