I’m taking advantage of more time home than I usually experience (grrr think Covid-19 and “stay home”) to keep working my way through the Cornell University online ornithology course textbook. It is a wonderful text and course, with lots of online video aspects too. I hope to put out summary posts of some of what I’m learning using examples to make learning fun. On the section on thermoregulation Bushtits are used as an example, tweaking my interest in researching Bushtit trivia. Birds, like only mammals in the animal kingdom, are warm blooded, a.k.a. are endothermic homeotherms. This means that when they are in an environment warmer or cooler than their core temperature birds, just like us, need to do something to maintain their stable body temperature. There are exceptions, namely torpor-like states where birds allow themselves to cool down and slow their metabolic temperatures, but most birds find ways to maintain a body temperature without burning external fuels to heat their environment, or wearing warm clothes other than their own feathers.

I’ll focus here on birds staying warm, though how they stay cool is maybe even more interesting. The example used in the text was Bushtits. These tiny hyperactive birds weigh about 5.5 grams. A teaspoon of water weights 5 grams for comparison. This is only slightly more than the Anna’s Hummingbirds that also winter around our area. (4.3 grams) A study looked at Bushtits in the south and showed that they need to consume about 80% of their body weight in animal matter daily to maintain their body temp and metabolic demands at 20 degrees C, a pretty nice summer day here (68 F). It has to be higher in our near freezing winter weather.
Here are some tidbits about Bushtits and things they do to stay warm:
• There is some evidence that Bushtits sometimes build winter nests that are warmer than their breeding nests.
• Like other small birds they often huddle together in tightly packed protected areas on cold nights.
• In the daytime they are constantly moving, both to find enough food to burn to generate heat, but also to use their larger muscle masses to generate heat.
• There is no evidence that they gain body fat in the winter to add a layer of fat for warmth.
• It is not felt that they intentionally allow their body temp to drop at night.

Here is a table comparing some other common wintering birds in our area, with their weight in grams, a calculation of their surface area as if they were sphere shaped (obviously they are not, but for comparison I assume that the ratios are relatively appropriate as birds generally have the same body parts and general shape), and the ratio of surface area to mass.

It seems cool to me that intuitively it seems that the bigger birds can remain relatively still, and the smaller birds seem to need to move around a lot. Likely lots of reasons, but thermoregulation has to be among the more important of these reasons.

Until next time, good birding, and good day.