By Bob Engel, Marlboro College Professor Emeritus of Biology and Environmental Science
Judging from the houses that I see every fall, most fairies are pretty small. Some might be even less than an inch tall. It's a real challenge to be that small and be warm blooded, which I'm pretty sure all fairies are.Take a hummingbird, the closest thing to an areal fairy I know. The smallest of these is the Bee hummingbird of Cuba, weighing in at less than 2.5 grams. "Ours," the Ruby-throated hummingbird, weighs about 3.5 grams (that's about 60% of a quarter!), though that weight might go up or down as much as 30% over a day.
Because animals lose and gain heat from the environment as a function of their surface area, small animals cool and heat quickly because their surface area is large compared to their volume. If they produce heat in an attempt to maintain a more-or-less constant body temperature, tiny creatures have to be real furnaces because their (almost always) cooler environment sucks their heat away. So what is a fairy, a hummingbird, a shrew, or a tiny mouse to do? There are only two solutions, maybe two and a half.
The first is to eat incessantly. I'm not sure about fairies, but all the rest of the little people eat most of the hours of the day (and some, the night). A hummer would eat more than is does, except for its crop, which inconveniently fills up. How annoying to have to sit on a perch and wait for the contents of the crop to empty into the rest of the gut. In the meanwhile, why not aggressively defend your food source (only a male option, really). A feeder is like a large grocery store for a hummingbird: with any luck, it always has food. If the store owner mixed the contents correctly, the "nectar" is at least 25% sugar, so an ounce might contain close to 25 Calories. Just catch a few gnats to add real nutrition.
Another example of this endless quest for calories among the tiny and hot-bodied was a poignant passage from a book by the Nobel laureate, Konrad Lorenz. He had a family of small shrews living in a terrarium. One night he failed to feed them at the three hour intervals they required and they all starved...in just three hours.
The 0.5 option, by the way, is to eat highly calorific food if available. Many desert rodents do this by eating seeds with a lot of fat. Think nuts. Fat has twice the calories, gram for gram, of either protein or carbohydrates. That's why many desserts are toxic to human diets.
The other major solution to the small, hot dilemma is to drop the body temperature whenever possible. By doing so, heat loss is reduced because it depends on the difference between the body and ambient temperatures. Hummingbirds, chickadees, and some small rodents do this every night, or with the rodents, even for a few days.
So what about the fairies? I can only guess, but I propose the following. First, I would guess that fairies love fatty food, and are constantly stealing from places like ice-cream stores. Second, I think they sleep often and deeply, dropping their body temperatures when they do so. That might be why they are so hard to see.