Nature up close: Feathers
By "Sunday Morning" contributing videographer Judy Lehmberg.
We heard a thin, high-pitched sound repeated several times while walking up a snow-covered hill in a spruce forest. Could it be a golden-crowned kinglet surviving in these sub-freezing temperatures? As we got closer and continued to hear the same calls, I knew it had to be at least three kinglets. We finally saw them flitting around in the branches of a spruce tree gleaning small caterpillars.
We had been warmed by walking (actually, slogging) through the snow, but we were still really cold in spite of our thick coats and gloves. How were these tiny birds, the smallest bird in the northern woods weighing in at all of one-fifth of an ounce -- the weight of a quarter! -- able to survive well-below-freezing temperatures?
The further north an animal lives, generally the bigger it is. Temperature-wise, that makes sense. A large kettle of hot soup cools down very slowly as compared to a piece of toast. But here was a group of tiny birds doing just fine. How?
An English major might quote Emily Dickinson and say "Hope is the thing with feathers." A biology major would (quite unpoetically) say, "Feathers are unbelievably well-adapted by millions of years of evolution to allow birds to fly and stay warm."
Feathers truly are some of the most amazing structures in the natural world; they are extremely lightweight (which is helpful if you fly); replaceable (when one is lost a new one grows in to replace it); they are fantastic insulators; and they provide superb waterproofing.
Although they can be split up into a number of categories, birds have two major feather types: flight feathers and down feathers. Flight feathers are stiff and, because birds can control their positions, act almost like individual wings. The individual feather "branches" (or, more correctly, barbs) have their own barbules with thousands of little hooks. These hooks hold the barbs together, much like Velcro, to form an air-resistant surface that catches air and allows birds to fly. They also provide a waterproof surface -- especially important for aquatic birds. The waterproofing is maintained by birds, such as ducks, when they preen. They have an oil gland on their back, the uropygial gland, where their body meets the tail. When they preen they get oil from the gland and spread it on their outer feathers.
Downy feathers make up the majority of feathers on a bird and provide excellent insulation because they trap air between the bird's body and the air around it. Downy feathers have barbs and barbules just like flight feathers, but a down feather's structure is different. These barbules don't form hooks, but instead branch to create more air space, and thus more insulating ability. In especially cold weather, birds can fluff their feathers to provide an even thicker layer of air.
Consider the pink fiberglass insulation common in houses made up of millions of individual fibers with air in-between. The thicker the insulation, the higher the R-value. When birds fluff their feathers they are increasing their R-value. None of the synthetic fibers made to imitate down in coats comes close to the insulating properties of bird down, but then nature has been working on feather design 150 million years longer than we have been making synthetic down coats!
Judy Lehmberg is a former college biology teacher who now shoots nature videos.
For more info:
- Judy Lehmberg (Official site)
- Judy Lehmberg's YouTube Channel
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