That's very interesting. I wonder why evolution did not favor a fur-covered bird with feathered wings. What use are feathers on it's body compared to the insulation superiority of fur?
Although feathers and fur do both provide insulation, feathers offer less drag. Ever see any fast swiming under sea creature, with fur? Several feathered birds are very good underwater swimmers. - Same is true in air. (Bats are probably developed from tree climbing, fruit eating, rats and got trapped with their evolutionary gift of fur.)
Perhaps more important is the difference in the way fur and feathers work thermally in sunlight. People from Africa (and most other animals) with fur* have black fur.
Not nearly as many animals with fur near the poles, but their fur is normally white or of a light color at least. This is usually explained by the presence or absence of snow, but that is the factor of lessor importance.
At the pole, where it is cold and the sun is weak, one "wants" to both have an insulation layer and to forward scatter each photon down to the skin, thur the insulation layer and absorb it there. Near the Equator, one "wants" to absorb the intense solar photons in dead tissue at least a centimeter away from the skin and in a region where the resulting heat is more likely to be given to the surrounding air, instead of the body.
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*As part of the unconscious effort to make humans unique, not just another animal, we call human fur "hair."
PS I doubt that your simple statement about fur having superior insulation quality encompesses the problem, is true, etc. You must at least speak of what type of energy flux you are trying to impead, where you want the energy incident to be disipated, etc. Fur is better for solar flux control, but I think if your body is significantly hotter (as is case of birds) than the air and the insulation is to prevent loss of body heat with the least thickness of insulation layer, feathers are greatly superior to fur.
As a general rule "mother nature" knows and has found the optium solution to all problems. I like to say: "My body is smarter than all the doctors in the world." - I avoid going to doctors unless my body is getting into significant trouble. I.e. I suffer with my alergies, instead of take pills etc. In last 30 years, I have had one systemic infection that required an anti-biotic. - This is not an exception to my idea that "mother nature knows best." - She built that dam bug also. I waited with fever* for almost a week, before concluding that she had given that bug a better set of gifts (weapons for our mutual war) than me. Most people my age take half a dozen pills each day - I take none and am in great physical shape, very rarely get sick. - My immune system is strong as it has had to work without a lot of medical help.
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*Good idea to keep careful watch on your fever temperature. If it goes above 102F think seriously about taking some asprin, above 103 definitely take some. Above 104F, get in tub with cold water, ice in it, if you need to. Mine stayed about 101 for days so I did nothing. If my "smarter-than-doctors" body wants to set my temperature at 101F to make conditions tough on the invador - I would be stupid to stop it.