Like many birds, Barn Owls show variation in their plumage coloration. There could be several reasons, but there are at least two that I know of.
I’m sure you have driven at night when a white owl flies across the road in front of your car. Often they look bright white. But look at a Barn Owl up close and it’s more or less a tan-tawny color on a whitish background. And the coloration varies a lot. Besides streaks of tan color, there are spots. One study some years ago discovered that females with the most spots are favored by males. Why are spotted females are more attractive to males? It turns out that females with the most spots have the fewest number of feather parasites.
This is the thing about science: one study seemingly disconnected to anything later becomes relevant. So someone at one time studies the feather parasites of Barn Owls, discovering that the more spots a female has the fewer parasites she has. Ok, a simple fact. Later someone studies Barn Owls and finds that males prefer more spotted females. Then another scientist puts two and two together and finds that the reason males prefer the more spotted females is that they are healthier with fewer parasites. How do the males know that more spotted females have less parasites? Maybe they really don’t but natural selection has – males who mate with more spotted females have more offspring because the females are healthier.
Now a study comes out demonstrating that color variation in Barn Owls affects hunting success. Barn Owls hunt at night and can see well in the dark, as we all know. But some nocturnal prey also have good eyesight or hearing in order to detect predators. Barn Owls show color variation in a tan to whitish range, dark to light. Now you would think that the darker the plumage of the owl the more camouflaged it would be against the dark sky, the lighter birds standing out, especially with a full moon.
Voles, one of the owl’s prey species are mouselike rodents somewhat similar in appearance to pocket gophers. They have a compact, heavy body, short legs, a short-furred tail, small eyes, and partially hidden ears. Fully grown they measure 5 to 8 inches head to tail. Although voles spend considerable time above ground, they spend most of their time in their burrow system. They don’t have particularly good eyesight, but they can see a predator like an owl.
Well, when a vole sees an owl, it takes a second or two to focus on it and determine it is a threat and decide to skedaddle back to the burrow. But the whiter the owl, the more confused (or maybe fascinated) the vole is, so the more time it takes in deciding to run off. So like a deer in the headlights, the vole is frozen in place for a few more seconds and becomes food for the owl.
But the story is more complicated. Whiter owls are only more successful in catching voles on bright, especially full moon nights. On dark or new moon nights, there is no difference in the hunting success of dark, tawny Barn Owls and the whiter version. Ain’t nature grand?