The story of bird plumage is subtle and complex

The story of bird plumage is subtle and complex

One obvious reason why people enjoy the existence of birds so much is their bright and varied colors. Humans — and most mammals — seem boring in comparison. But why are birds the colors that they are? The competing evolutionary directives toward camouflage and mate attraction are two of the clearest forces, although the complete story of avian plumage patterns is considerably more subtle and complex.

Brown creepers are among our greatest masters of camouflage. Photo by Mick Thompson

Camouflage first. Many of our local birds are not dressed in shockingly bright colors — for good reason. Bright reds, yellows and multi-colored patterns make a bird more visible to predators, especially when considered against the relatively subdued color palette of dry California. Many birds are dominated by browns and grays and streaky in-betweens: think of California towhees, titmice, wrens, sparrows and many more of our common birds.

Some of the most colorful birds we do have — orioles and tanagers, for instance — migrate here in summer from more tropical latitudes, where evergreen tree canopies with broken rays of sunlight make bright yellows a more effective general strategy of camouflage.

Awareness of some basic patterns reveals meaning and interest even in the least-colorful birds. For instance, many species have lighter bellies than backs: think of juncos, spotted towhees, black phoebes, wrens, jays and many sparrows. This makes sense. When birds are on the ground, a darker back is good camouflage against plants or shaded soil, while when they are in trees overhead, predators might see them against the brighter sky, in which case the lighter bellies would be more effectively cryptic.

Another common feature is the use of streaking and various subtle patterns, rather than uniform color. While these patterns can appear mesmerizingly rich in the abstract, they help birds to blend in with their surroundings as well, as can be seen in our greatest masters of camouflage such as great horned owls, brown creepers or Wilson’s snipe.

There’s another obvious trend in bird camouflage: males are often less cryptically plumaged than females. Part of the reason for this is that females have, on average, a greater need for cryptic plumage than males, because they do a larger share of the incubating of eggs and brooding of young — their camouflage is essentially for concealing both themselves and the next generation. One way we can tell this is a factor is that birds that nest in enclosed cavities rather than open cups tend to have less “sexual dimorphism” — male and female titmice, chickadees, wrens, nuthatches, and woodpeckers are all generally similar in appearance.

The other part of the male-female difference in bird color relates to the males: why are they so extravagantly colored, at least in some cases? Male turkeys, quail, goldfinches, house finches, orioles, grosbeaks, tanagers and more all show clear differences between male and female birds. The bright colors or other extravagances of these males would seem to be directly counterproductive to their safety and discretion.

And that’s the point. The standard theory is that the harmful fitness effects of extravagant display plumage are part of how the display works: if a male bird is able to survive and thrive despite carrying around a large, non-removable billboard to predators shouting “here I am!” then it must be a healthy, effective individual. The same mechanism is true for the other famous avian display, song. Bright plumage and singing both also reveal physical health directly: an adequate and suitable diet, a lack of disease, and general vigor are all conveyed by a fresh and vibrant set of feathers in good condition. Such signals are crucial for mate attraction in spring (hence the frequent pattern of molting into bright plumage at the beginning of the mating season), but can also play a role in signaling health in flocking birds in winter.

These two features — concealment from predators and display of health to potential mates and rivals — are the biggest influences on why bird plumage has evolved toward the patterns we see today. Other features have other functions: some birds suddenly reveal concealed patches of bright plumage to startle predators, startle prey, or to help stay in touch with partners or flock members, for instance. Birds fill our lives with color. And those colors reveal not just abstract and random rainbows, but keys to understanding how they live their lives.

Jack Gedney’s On the Wing runs every other Monday. He is a co-owner of Wild Birds Unlimited in Novato and author of “The Private Lives of Public Birds.” You can reach him at jack@natureinnovato.com.