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The latest from Dr. Bird


I was about halfway through scarfing down some beans on toast for my supper on a gorgeous summer evening in North Saanich, British Columbia when my next-door neighbor appeared through the patio door with a tiny brown bird cupped in her hands and a long, sad look on her face. I surmised right away that it was a Brown Creeper and that it had hit her window. Our yards are loaded with humongous Douglas fir trees and we commonly see these little mouse-like birds spiralling upward from the base of a tree. I also knew from experience that a bird smacking into a window probably has about a fifty-fifty chance of making it, depending on the speed of impact. Anyway, I placed the little tyke on some rough bark about five feet up in a nearby tree and kept an eye on it to see if it would shake off the impact and fly off to live again.

But here is the interesting part. Once I walked just a few feet away from the creeper, it literally melded into the bark. Sitting frigidly still, it was incredibly hard to see! And that got me thinking about --- camouflage in birds.

While some birds are brilliantly colored and really stand out in their environment, they also likely have some other means to avoid capture, e.g. speed. Some birds, however, rely totally on being cryptic and hence, not being seen. By matching their background in coloration and pattern, they make it hard for a predator to distinguish them from other objects in their environment.

Take Whip-Poor-Wills for instance. They basically hunt insects by night and sleep during the day in the leaf litter on the ground. While resting though, they could easily fall victim to a number of four-legged predators such as raccoons, skunks, coyotes and foxes. That is, if they are detected! The color and pattern of these odd-looking birds matches perfectly the appearance of dead leaves; even if one does spot the feather detail, it is hard to discern where the vulnerable head is! The cryptic pattern of their plumage basically breaks up their outline and allows them to blend in nicely with their background. A close relative of the Whip-Poor-Will, the common Nighthawk, also prefers to sleep on terra firma during broad daylight, but their mottled gray feathers are more conducive to their habit of roosting on bare areas of gray pebbles or sand.

One of the coolest examples of camouflage in the avian world is the American Bittern. Because it inhabits mostly marshy habitat, this Bitterns is blessed with strong vertical streaking to match the dry brown bases of the cattails in their environment. However, when it senses the presence of an approaching predator, it acquires its best match to its background by stretching its neck upward and pointing its beak straight up into the sky. It will even sway from side to side to mimic the movement of the reeds in the wind! This behavior is so ingrained into this bird’s defensive mechanisms that it will actually adopt this posture even if it is threatened by danger while standing in the middle of a lawn. Other birds that share that reedy habitat with the bittern include the Marsh Wren and the Sedge Wren, but they rely solely on their streaked brown plumage and not their posture to avoid detection.

Owls rely heavily on camouflage too. The majority of these birds hunt at night and sleep during the day and their worst nightmare is being discovered by a crow or a jay or any number of small songbirds while napping in a tree, resulting in an annoying, intensive mobbing behavior to drive it off. If one peruses the pages of any field guide to peek at the owls, one quickly sees that their plumages are mostly composed of browns, grays and black colors in various barred, mottled, striped or streaked patterns. Like the bittern, many roosting owls compress their plumages to stretch themselves taller and thinner so as to resemble branches. Some, like long-eared, great horned, and screech owls, are even equipped with feather tufts on their heads which partly serve to break up the outline of their bodies and enhance the branch-like appearance. Snowy Owls, on the other hand, are day-time hunters and generally dwell in snow-laden environments. Their white feathers, sometimes speckled with black spots, give them a perfect match to their background. Nevertheless, they remain white year-round and those individuals that head to southern climes during winter and then delay their migration northward until after the snow has melted sometimes end up as illegal stuffed trophies on someone’s mantle shelf.

Along these same lines, perhaps the most amazing examples of birds attempting to look like something else but perching in plain view are seen in the great Pootoos and Frogmouths. Using a plumage color and pattern that looks like tree bark, assuming an upright posture to resemble a branch, and sitting absolutely still for literally hours during the daylight can render these birds totally invisible.

Not all birds relying on camouflage for survival retain the same plumage color and pattern year-round. Ptarmigan live high above the tree line, either at high elevations in the mountains or in the far north. In the winter months with nothing but the snow to hide in, these grouse-like birds sport a snowy-white plumage without any patterns whatsoever. When the snow retreats in summer, these birds change their plumage into a dark, mottled appearance that perfectly matches a landscape of tundra and rock. To achieve these remarkable transformations, Ptarmigan undergo two complete body molts each year.

Birds resort to other tricks too for blending into their habitats, including even adopting bold and striking color patterns. Disruptive coloration, whereby dark bands or bars in the form of rings around the upper breast of plovers like killdeer, help to make them harder to pick out in their environment. Even bold white patches on a dark bird, e.g. Lark Bunting, or the reverse, e.g. Snow Bunting, serve to draw one’s eye away from the bird’s body and help to conceal it. Other birds like Cedar Waxwings employ the ruse of hiding their eyes in black facial patterns in the form of masks. False eyes in the form of dark patches on the back of the head, as seen in ferruginous Pygmy Owls and American Kestrels, serve to draw attention away from the bird’s real eyes, which could present a vulnerable target to both mobbing songbirds and even predators.

Finally, countershading is widespread among a wide variety of birds frequenting watery habitats, e.g. Grebes, Loons, Alcids and Penguins, to name but a few. Many soaring hawks also use this strategy to avoid detection by prey. The idea is to present a color pattern that counters the normal shading caused by overhead lighting, i.e. the top surface being darker and the underside being lighter. This rather abnormal pattern of countershading gives these birds a more two-dimensional look, one that proves to be much harder to discern by the brains of predators, or even prey for that matter, which are more used to searching for three-dimensional images.

Camouflage has recently drawn some attention from ornithologists in yet another aspect of the world of birds --- their nests. It is well known that many bird species first choose to build their nests in locations that are hard to find and some birds like Tits and Hummingbirds even decorate the outer layers of their nests with naturally concealing materials like lichens and spider cocoons. While these efforts might seem to be obvious ploys to avoid the loss of eggs, young, and even the attending adults from a whole host of predators, including mammals, birds and reptiles, there is little direct evidence of the benefits of building a well-camouflaged nest. As an example, observations in the wild found that diamond firetail nests that were more heavily festooned with brightly colored flowers suffered from no more predation than those covered less liberally. In a more artificial experiment, wicker basket nests of Japanese Quail lined with leaves experienced less predation than nests lined with moss inside and out to match the surroundings.

This begs the question -- do nest-building birds actively use materials that render their nests harder to detect by predators or do they just use local materials that happen to match the nest’s surroundings simply by chance? In an aptly titled study, “Birds Build Camouflaged Nests”, appearing in the January 2015 issue of The Auk, Ida Bailey of the University of St. Andrews in the U.K. and four other colleagues sought to answer that very question by using captive Zebra Finches, an often used bird species for behavioral research.

Six to 10 days after housing 21 pairs of these finches in wooden aviaries, Bailey’s team presented the birds with open-topped nest cups covered with color paper on both the inside and outside. Using the same colored paper, they cleverly covered the two walls in that half of the cage where the nest cup hung. After giving the birds 24 hours to acclimatize to their surroundings, the researchers presented each pair of birds with 50 small strips of colored paper, i.e. pale yellow, pale pink, mint green, cream, and pale blue, with which the males could use to build their nests. The scientists deliberately chose colors that bore no resemblance to natural nest materials. Half of the strips matched the background color of their nest cup, while the other half were of a contrasting color. The paper strips were offered in bundles on the floor and their choices of nest materials were subsequently filmed by video cameras.

Twenty of the 21 male finches built nests with the colored paper. A statistical analysis showed that the birds significantly chose nest materials of the same color as the nest site’s background rather than a contrasting one. Pink was the most preferred color and yellow the least.

“Nest-building male zebra finches chose material that matched the background color of the nest site,” they wrote, “It appears, then, that birds can both choose the location of their nest in order to reduce predation risk and also choose nest materials that reduce the visual conspicuousness of the nest.”

The researchers do acknowledge that their assertion should be tested in wild birds, but they readily point out that their captive zebra finches are many generations removed from their wild counterparts and without the threat of predation, really have no need to actively adopt anti-predation strategies. So the tendency might even be stronger in wild birds. On the other hand, wild birds may instead resort to other tactics such as secretive nest site choices and defensive behaviors like mobbing to minimize predation. And there is also the matter of texture as opposed to just color in the choice of nest materials. Finally, background color matching has also been shown to be most effective when combined with the disruptive coloration strategy discussed earlier. Many of the finches chose a small proportion of paper strips that were not the same color as the cage walls, suggesting that weaving in small bits of non-camouflaged, contrasting material, can serve to break up the nest’s outline, making it appear less like a nest.

To my mind, this was a well-crafted study, simple yet elegant, and of course, raising new questions and likely sparking a whole host of new experiments by ornithologists all over the world.

Oh, as for that little Brown Creeper I mentioned at the beginning, it flew off alertly on strong wings about ten minutes later.

I just thought you might want to know.

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