by Greg Mayer
Ground nesting birds are more vulnerable to predation of both themselves and their eggs because the ground is accessible to a larger variety of predators than are nests built in trees. There are a number of ways of dealing with this. One is for the bird, its eggs, or both, to have concealing coloration. This is very common, and such cases constitute a large class of examples in the classic work establishing the principles of adaptive coloration.
I saw this myself recently during a stop in New Madrid, Missouri, where I heard a bird yelling in my ear. But it took some time to find the bird.
Eventually I did spot it (I had binoculars), sitting on the ground. A second killdeer was running about on the grass not far away.
As I approached, it did not attempt to lead me away in a distraction display (which killdeer will do), but once I was close enough it stood up and displayed its more strikingly marked tail feathers, although not as vigorously as did one photographed by a WEIT reader earlier this summer.
According to the Cornell Laboratory of Ornithology, which of the two possible displays is used—distraction (which leads the interloper away from the nest), or tail (which alerts the interloper to the location of the nest)—depends on the nature of the interloper. If perceived as a predator, the distraction display is used to lead the predator away; but if perceived as a blundering ungulate (bison in the old days), the tail display is used to make an annoying spot on the ground that the ungulate will walk around (rather than on top of). So, she perceived me as a lumbering, dumb, brute, rather than an egg predator; clever girl!
There were two eggs, both camouflaged with dapples and spots, and no apparent nesting materials, but I didn’t want to bother her enough to move her off the nest to get pictures of the eggs.
Another common way of dealing with the problems of a ground nest is to use a less accessible piece of ground, such as an island or a cliff, as the nesting site. Seabirds frequently do one or both of these. In my part of Wisconsin, Canada geese have become cliff nesters over the past twenty years, building their nests on ledges and roofs of buildings, a behavioral change that has resulted in a huge increase in nesting success and nest abundance. It would be interesting to determine how much of this new nesting behavior is an evolved adaptation or part of a learned repertoire.
Cott, H.B. 1940. Adaptive Coloration in Animals. Methuen, London.