By Shani Friedman
When a predator approaches most nesting species, the parent bird usually makes no attempt to cover the nest or young. Instead, it makes itself as prominent as possible. The intruder’s attention is thus drawn away from the nest and toward the adult bird’s exaggerated motions. These actions occasionally may be mildly aggressive, as in the case of the surfbird, a shorebird that “explodes” into the face of either man or animal that steps too close to its nest. More often, the threatened bird pretends to be injured, sick, or exhausted in a distraction display. This kind of behavior has been noted in many groups of birds from sparrows to ostriches.
A great number of species from many families of birds use a pretense of injury to protect their nests. It is most elaborate in birds that nest in the open on the ground, where the eggs, nest and young are more vulnerable than those of tree-nesting species. The principal movements consist of beating, fanning, or dragging one or both wings, spreading and depressing the tail, fluffing the back and rump feathers, giving distress calls, and moving alternately parallel to and away from the predator. If followed, the bird keeps out of reach and increases the distance between itself and the predator as it gets farther and farther from the nest. When it is a considerable distance from the nest, it flies off in a perfectly normal manner.
To some extent, the habitat determines which type of movement will be used. Most shorebirds, water fowl, and other ground nesting birds use a broken wing display. Species that nest in trees or marshes use some of the movements and adapt them accordingly. For example, most warblers feign illness rather than injury in their distraction displays.
Species that nest on the ground in thick grassy cover characteristically mimic a small running mammal. The parent bird sneaks off the nest, and runs quickly away from the predator, occasionally hopping and fluttering its wings as it disappears from sight. In its “rodent-running” display, the Purple Sandpiper, an arctic tundra nesting shorebird, drags its wings to create the illusion of a second pair of legs, holds its feathers erect to provide a resemblance to fur, and squeaks like a mouse while it dodges between imaginary barriers. While this behavior is sure to lure away a fox or weasel, there is a flaw in it when a human being is involved because if the sandpiper is not pursued, it will rush back, whistle loudly for attention and repeat the performance a second time.
How these antics developed remains a controversial subject among animal behaviorists. Leading a predator away from the nest would undoubtedly be favored by natural selection. Some believe that distraction displays result from conflicting desires of the parent bird to flee from danger, to return to the nest to continue incubation or to brood young, and to approach the predator aggressively. Others postulate that the displays evolved as a direct predator defense, with more ritualized behaviors occurring in species that undergo heavy predation. Either way, an element of fear is involved. Tame or semi-tame birds cannot be induced to give a distraction display. They have become habituated to man, and/or repetitive non-lethal invasion by predators.