THE PARENTAL STRATAGEMS OF BIRDS

Abstract

Summary Sonic birds steal unobtrusively from the nest while a man is a long way off, others wait until he is almost within arm's length. The same individual may abruptly shift from one of these procedures to the other; or one member of a pair may follow the first method and the other the second. Both of these modes of departure arc strategically sound; but any intermediate course needlessly exposes the nest to detection. If a bird delays with its eggs or young until an enemy has come very close it has one last resource: it may try to lure the intruder away by a distraction display, or “feigning injury”. Typical injury‐simulation is merely a highly developed form of conspicuous departure from the nest, of which many examples are given. Such departure, which does not suggest disablement, may be either over the ground or by a peculiar slow flight. Injury‐simulation may begin after an unsuccessful attempt to defend the nest. Birds are careful to select unobstructed ground for this display, which may be interrupted while they pass over tangled vegetation and resumed in a clear area on the farther side. Endless variations in the mode of giving distraction displays suggest great discrimination and the intelligent adaptation of an essentially innate pattern to fluctuating circumstances. Injury‐simulation is on the whole confined to species whqse nests are so placed that it is likely to be effective, chiefly to those whose open nests are on or near the ground. It is rare in birds whose eggs are placed high in trees, in holes, in elaborate closed nests, or in crowded colonies. There are many records of the effectiveness of distraction displays with men and other mammals. But a Sclater's Antbird Myrmeciza exsul tried unsuccessfully to lure an Agouti Dasyprocta from its nest. Of other parental stratagems, “freezing” in an upright posture on the nest's rim has been observed only in the Cedar Waxwing Bombycilla cedrorum. Nest‐building to distract attention from a near‐by occupied nest, and stuffing with fresh material the door of an occupied nest, have been reported for certain wrens. Eggs are seldom covered over during the parents' absence, although the expedient would seem to afford some protection. As fledglings fly weakly from the nest, the adults of some species fly close above them in “shielding flight”, which may give a measure of security from aerial predators. Distraction displays appear to be acquired with relative ease by species whose mode of nesting makes them effective, and are perhaps almost as easily lost when changed nesting habits render them useless. Hence it is necessary to postulate an innate foundation, widespread in birds, upon which these displays can be built with readily acquired genetic modifications. It is suggested that this foundation is the bird's intelligence or ability to learn by experience the effectiveness of this ruse. In a race of birds in which individually acquired distraction displays are somewhat frequent, genetic mutations tending to reinforce or elaborate such behaviour, at the same time making it heritable, would at once have “survival value”, which otherwise they might lack. It is probable, then, that distraction displays have often been built up by “organic selection”, or the gradual replacement of individually acquired traits by genetically transmitted characters.