Responses of Brooding Nighthawks to a Disturbance Stimulus

A great number and variety of birds, when brooding, respond to an intruder at or near the nest by engaging in behaviors tending to make the bird conspicuous to the intruder and to divert the intruder from eggs or young (after Simmons, 1955)."Distraction display" is tke term most commonly used to denote this type of response, although other names, such as "diversionary display" (Armstrong, 1949), have also been employed."Diversionary behavior" is used in this paper because the term is most descriptive of the apparent function of the responses described and because it is doubtful whether some of these responses are displays in the sense of Tinbergen (1952: 28-30).Diversionary behavior has long intrigued students of birds.Armstrong (1947, 1949, 1954), Simmons (1952), and Skutch (1955) have surveyed and discussed muck of the literature in this area.Useful as some of this literature is, most of it is of an anecdotal nature, based upon scattered and casual observations, often lacking precision and objectivity.Simmons (1955) has stressed the need for systematic, objective investigations of diversionary behavior.The studies of Williamson (1949), Simmons (1951), Armstrong (1952), Brown (1962), and Stephen (1963) represent the type of extended investigation and careful analysis which is needed.The aims of the present study were to describe in detail the spectrum of responses seen in the diversionary behavior of the Common Nighthawk (Chordeiles minor) and to describe and measure the variation in response, througk the nesting cycle, of these birds responding to a standardized stimulus.Such data may allow clearer insights into the functional aspects of diversionary behavior.The Common Nighthawk was chosen because it exhibits well developed diversionary behavior and because it often nests on flat, gravel roofs, permitting unobstructed observation in a relatively constant environmental situation.All observations were made in Madison, Wisconsin, in the nesting seasons of 1963 and 1964.PROCEDURES In all, 11 nests were studied, 5 of them from 12 June to 14 July 1963, and 6 from 22 May to 17 July 1964.(The term "nest" is used in this paper to denote the site of egg[s] or young; the nighthawk does not construct a nest but lays its egg[s] directly upon the substrate.)The nests were studied from the time they were discovered until they were abandoned or the young fledged, thus some were 72The Auk, 84:72 86.