Spacing out at Mono Lake: Breeding Success, Nest Density, and Predation in the Snowy Plover

--Snowy Plovers (Charadrius alexandrinus) nesting on the exposed lake bed surrounding Mono Lake, California lose up to 40% of their clutches. Most are destroyed by predators, predominantly California Gulls (Larus californicus). In 1978 Snowy Plover reproduction was estimated at 0.49-0.70 fledged young per female. Population stability was estimated to require 0.80 fledged young per female. A series of experiments with artificial clutches placed at different densities in the nesting area demonstrates that the predators can have an effect on the plovers' nesting success that is dependent upon their nest density. The maintenance of low nesting density is an important antipredator adaptation. We consider predation on clutches and broods to be the major limiting factor on the Snowy Plover population at Mono Lake. Received 18 January 1982, accepted 27 May 1982. MANY workers have hypothesized that shorebird mating systems, territorial behavior and nest-dispersion patterns evolving together have increased the species' ability to exploit food resources or thwart predators, thereby enhancing the chances of successful breeding (e.g. Holmes 1966, 1972; Oring and Knudson 1972; Graul 1973; Pitelka et al. 1974; Safriel 1975). The effect of food resources and predators on shorebird breeding success has largely gone unmeasured. In fact, few workers have measured breeding success, because shorebirds' precocial young are almost impossible to relocate after they leave the nest. Tinbergen et al. (1967) conducted the first experiments demonstrating that the dispersion of birds' nests may affect their survival. They found that the rate at which Carrion Crows (Corvus corone) took camouflaged, scattered chicken eggs increased with increasing egg density. Goransson et al. (1975) exposed camouflaged dwarf hen eggs to Carrion Crows and Herring Gulls (Larus argentatus) and obtained similar results, except when the eggs were in an area defended by Lapwings (VaneIIus vanellus), which drove the predators from the area. Gottfried's (1978) experiments with imitation landbird nests in old-field habitat showed no relationship between the rate of nest loss and nest density. He suspected that the differences 13 between his and the previously mentioned experiments may have been related to the types of predators. Snakes were responsible for most losses at his study site. The results of these and other experiments (Andersson and Wiklund 1978) indicate that a wide dispersion of nests may be the best strategy for birds unable to defend their nests against visually cued avian predators. We have measured breeding success of the Snowy Plover (Charadrius alexandrinus nivosus) at several localities in California as part of a larger study of the species' breeding status in the state. Noting a particularly wide spacing of nests and a high level of nest loss to predators at Mono Lake, we undertook experiments with artificial nests to determine the value this wide spacing might have in reducing the predators' effects on nesting success. This paper describes how we estimated breeding success, how the estimated success relates to the level of production needed for maintenance of population size, and how predators limit nesting success and possibly regulate the size of the breeding population. STUDY AREA AND METHODS Study area. --The study was conducted at Mono Lake, Mono County, California. This nearly elliptical, 15-km by 20-km, alkaline lake lies in a closed The Auk 100: 13-24. January 1983 14 P^GE ET ̂ L. [Auk, Vol. 100