Feeding Rates and Prey Selection of Oystercatchers in the Pearl Islands of Panama

Abstract

The feeding pattern of the American oystercatcher (Haematopus palliatus) was examined in the Pearl Archipelago, on the Pacific coast of Panama. Twenty species of mobile molluscs from intertidal rock benches formed more than 97 percent of the diet, but most species were underrepresented as prey relative to their abundance in nature. At middle to high shore levels, oystercatchers avoided three abundant but small-bodied snails, and fed primarily upon larger size classes of two other, large-bodied herbivorous molluscs. Pairs of birds averaged more than 80 prey items per low tide. This predation rate significantly affected the local abundance and size structure of the two preferred species. Lower on the shore foraging was limited, and relatively few prey of any species were taken. Prey utilization by oystercatchers resulted from the behavior of prey species, their position on the shore, and active choice by birds. Local mortality of gastropods due to predators such as oystercatchers may contribute to observed, intersite variability in the distribution and abundance of intertidal organisms in Panama. RARE, PATCHILY DISTRIBUTED, OR INTERMITTENTLY PRESENT PREDATORS play important roles in a number of habitats, including coral reefs (Glynn 1976), subtidal kelp beds (Duggins 1980), tropical forests (Gradwohl & Greenberg 1982), and tropical rocky intertidal shores (Garrity & Levings 1981). Such predators may be difficult to examine (Edwards et al. 1982); consequently, their effects can be considerable, yet easily overlooked (Holmes et al. 1979). One such group, shorebirds, has been examined in a variety of intertidal habitats (Schneider 1978, Pitelka 1979, Frank 1982, Goss-Custard et al. 1982). They have been linked to changes in the distribution, abundance, and size structure of prey populations (O'Connor & Brown 1977; Schneider 1978; Frank 1982; Sutherland 1982a, b; Marsh 1983; Quammen 1984). Oystercatchers (Haematopus spp.) and other shorebirds that forage on rocky shores have been studied thoroughly in temperate locales (Feare 1971; Heppleston 1971a, b; Hartwick 1974, 1976, 1978, 1981; O'Connor & Brown 1977; Hartwick & Blaylock 1979; Frank 1982; Goss-Custard et al. 1982; Hulscher 1982; Sutherland 1982a, b), but not at lower latitudes. We have recently studied the feeding activity of the American oystercatcher, Haematopus palliatus (Temminck), along rocky intertidal shores of the Pacific coast of the Republic of Panama. The rocky shores of this region have been described (Lubchenco et azl. 1984 and included references). Tides are semidiurnal with a range of approximately 6 m. The beds of sessile bivalves or of erect algae typical of most temperate areas are absent here (Lewis 1964, Dayton 1971, Paine 1971, Stephenson & Stephenson 1972), but there is a diverse and abundant assemblage of snails, limpets, pulmonates, and chitons (Lubchenco et al. 1984). The distribution of oystercatchers along Panama's Pacific shore was estimated in the course of other studies (1976-1983). Their presence is easily noted since these birds are noisy and conspicuous in flight, nest or roost in open areas just above the high tide line, and forage on rocky intertidal shores. Our observations were reinforced by reports from other scientists and birdwatchers, and covered virtually the entire coast. Birds have been reported in Los Santos (Ridgely 1976), and occasional sightings have been made elsewhere in Panama (P. W. Glynn & D. C. Schneider, pers. comm.), but oystercatchers appear commonly only in the Pearl Archipelago (ca 825 'N, 79?W). We roughly estimate a resident population of 250-350 birds, based on two surveys of islands within the archipelago (October 1982, April 1983). This study examines feeding rates and foraging behavior of oystercatchers within the Pearl Archipelago, describes species of prey, and suggests some potential effects on intertidal molluscs and upon the intertidal community.