Effects of Winter Feeding on Limpets by Black Oystercatchers, Haematopus Bachmani

Black Oystercatchers fed on limpets in an intertidal boulder field at a rate of 2 limpets°m — 2 °yr — 1 . The birds exert a major influence on the microdistribution of these molluscs. Where accessible to oystercatcher predation, limpets are unlikely to survive over a winter once they reach moderate size (10—14 mm). Collisella pelta are exceptional in having refuge in large size. Oystercatchers thus produce and reinforce patchy limpet distributions. Adaptations of the limpets include crypsis and habitat selection of setting larvae. Spatial heterogeneity on local, regional, and broader geographic scales complicates these effects, especially since settlement behavior is constrained from making distinctions between all levels of heterogeneity, e.g., between cliffs where limpets will be relatively safe and boulders where they are at high risk. Locally the effects of bird predation are most notable for Notoacmea persona, but N. scutum, C. digitalis, and C. pelta are also consumed in significant numbers. The resulting microdistribution of limpets affects algal cover, which tends to be effectively removed by their grazing on steep and vertical walls, but which can maintain itself on large horizontal and gently sloping surfaces. Feeding in this locale was confined to the birds' nonbreeding season, fall and winter. By its end the limpets were reduced in number and the birds foraged elsewhere and on other prey. Limpet recruitment may be insufficient to allow year—round feeding in such areas by these birds, whose foraging may represent an adaptive seasonal adjustment to local conditions.