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.

Numbers, deaths and growth rates were observed for three years in a delimited population of Acmaea digitalis whose members had been individually marked. The zone of vertical distribution of the species is determined by the behavior of these snails. In the fall and winter they ascend in the intertidal, and descend a smaller distance in the spring. This leads to a differential distribution of size classes. The oldest and largest animals are found highest in the intertidal zone. That the vertical movements are adaptive is suggested by death rates which are highest in the upper portions in summer, but rise in the lowest zone in winter. Probability of survival improves with age. Growth rate was remarkably consistent during the period of observation. It was fastest in the fall and winter months. During the month of July, and in part of June and August growth ceased, at least as measured by a change in shell length. Crowding decreased growth slightly but significantly. Growth rates in different areas suggest that time available for feeding may be a significant variable. Experimental manipulation of density indicates that emigration rates are density—dependent. The mechanism of density regulation is postulated as operating even at low overall densities because of the behavior pattern of the species.

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