A study on parental behavior in American Black Oystercatchers (Haematopus bachmani) in British Columbia revealed sexual differences and seasonal variation in behavior. Before the egg-laying period, females tended to forage more, whereas males were more aggressive and alert. Females initially incubated more than males, but the sexes shared incubation duties almost equally toward the end of the incubation period. Males remained aggressive throughout incubation, but female aggression declined after eggs were laid. Females did most of the brooding, but the sexes participated equally in foraging for chicks and feeding them. After their eggs hatched, parents were more alert and aggressive toward other species. Patterns of parental care in oystercatchers appear to be related to their relatively large size, long-term mate fidelity, longevity, specialized feeding behavior, and sedentary life style, compared with other shorebirds.