Black skimmer parental defence against chick predation by gulls

Predation by laughing gulls, Larus atricilla, on black skimmer, Rynchops niger, chicks was observed in a Texas nesting colony. The patterns of predation differed from random in two respects: (1) significantly more attacks (both attempts and successes) were directed toward nests with two parents present than toward nests with only one parent and (2) significantly more attempts occurred when female parents were brooding than males. Differences in gull attack rates according to sex of the parent skimmer may have been due to the considerable sexual size dimorphism in skimmers. Male skimmers are larger than laughing gulls, while female skimmers are smaller. Predation on skimmer eggs was uncommon. Solo incubating by females was significantly less frequent after the first egg's hatching than before. The decreased frequency of female solo attendance after the first hatching may represent an adaptive response to predator pressure at a time when chicks are most vulnerable to predation by gape-limited predators such as laughing gulls and would be better protected by males or both parents. The higher observed frequencies of both predation attempts and successes directed toward nests with bi-parental attendance was not predicted and may be attributable to certain unavoidable high risk activities, such as brooding exchanges and chick feeding when broods are young, which typically occur only when both parents are present.