Distribution of parental investment and sibling competition in the herring gull, Larus argentatus

Herring gulls, Larus argentatus, typically lay three eggs. The third laid egg is smaller, hatches later, and hatches a lighter, smaller chick than the first two. The third hatched chick also has a lower chance of survival. Observations of parent-chick and chick-chick interactions were conducted at two Ontario sites in the Great Lakes to determine whether and how parents might behaviorally bias investment in their chicks after the incubation period, and how interactions among chicks might add to the third chick disadvantage. Little evidence was found for behavioral discrimination against the third chick by either its parents or its siblings. The last hatched (C) chick was not less likely than its older siblings (A or B) to get to the food first, and while A and B were more likely to target C when initiating tugs-of-war over food, they were not more likely than C to initiate tugs-of-war in general. C pecked siblings the most, and, despite its younger age and smaller size, was not more likely to lose tugs-of-war with its siblings. It is likely that the locations where this study was conducted represent relatively benign environment in which to raise chicks compared to the marine populations observed in other studies. Given this, parents might be expected to reduce chick asymmetries if they are able. There was a high degree of hatching synchrony in Hamilton Harbor. Ln 7 out of 28 nests the C chick hatched on the same day as the B or A and B chicks.