Eggshell removal by the laughing gull ( Larus atricilla): Normative data and visual preference behaviour

Abstract

After the eggs hatch, about 63% of the large eggshell fragments can be found near laughing gulls' nests. For the shells that were found, the mean distance from the rim of the nest was less than 80 cm at three study sites. The removal distance was positively correlated with the size of the nest platform, which suggests that birds often walk to the edge of the platform and simply drop the shell. Within the colony, eggshells have a natural variation in darkness that is related to the range of darkness of the nest material. In preference tests we have matched shell models to the darkness of the eggs in the nest. When both natural shells and orange shells pegged to the ground were presented simultaneously, gulls preferentially attempted to remove the orange shell. The strength of the preference for the pegged orange shell varied from site to site within the colony, so the results were validated in a separate experiment by placing shells in the nest bowl one at a time to be removed in as natural a manner as possible. Orange was stiff preferred; fear of the orange model seemed to promote the removal response. Contrary to the expectations from prior studies, the natural model was also no stronger stimulus than a solid white model in pegged shell tests. Control tests with white, grey, and black models indicate that brightness plays a very weak role, so the removal preference must be based on colour. When pegged models of any colour were on the nest and the bird engaged in nest building, the nest building was usually biased toward the models, but not precisely directed at them. Our results suggest that removal behaviour can be elicited via different perceptual mechanisms. One mechanism specifically recognizes the egg-shell, while the other mechanism is stimulated by a broad range of foreign objects that contrast with the nest background. In the laughing gull, the contrast mechanism appears to be at least as strong as the recognition mechanism. This may be related to the low level of predation experienced by these gulls.