Determinants of Cliff Edge and Escape Responses in Herring Gull Chicks in Nature1)2)

Abstract

AbstractHerring gull chicks from nests on a) cliff ledges and b) vegetated plateaus showed different types of escape and edge response when tested on an elevated platform. The birds from cliff sites, though bolder at the platform edge, were more reluctant to jump. Those from the plateau nests leapt to the ground with relatively little hesitation. Chicks from cliff eggs, hatched and reared in the plateau by foster parents, resembled native plateau chicks in their escape responses while chicks of plateau origin reciprocally transplanted to and reared on cliff sites showed the reluctance typical of cliff chicks. The posssibility of two genetic types reproductively isolated on the two breeding situations is thus essentially eliminated. The likelihood of a stable genetic polymorphism is considered and tentatively discarded as less plausible than an explanation based on learning. Although conditions favorable for simple instrumental conditioning along divergent lines are present in the two rearing situations, a theory is favored which attributes the divergence to major differences in the amount and variety of experience afforded by the two environments. Modest support for this theory is provided by experiments in which plateau chicks confined by fencing to the immediate nest vicinity tended to resemble the naturally deprived cliff chicks in their reluctance to jump. In conclusion the concept of species-characteristic responses is considered inapplicable to the cliff edge and escape patterns of herring gull chicks except as it incorporates recognition of a determining role in factors of the rearing environment.