THE POSSIBLE FUNCTION OF STONE RAMPARTS AT THE NEST ENTRANCE OF THE BLACKSTART

Blackstarts, Cercomela melanura, Turdidae, construct a rampart of ston es at the entrance to their nests. These ramparts may reach remarkable proportions, containing hundreds of flat rocks. We investigated sever al hypotheses regarding the function of stone ramparts, by monitoring individually marked blackstarts at the En-Gedi Nature Reserve, Israel. Stones were collected solely by females, who carried them in their be aks, while flying to the nest, at a rate of up to one stone per min, a fter pair formation had occurred. The number and total weight of stone s as well as rampart height showed a highly positive correlation with the size of the nest cavity opening. The rampart decreased the size of the cavity entrance to some nests by as much as 67%, which suggests a n antipredator barrier function. Survival rates of eggs and chicks wer e extremely low and the major cause of reproductive failure was predat ion. Successful nests tended to be located higher off the ground than predated nests, and often contained fewer stones. Furthermore, larger females in terms of wing and tail length nested in cavities higher off the ground: and built smaller ramparts containing lighter stones. An artificial nest predation experiment did not reveal a difference in pr edation rates between nests with and without stone ramparts. Spiny mic e, Acomys sp., were the main egg predators. However, in 37% of nests w ith ramparts that were predated, the perpetrator flattened the rampart , suggesting that they may serve as a barricade, forcing the predator to invest time in clearing the stones in order to gain access to the n est, and perhaps allowing the nesting female sufficient time to escape . We propose, therefore, that owing to such high nest predation rates, females nesting close to the ground build stone ramparts as an 'early warning' mechanism to prevent themselves from being trapped inside ne st cavities and predated. (C) 1998 The Association for the Study of An imal Behaviour.