EXPERIMENTAL INVESTIGATIONS OF MASS-DEPENDENT PREDATION RISK IN THE EUROPEAN STARLING, STURNUS-VULGARIS

The influence of mass on escape ability in the European starling was investigated with reference to aerial manoeuvrability and take-off ability. Experimental groups of higher body mass were less manoeuvrable. In the take-off experiments, manipulations of mass influenced predominantly trajectory rather than velocity or distance covered in a given time. The take-off strategies observed may represent a trade-off between maximizing rate of ascent and maximizing escape velocity. Under increased load, the birds appeared to 'defend' escape velocity at the expense of rate of ascent. It is argued that if these changes in escape performance are important determinants of predation risk then changes in body mass, or the effects of such changes, should be incorporated into decisions that might result in exposure to predation. Consistent with this proposition, manipulations of body mass influenced the decision to leave protective cover; alternative interpretations of this result are also considered. In a separate procedure, the hypothesis that perceived predation risk influences the amount of stored fat was investigated by varying the availability of protective cover. Where most cover was available, fat reserves, scored from visible subcutaneous fat deposits, were highest. There was no relationship, however, between wing loading (body weight/wing area) and protective cover, suggesting that other components of body mass may change in association with fat reserves. The relevance of these results for regulation of body mass and mass-allocation strategies is discussed. © 1993 The Association for the Study of Animal Behaviour.