The Economics of Winter Fighting in Silvereyes

Abstract

This paper investigates winter fighting by the Silvereye population of Heron Island in natural environments. Most fights were less than five seconds in duration, though some exceeded 20 seconds. Fights over food accounted for about two-thirds of all fights and were longer than fights not involving food, since the former consisted of more component interactions each of which lasted longer. Fights not involving food were mainly defence of the summer breeding area. The main cost of fighting involved time and energy, and the calculated net energy cost per second of time spent fighting varied considerably, depending on the fight duration and intensity. The energy value of some food items (especially bites of fig and large insects) was usually greater than the cost of fighting. Fruits were generally fought over more than insects, and for both fruit and insects the frequency of fighting increased in relation to the item's energy content. These patterns are consistent with theories which suggest that individuals should adjust their fighting so that the cost (in survival) is usually less than the potential benefit.