Territory size and polygyny in the spotless starling: resource-holding potential or social inertia?

Abstract

In male birds that base their acquisition of mates on the defence of resources essential for female reproduction, the size or quality of the territory decisively affects polygyny rate and breeding success. However, there is contradictory evidence concerning the importance of resource-holding potential (RHP) versus factors not related to individual quality on between-year consistency in territory occupancy and hence on long-term reproductive success. We manipulated the aggressiveness (a major determinant of RHP) of male spotless starlings (Sturnus unicolor) by means of androgens and antiandrogens to investigate the importance of RHP in breeding-territory acquisition and maintenance, polygyny rate, and breeding success over 3 years (1996–1998). The number of males that held a territory for the first time was high in 1996 with respect to the year before the experiment began. Most of these males still bred in the colony during the third study year, though new owners established territories to the detriment of the former males, especially in 1998. Manipulation of testosterone activity during the first year affected the number of nest boxes defended by males and also the number of mates gained and the number of successful breeding attempts made throughout the 3 study years. The results were in the expected direction: an increase in testosterone activity led to greater success. The results suggest that RHP may be important in the acquisition of territory and mates, at least under conditions of recently established interindividual relationships. However, social inertia, i.e., the maintenance of dominance status irrespective of changes in RHP, seems to be of crucial importance in the long-term maintenance of territory resources and polygyny rates.