EFFECTS OF INTERFERENCE COMPETITION ON THE FORAGING ACTIVITY OF TROPICAL ROSEATE TERNS

Although difficult to detect, interference competition among foraging birds may affect the fitness of an inferior competitor by lowering foraging success. We studied foraging flocks of Roseate Terns (Sterna dougallii) at Culebra, Puerto Rico, in 1990 to examine the effects of small and large monospecific and mixed-species flocks on the foraging success and behavior of this threatened species. Roseate Terns mostly foraged in small flocks with Brown Noddies (Anous stolidus). The frequency of foraging attempts in Roseate Terns decreased with increasing size of mixed-species flocks, primarily due to passive interference by Brown Noddies, which foraged just over the water surface and limited the area available to Roseate Terns. The size of monospecific Roseate Tem flocks had no effect on foraging attempt frequency. Roseate Terns also aborted more feeding dives and foraged more by dipping when in flocks with noddies, relative to flocks of only conspecifics. Proportionally fewer Roseate Terns than expected foraged in large, mixed-species flocks (>8 birds), suggesting that they preferentially avoid large flocks that result in lower foraging success. Although Brown Noddies caused a reduction in Roseate Tem foraging attempt frequencies and an increase in aborted dives, we were unable to detect a negative effect on adult survival or reproductive success of this population.