COMPETITIVE DISPLACEMENT? AN EXPERIMENTAL ASSESSMENT OF NEST SITE PREFERENCES OF CLIFF-NESTING GULLS

Abstract

Red-legged Kittiwakes (Rissa brevirostris) are colonially breeding seabirds that are endemic to the Bering Sea, where they are sympatric with the more widely distributed Black-legged Kittiwake (R. tridactyla). Moreover, these two species breed interspersed on cliffs and potentially compete for nest sites on St. George Island, Alaska, which is the largest of only four known breeding locations of the Red-legged Kittiwake. On St. George Island, nesting ledges used by Red-legged Kittiwakes are narrower than those of Black-legged Kittiwakes, are more often located beneath overhangs, and are concentrated on high-elevation cliffs. To determine whether the nesting distribution of Red-legged Kittiwakes on St. George Island is shaped by competitive displacement by larger bodied Black-legged Kittiwakes, I compared realized niches of the two species (determined by measuring physical characteristics of natural nest sites) with fundamental niches of the two species (determined by observing nest site preferences of kittiwakes on artificial nesting ledges that differed in width, overhang cover, and elevation). The displacement hypothesis predicted that interspecific overlap in physical characteristics of nest sites would be greater for artificial than for natural nesting ledges (fundamental niches would overlap to a greater degree than realized niches). In contrast, nest site preferences of kittiwakes on artificial ledges differed markedly between species and mirrored the characteristics of natural nest sites: Red-legged Kittiwakes preferred narrower ledges, Black-legged Kittiwakes preferred wider ledges, and both species preferred ledges located within high-density clusters of conspecifics. Thus, I rejected the competitive displacement hypothesis and concluded that interspecific competition does not currently shape the breeding distributions of kittiwakes on St. George Island. Although interspecific competition in the past may have shaped current nest site preferences of kittiwakes, the concordance of habitat preferences with interspecific differences in body size, clutch size, and nest size and structure indicate that Red-legged Kittiwakes are uniquely adapted to exploit small ledges.