A Comparsion of Nesting‐Ledges Used by Seabirds on St. George Island

Abstract

Measurements of 288 nesting—ledges of six species of seabirds on St. George Island, Alaska, revealed significant interspecific differences in the size, shape, and overhang of ledges used. Typically, Red—legged Kittiwakes used ledges 1 dm deep; Northern Fulmars, Black—legged Kittiwakes, and Thick—billed Murres used ledges of an intermediate 2—4 dm depth; Red—faced Cormorants used ledges 3—6 dm deep; and Common Murres occurred singly on shallow ledges or in groups on deeper ledges. Fulmars and murres, which did not build nests, used nearly horizontal ledges exclusively; the nest—building species occurred on a wider range of slopes. Only the Red—legged Kittiwake regularly used ledges with over 50% cover by overhanging cliff within 5 dm of the ledge. Classification of the measured ledges of the cormorant, the two kittiwakes, and the Thick—billed Murre by a discriminant function analysis revealed significantly greater overlap than expected between the species pairs of cormorant with Thick—billed Murre, Black—legged with Red—legged Kittiwake, and Black—legged Kittiwake with Thick—billed Murre. Observations of 43 interspecific exchanges of nest sites during and between three breeding seasons agreed with the predictions by the discriminant analysis of overlap or lack thereof in three of four significant cases. Exchanges occurred significantly more frequently than expected between the species pairs of cormorant with Black—legged Kittiwake, cormorant with Thick—billed Murre, and Black—legged with Red—legged Kittiwake.