Sexual Size Dimorphism and Assortative Mating in Razorbills (Alca torda)

Abstract

Razorbills (Alca torda) are relatively well-studied seabirds, but detailed morphometric data are lacking. Like other members of the Alcidae, the sexes are similar in size and indistinguishable in plumage, making it difficult to obtain measurements of live birds of known sex. The size of individuals has been reported from several museum collections of skins from diverse populations (Cramp 1985), samples of which should not be combined because of clinal variation (Bedard 1985). Hope Jones et al. (1985) measured a large number of corpses that were retrieved along the British coast but only reported bill and wing lengths of specimens of undetermined sex. Lloyd (1976) reported two body-size variables of live birds from a single population of Razorbills in Wales, but the sample sizes were small, and the sex of most individuals was unknown. My primary aim here is to report the first large data set of live, known-sex Razorbills and to identify the degree of sexual size dimorphism in a number of body measurements. This study is part of a wider project on mating behavior of Razorbills based on color-banded breeding pairs (Wagner 1992). Morphometric data of pairs can provide the answer to a second question, namely whether Razorbills mate assortatively by some aspect of body size. Before I began measuring Razorbills, I was especially interested in bill depth because it coincides with one of the striking physical features of the species, namely its mainly black bill, which is indented by three or four grooves, the largest of which is white-colored. The decorative appearance of this marking suggests that the white coloration of the groove is a sexually selected ornament. In sexually monomorphic species, Darwin (1871) suggested that ornaments are produced by mate choice by both sexes, as has been illustrated by another alcid, the Crested Auklet (Aethia cristatella), in which both males and females are attracted to each other's identical ornaments (Jones and Hunter 1993). Methods.-I measured Razorbills in the Basin study colony on Skomer Island, Wales (51'40'N, 05?15'W), in 1987 and 1988. To avoid disturbance during the prelaying period, almost all breeders were trapped late in incubation or during the nest-