COMMON MURRES PROSPECTING WITHIN CORMORANT COLONIES IN DEL NORTE AND HUMBOLDT COUNTIES, CALIFORNIA, 1996 to 2004

Abstract

Between the 1940s and 1960s, common murres (Uria aalge) colonized or recolonized several islands in Del Norte and Humboldt Counties, northern California, during an extended period of population growth following heavy prior impacts from early settlers and native peoples (Carter and others 2001). The exact timing of these colony formations and various factors influencing them were not well documented. Since the 1st complete northern California seabird colony survey in 1979–80 (Sowls and others 1980), almost annual June aerial photographic surveys have been conducted of breeding colonies of murres and cormorants (Brandt’s and double-crested, Phalacrocorax penicillatus and P. auritus) in northern California (Briggs and others 1983; Takekawa and others 1990; Carter and others 1992, 1995, 1996, 2001; Capitolo and others 2004a; US Fish and Wildlife Service [USFWS], unpubl. data). While several murre colony formations have been documented in Mendocino County primarily with aerial photographs since 1979–80, no colony formations have been documented in Del Norte and Humboldt Counties. In 1996–2004, we documented (1) the 1st known attendance by murres on 4 offshore rocks in Del Norte County (High Bluff South, White Rock, Prince Island, Unnamed Small Rocks); (2) continued sporadic attendance by murres without recent confirmed breeding on 2 other offshore rocks in Del Norte County (Rock R and Sister Rocks); and (3) the 1st recent attendance by murres on 1 offshore rock in Humboldt County (Sea Lion Rock). In this note, we describe these observations of prospecting murres, show relationships with nesting cormorants, and discuss their potential importance to understanding possible future colony formations. We use the term ‘‘prospecting’’ to describe murres attending potential breeding habitats on land where murre breeding has not been recently documented, though we usually did not know what activities occurred on land nor how long murres were present on land. Because murres do not attend such habitats without a breedingrelated purpose, we consider that prospecting for a breeding site or mate are the chief reasons for such attendance (see later discussion). However, without marked birds or access to colonies, source colonies for these prospecting murres cannot be specifically determined.