BIOLOGY AND CONSERVATION OF THE COMMON MURRE 35 



Small numbers of murres breed in British Columbia and there is no evidence to suggest they are more 

 numerous than in 1900. About 8,300 breeding birds (i.e., less than 1% of the U. a. californica population) 

 were estimated at two active colonies in 1 989, although five other small colonies had been active in the 1 970s. 

 The northernmost colony of U. a. californica is at the Kerouard Islands at the southern tip of the Queen 

 Charlotte Islands. In British Columbia, the vast majority of murres breed at the large colony at Triangle Island 

 off the northern tip of Vancouver Island. Population trends at Triangle Island have not been well assessed. 



Key words: Alcidae, breeding colony, breeding distribution, British Columbia. California, colony disturbance, 

 colony extirpation, colony formation, common murre, egging. El Nino, gill net, habitat change, oil spill. 

 Oregon, population size, population trends, predators, seabird, Uria aalge, Washington 



Information on populations of the common murre 

 (Uria aalge californica) in California, Oregon, 

 Washington, and British Columbia are of two types, 

 whole-colony counts of birds, which can be adjusted to 

 derive estimates of the number of breeding adults at 

 each colony, and transect counts of birds at sea, which 

 describe at-sea densities. These two types of population 

 data serve as primary baseline information for monitoring 

 and assessing trends in populations of murres in various 

 geographic areas in western North America (Sowls et al. 

 1980; Briggs et al. 1987, 1992; Speich and Wahl 1989; 

 Takekawa et al. 1990; Rodway 1991; Wilson 1991; 

 Carter et al. 1992, 1995; Byrd et al. 1993; Tyler et al. 

 1993). 



Standardized whole-colony counts include a large 

 proportion of breeding birds (i.e., each egg or chick had 

 one or two attending adults) and some nonbreeding 

 individuals attending the colony. Thus, whole-colony 

 counts of all colonies in a geographic area constitute a 

 primary population index wherein most of the 

 population is counted directly rather than sampled. This 

 kind of population index increases our ability to measure 

 trends by greatly reducing potential variation or bias 

 from sampling. At each colony, this index is related 

 directly to the number of breeding adults or the total 

 number of murres (breeding and nonbreeding) attending 

 the colony, but the exact relation has not been 

 determined. Estimates of the number of breeding birds 

 at a colony can be derived from whole-colony counts 

 with the use of a correction factor k (see Appendix A). 

 Similarly, estimates of the numbers of nonbreeding birds 

 can be derived through population modeling. However, 

 k correction factors and demographic variables used in 

 population models have been determined in only a few 

 studies at certain locations and may not apply widely. 



We considered trends in sums of standardized 

 whole-colony counts from aerial photographs for all or 

 many colonies in a geographic area to best reflect trends 

 in murre populations over time. Within the range of U. 

 a. californica, whole-colony counts can be conducted 

 at all colonies, which reduces the potential for sampling 



error (e.g., if one colony were selected for monitoring in 

 an area). Source colonies of birds also are known for 

 colony counts but must be interpreted using various 

 sources of information for at-sea counts. In general, 

 standardized whole-colony counts are less variable, 

 more repeatable, and subject to fewer biases than at-sea 

 counts. However, numbers of murres attending colonies 

 during the breeding season are subject to variation 

 because of several factors, especially time of season, 

 time of day, and colony disruption by human 

 disturbance or interactions with other seabirds or marine 

 mammals. 



Transect counts of birds at sea also can be 

 extrapolated over large areas to derive estimates of total 

 population (i.e., adults and subadults) in a defined 

 geographic area, but must account for murres attending 

 colonies. At-sea counts and total-population estimates 

 provide important data on the density, distribution, 

 abundance, and movements of murres at sea, which are 

 important in connection with various conservation 

 issues. Significantly more baseline population 

 information useful for monitoring purposes is available 

 for colonies than for at-sea murre distribution and 

 abundance. 



Since 1979, monitoring of murres in California, 

 Oregon, and Washington has focused primarily on 

 standardized whole-colony counts from aerial 

 photographs of birds attending colonies during the 

 breeding season. The monitoring is so focused because 

 (1) most colonies are comparatively small (fewer than 

 20,000 breeding birds) and are present on small islands 

 with open habitats that can be aerially photographed 

 on a regular basis, (2) intensive monitoring of plots 

 within most colonies is impractical because most 

 colonies are inaccessible or cannot be accessed without 

 extensive disturbance to breeding birds, (3) potential 

 biases are associated with monitoring plots from the 

 ground (i.e., plot selections, number of plots, variation 

 between plots, and counting error [Harris et al. 1986; 

 Mudge 1988; Harris 1989]), and (4) monitoring plots 

 can involve high cost and effort. The U.S. Fish and 



