68 USGS/BRD/ITR-2000-0012 



the availability of open breeding habitats on islands in 

 British Columbia are reduced. Washington and southern 

 British Columbia also are located at the northern end of 

 the California Current upwelling system where it meets 

 the Alaska Current during spring and summer (Morgan 

 etal. 1 991; Tyler etal. 1993;Wahletal. 1993). Different 

 prey resources are associated with the estuarine 

 conditions within the extensive fiord system along the 

 coasts of British Columbia and southeastern Alaska. 

 Also, large populations of native people were present 

 for at least thousands of years in British Columbia prior 

 to the mid-nineteenth century (Duff 1997), and probably 

 visited many seabird nesting islands to obtain eggs and 

 birds for food. Certain murre colonies may have been 

 extirpated prior to 1 800. However, reduced availability 

 of breeding habitats and change in prey resources may 

 be the primary factors contributing to the current 

 geographic gap between murres breeding in Alaska (U. 

 a. inornata) and from southern British Columbia to 

 California (U. a. californicd). 



During glacial periods in the last 1 million years 

 (i.e., the last period ended about 10,000 years ago), 

 continental ice sheets extended to sea level from 

 northern Washington through much of southern Alaska, 

 which coincides to a large degree with the current 

 geographic gap and is related to changes in coastal 

 topography. Glacial history probably is a major factor 

 underlying the current gap in distribution, and also may 

 have strongly influenced the current location of the 



major portion of the population of U. a. californica in 

 Oregon and California. However, the fossil history of 

 Uria extends back at least 5 million years in southern 

 California (Barnes etal. 1981; Howard 1949, 1981, 1982; 

 see Bedard 1985). Various changes in seabird 

 communities, marine environments, and coastal 

 topography have occurred in the North Pacific over 

 millions of years and influenced the distribution and 

 abundance of the common murre. 



Recent and Historical Population Trends 



The numbers and distribution of common murres 

 in the Oregon and northern California "core" population 

 between 1979 and 1995 seem to represent relatively 

 stable high levels, possibly indicative of near carrying- 

 capacity levels and distribution (Figure 2.16). In this 

 area, murre numbers have stabilized for several decades, 

 apparently in relation to available breeding habitat, prey 

 resources, and relatively low levels of human 

 disturbance at colonies. Most suitable breeding habitat 

 is occupied, although some habitat has been removed 

 historically by either connecting islands to the mainland 

 with breakwaters or modifying islands for lighthouses 

 or other structures. The abundance and availability of 

 prey resources have not been well studied, but evidently 

 have been adequate to maintain populations at current 

 high levels. Few natural factors are known that would 

 disrupt this stability. Lower numbers of breeding birds 



Washington 



Oregon 



30 



Figure 2.16. Changes in whole- 

 colony counts of common 

 murres at breeding colonies in 

 central California (all colonies), 

 northern California (excluding 

 the Castle Rock colony), Oregon 

 (15 sample colonies), and 

 Washington (excluding Tatoosh 

 colonies), 1979-1995. 



13 



e 

 - 



60 

 40 

 20 



1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 



Central California 



140 

 120 

 100 



80 



60 



40 



20 



1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 



Northern California 



100 



40 

 20 



r 



1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 



1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 



Year 



