Technical Report

Whole-Colony Counts of Common Murres, Brandt's Cormorants and Double-crested Cormorants at Sample Colonies in Northern and Central California, 1996-2004

From 1996 to 2004, annual aerial photographic surveys of all northern and central California breeding colonies of Common Murres (Uria aalge), Brandt’s Cormorants (Phalacrocorax penicillatus), and Double-crested Cormorants (P. auritus) were conducted by the Common Murre Restoration Project (CMRP), in cooperation with the California Department of Fish and Game. These surveys have continued a long-term dataset (initiated in 1979) of whole-colony counts derived from aerial photographs; these counts have been used as indices of population size for estimation of breeding population size and for assessing long-term population trends. In this report, we present a summary of whole-colony counts in northern California for 1996 to 2004 and in central California for 2002 to 2004, and briefly discuss population status and general trends evident for murres and cormorants during these periods. For northern California, we counted all murre breeding colonies for 1997, 1999, 2001, 2003 and 2004, and certain sample murre colonies for other years. For central California, we counted all murre breeding colonies and other sample Brandt’s Cormorant colonies for 2002 to 2004. Numbers of California Brown Pelicans (Pelecanus occidentalis californicus) roosting within murre and cormorant colonies also were counted to provide information on use of coastal rocks by this endangered species during the late May and early June survey period. For Common Murres in northern California (defined as north of Point Reyes to the Oregon border; 38-42°N), a total of 211,421 birds were counted at 16 active colonies and one irregularly attended colony in 2004, corresponding to a rough estimate of 353,100 breeding individuals or 176,550 breeding pairs. The 2004 total count was 28% higher than the 2003 count, and total counts were similar in 1999, 2001, and 2003. Lower numbers in 1997 seemed to partly reflect reduced photograph quality. Less than 10% of the northern California murre population occurs in Mendocino County, but numbers there increased steadily since 1996 and two new colonies formed. No murre colonies occur in Sonoma and northern Marin counties. Overall, the murre population in northern California from 1996 to 2004 appeared to be slowly increasing and expanding farther south, continuing the trend noted in 1979-1995. For Common Murres in central California (defined as Point Conception to Point Reyes; 34.5-38°N), a total of 161,572 birds were counted at 10 active colonies in 2004, corresponding to a rough estimate of 269,800 breeding individuals or 134,900 breeding pairs. The total count for nearshore colonies in 2004 was 14% higher than in 2002, following lower numbers in 2003. Total counts for offshore colonies at the North and South Farallon Islands were very similar from 2002 to 2004. Overall, the murre population in central California from 2202 to 2004 appeared stable, but had increased earlier. For Brandt’s Cormorants in northern California, a total of 4,728 nests were counted at 13 active sample colonies in 2004. Sample colonies accounted for about 70% of the total number of nests in northern California in 1989 abd 2003. The 2004 nest total for sample colonies was similar to the 1989 total despite reduced breeding populations in the late 1990s due to impacts from El Nino conditions in 1992-1993 and 1998. Sample colonies of Double-crested Cormorants accounted for only 10% of the northern California nest total in 2003 and trends were not determined. For Brandt’s Cormorants in central California, a total of 12,784 nests were counted at 13 active sample colonies in 2004. Sample colonies accounted for about 50% and 63% of central California nest totals in 1989 and 2003 respectively. The single sample Double-crested Cormorant colony accounted for only 13% of the central California nest total in 2003 and trends were not determined. Additional statistical analyses and consideration of factors affecting different colonies are needed to better describe murre and cormorant population trends in northern and central California since 1996 with available count data. For complete assessment of Common Murre population trends in California, aerial photographs of certain northern California colonies from 1996, 1998, and 2002 would need to be counted. For complete assessment of population trends of Brandt’s and Double-crested Cormorants in northern and central California, aerial photographs of certain colonies from 1996 to 2002 and 2004 still need to be counted and analyzed.

Relationships