We were unable to distinguish between 

 breeding and non-breeding Common Murres on 

 the photographs. Our Common Murre estimates 

 are actually the numbers of birds at a colony 

 rather than the numbers of breeding birds. 



Common Murres are extremely difficult to 

 census and our estimates of their populations are 

 probably the crudest of all estimates in the 

 catalog. These birds pack densely on islands and 

 it is often impossible to distinguish between 

 individuals, even on high-quality photographs. 

 Comparison of our numbers with those obtained 

 in future surveys will be difficult, but direct visual 

 comparisons of photographs could easily show 

 whether the extent and density of a colony has 

 markedly changed. To make comparisons simpler, 

 we have archived data and photographs at several 

 different sites (see Appendix A). 



Storm-petrels and Cassin's Auklets are 

 nocturnal and difficult to census. We made no 

 attempts to census colonies of nocturnal seabirds 

 which we knew existed on Castle Rock (325 

 006), Green Rock (325 020), and Little River 

 Rock (325 035), but instead relied on population 

 estimates in the literature. Other colonies of 

 nocturnal birds in our study area which we 

 censused were eiiher small in magnitude or small 

 in area and did not warrant intensive survey 

 efforts such as establishing sample plots and 

 determining the ratio of active to inactive burrows. 

 To obtain more refined data on these sites would 

 have caused more disturbance than we felt was 

 justified. 



Besides searching many islands for storm- 

 petrel and Cassin's Auklet colonies during the 

 day, we made overnight surveys one or more 

 times on Prince Island (325 003), Flatiron Rock 

 (325 023), Pewetole Island, Button Rock (325 

 054), Goat Rock (379 006), an island at Van 

 Damme Cove (379 027), and Fish Rocks (404 

 003). We conducted searches at night for nesting 

 storm-petrels and auklets, and on all but Pewetole 

 Island, Flatiron Rock, and Goat Rock erected 

 mistnets to catch nocturnal birds. We caught and 

 banded 174 Leach's Storm-Petrels on Prince 

 Island, Button Rock, and Fish Rocks combined. 

 Our population estimates for several storm-petrel 

 colonies were based on one or more of the 

 following: 1) the number and density of burrows, 

 2) the number of birds seen and heard on the 

 island at night, 3) the number of birds captured 

 and banded, and 4) the number of birds recaptured. 

 We discuss our population estimates for storm- 

 petrels at each site more fully in the species 

 accounts. We offer no new estimates of Cassin's 



Auklet numbers in the catalog. 



This catalog includes the best information 

 available for all seabird colonies along the coast of 

 California, but the information varies in quality 

 from species to species and from site to site. We 

 have devised a data quality code, described below, 

 as an attempt to rate our data by its quality. A 

 data quality code number follows each of our 

 population estimates in the maps and tables 

 section. 



Data Quality Code 



I. Total count of all nests (number of 

 nests x 2 = number of breeding birds). 

 Few if any errors were made in these 

 counts. Any changes in number of 

 breeding birds from year to year can 

 probably be detected. 



II. Count of nests. Because of omission or 

 misidentification of nests, counts may 

 be slightly higher or lower than actual 

 bird numbers. Small or moderate 

 changes in number of breeding birds 

 from year to. year can probably be 

 detected. 



III. An estimate of the size of a breeding 

 population based on counts of nests 

 and birds and on estimation of the 

 amount of available nesting habitat. 

 Census techniques vary considerably 

 among species. Only large changes in 

 populations from year to year can 

 probably be detected. 



DISCUSSION 



Populations containing nearly 700,000 sea- 

 birds, representing 23 species, are discussed in this 

 catalog. All major and most minor seabird 

 colonies along the California coast have probably 

 been. identified, and present data seem adequate 

 to evaluate the importance of even small sections 

 of coastline to breeding birds. 



Unfortunately, sufficient historical data do 

 not exist to evaluate population changes or trends 

 that may have occurred or are occurring along 

 much of the California coast. Only on the 

 Farallon Islands (429 012), Santa Barbara Island 

 (524 008), Anacapa Island (502 007), and at 

 Least Tern colonies in southern California have 

 long-term studies been initiated. 



Although California's breeding marine avi- 



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