counts of nests and individual birds were made. 

 Usually, the boat was slowed or stopped and both 

 observers counted birds or nests at the same 

 colony or section of cliff. Counts were often 

 repeated to ensure accuracy. 



Except for the Farallon Islands (429 012) 

 and Redding Rock (325 013), all islands and 

 offshore rocks north of Point Conception are 

 within one kilometer of the mainland. Counts of 

 seabirds on offshore rocks, islands, and mainland 

 cliffs were often made from shore promontories. 

 Generally, these counts underestimated popu- 

 lations since only the landward sides of colonies 

 were visible. In many instances, our population 

 figures for a particular colony are derived from a 

 combination of different survey techniques, 

 which may have included boat surveys, aerial 

 photography, counts from the mainland, and 

 observations made directly on an island after 

 landing. 



Landings were made on islands and rocks 

 whenever possible, except at colonies of Brandt's 

 Cormorants and Common Murres, which are 

 especially sensitive to disturbance. When we 

 landed on an island, we counted cormorant and 

 gull nests and searched for burrows of storm- 

 petrels, auklets, and puffins. Island visits were as 

 brief as possible to minimize disturbance. 



Counts made from boats, the mainland, and 

 the islands themselves were the principal methods 

 used to census Double-crested Cormorants, 

 Pelagic Cormorants, Western Gulls, Black Oyster- 

 catchers, Pigeon Guillemots, Marbled Murrelets, 

 Rhinoceros Auklets, Tufted Puffins, and, to a 

 lesser extent, Brandt's Cormorants. Most of our 

 surveys were conducted in 1979. In 1980 we 

 attempted to refine estimates made at locations 

 where surveys were hampered by poor weather 

 conditions the previous year. Also, many sites 

 which were accurately surveyed in 1979 were 

 resurveyed in 1980 to determine whether any 

 population changes had occurred. 



Cormorants and gulls build conspicuous 

 nests and are therefore the easiest species to 

 census. Since their nests were directly counted, 

 our population estimates for these species are the 

 most reliable. Most surveys for Pelagic Cormo- 

 rants and Western Gulls were made from a boat, 

 often very close to shore. Our counts of Double- 

 crested and Brandt's Cormorants and Western 

 Gulls were sometimes hampered by our inability 

 to see the tops of islands, the preferred nesting 

 habitat for these species. We circumvented this 

 problem by making counts from the mainland 

 where possible, or by taking aerial photographs. 



We counted and noted the locations of all 

 Black Oystercatchers, Marbled Murrelets, Pigeon 

 Guillemots, Rhinoceros Auklets, and Tufted 

 Puffins seen during our surveys. Nests of these 

 species are either difficult to find or, in the 

 case of burrowing species, difficult to identify 

 with certainty. Therefore counts of individual 

 birds and extrapolation from these counts were 

 our best survey method. 



Black Oystercatchers were counted during 

 nearshore surveys but in 1979 poor weather 

 hampered our efforts in some locations. Better 

 weather in 1980 allowed us to re-survey selected 

 portions of the coastline and improve our census 

 data. Because nests of this species are difficult to 

 find, the breeding status of many Black Oyster- 

 catchers was impossible to determine. Estimates 

 of populations were usually based on the presence 

 and behavior of birds rather than counts of nests. 

 Some non-breeding Oystercatchers were probably 

 included in our counts and may have inflated 

 population estimates for some regions. Fre- 

 quently, however, sections of coastline with poor 

 habitat for other seabirds were by-passed, even if 

 some habitat looked promising for Black Oyster- 

 catchers. For this reason, and because oyster- 

 catchers were sometimes hard to see among the 

 confusion of reefs, rocks, and breakers, we believe 

 our estimates of Black Oystercatcher populations 

 in California are conservative. 



Marbled Murrelets were counted opportu- 

 nistically in a narrow strip offshore of the coast. 

 The distances of these counts from shore varied, 

 depending on the nature of the coast, but never 

 exceeded one kilometer. The most critical factors 

 influencing our counts of Marbled Murrelets were 

 sea conditions and time of year. Our counts were 

 made from small inflatable boats, so we probably 

 overlooked some birds on the water in all but the 

 flattest seas. The best censuses of Marbled 

 Murrelets can probably be made in spring when 

 both members of a breeding pair are on the water 

 offshore of coastal forests. Our surveys, however, 

 were usually conducted later in the year. In 

 addition, our census strips were of necessity 

 narrow and this probably resulted in many missed 

 birds. Although our surveys of Marbled Murrelets 

 have provided few data with which future compar- 

 isons can be made, we feel they reflect the 

 species' breeding distribution in the state. 



Rhinoceros Auklets and Tufted Puffins were 

 frequently seen standing near and entering 

 burrows in the early morning. Time of day is an 

 important factor when censusing these birds, 

 since they are seldom seen at their colonies in late 



8 



