Abundance can be difficult to observe. Cassin's Auklet, for instance, breeds in San Mateo county, but its small size and pelagic status make it difficult to observe easily. Food availability, success or failure in breeding, and even relative wave action on a pelagic trip, can obscure their numbers. This Cassin's Auklet was one of very few seen on an Alvaro's Adventures pelagic trip, August 2015 (Mark Rauzon).
Each species has been giving a rarity rating, based on Peter Metropulos’ codes on the San Mateo County Checklist (download pdf file,) published by SAS.
1 = Common to abundant in appropriate habitat, always present in large numbers
2 = Fairly common, always present in moderate to small numbers
3 = Uncommon, usually present and in small numbers
4 = Rare, observed in very small numbers and perhaps not in each year
5 = Extremely rare, fewer than 10 records of occurrence in season indicated
6 = Accidental, recorded fewer than 10 times in San Mateo County and offshore waters
Percent of Total County List:
1 = 80 birds = 18% of list
2 = 91 birds = 20% of list
3 = 44 birds = 10% of list
4 = 94 birds = 21% of list
5 = 38 birds = 8% of list
6 = 101 birds = 23% of list
The county has 449 species on the San Mateo checklist. Of these, 102 are accidental or extirpated (Greater Roadrunner), leaving 347 species rated 1 to 5 (meaning, somewhat easier to see!). Nesting evidence exists for 172 species.