Breeding Biology of Western Gulls (Larus occidentalis) on San Nicolas Island, California, 1968

Although the breeding biology, ecology, and behavior of many gull species are well documented, Western Gulls (Larus occidentalis) have received little attention from ornithologists. This lack of data on such a conspicuous bird, the only gull breeding along the California coast, is perhaps not too surprising since Western Gulls nest primarily on islands to which ready access is difficult. Summaries in Bent (1921) and various state bird books, and short articles by Dickey and van Rossem (1925), Ferris (1940), Pearse (1946), Woodbury and Knight (1951), Bartholomew and Dawson (1952), and Bennet and Erickson (1962) are the only references I found to Western Gulls. As a member of the Smithsonian Institution's Pacific Ocean Biological Survey Program (POBSP), I studied the breeding biology of Western Gulls on San Nicolas Island, Channel Islands, California, on the following days in 1968: 10-23, 27-31 May; 5-10, 14-17 June; 2-5, 16-18 July, and 22 August. San Nicolas Island, the Channel Island farthest from the mainland, is situated at 33o14' N, 119o32' W, 62 statute miles from Point Mugu, California. It is oval in shape, with the long axis (9.8 statute miles) oriented approximately WNW-ESE; the maximum width is 3.6 miles. The island is 21,000 acres in area, with the highest point a broad plateau 906 ft above sea level. The south portion rises abruptly, eroded by deep gullies. The north side is less rugged and the northwest point is a sand dune of 200 ft elevation. Western Gulls nest on the western side of this dune in an area approximately 1 mile by 200 yards on a hillside rising from the beach cliff. This gradual slope is traversed by many gullies eroded to a depth of 20 ft. Waterand winderoded boulders up to 5 ft high are scattered across the area. Vegetation in the colony consists of low-growing ice plant (Mesembryanthemum crystallinum), ground heliotrope (Heliotropium currassavricum, var. oculatum), sand verbena (Abronia maritima), beach burr (Franseria chamissonis bipinnatisecta), alkali heath (Frankenia grandifolia), sea blite (Suaeda california pubescens), and lupine ( Lupinus hirsutus). An initial survey showed it impossible to census accurately the entire colony, so four study plots were established, comprising approximately one-third of the area and one-half of th nests of the entire colony. In these plots I marked each nest with spray-painted numbers that remained legible throughout the study. I surveyed the entire colony three times and checked marked nests every two or three day during May and early June, and on each visit in July and August.