Since Darling' s (1938) early observations of gulls, many studies have focused on spatial and temporal patterns of reproduction in seabird colonies. Reproductive success of birds differs among colonies at different sites (Hunt 1972, Harris 1973), among hab-itats within a colony (Brown 1967, Nettle-ship 1972), between nests positioned cen-trally or peripherally in a colony (Patterson 1965, Coulson 1966, Tenaza 1971, Dexhei-mer and Southern 1974), with the relative timing of breeding (Patterson 1965, Ver-meer 1970, Parsons 1975, Hunt and Hunt 1976), and between successive seasons (Fordham 1970). At traditional nesting sites, nests of White Pelicans (Pelecanus erythrorhynchos) are grouped into many spatially and temporally separate colonies (Hall 1925, Low et al. 1950, Behle 1958). Since many colonies oc-cur on a single island, climatic factors influ-encing reproduction affect all birds similar-ly. At Gunnison Island, Utah, food is equally available to pelicans from different colonies (see discussion by Orians 1961) at the limited foraging areas along the eastern shore of Great Salt Lake (Behle 1958). Pat-terns of reproductive success in pelicans at Gunnison Island, show basic aspects of the colony-nesting habit better than at many other places. In 1973 and 1974 I studied co-lonial nesting of White Pelicans at Gunni-son Island. I report here the spatial and tem-poral distribution of their nesting efforts, how the distribution was established, and variations in reproductive success. STUDY AREA AND METHODS Gunnison Island lies in the northwest arm of Great Salt Lake about 12 km from the western shoreline (Knopf 1974). The island, 1.6 km long, reaches a maximum width of 0.8 km and rises 85 m above lake level. Its 66 ha include a series of large hills with connecting ridges that separate four low, sandy areas. Vegetation is typ-ical of the cold desert community (Oosting 1956). Prominent forms include Bromus tectorum, Atriplex spp., Sarcobatus vermiculatus, Suaeda intermedia, Bassia hyssopifolia, Salsola kali, Opuntia fragilis, and Chrysothamnus spp. As many as 6,600 pelicans nest on the flatter, lower elevations of the island each sea-son. Behle (1958) provided details of the study area and its historical use by White Pelicans. Male pelicans are larger than females (Palmer 1962), and I was able to visually distinguish the sexes (as verified during copulations) by differences in bill lengths. In addition, pelicans undergo a presupple-mental molt during incubation (Knopf 1975a). The re-sulting supplemental plumage of the crown varies greatly among individuals as does the structure of the maxillary "horn." I was able to recognize individuals by using these features. I surveyed pelican colonies weekly from 1 April to 31 July in 1973 and 1974, using binoculars, from higher elevations of the island. (A colony was defined after Penny [1968] as a geographically continuous group of breeding birds with contiguous territories.) During surveys I recorded reproductive stage and total num-ber of nests for each colony. For each new colony I noted date of establishment and spatial location rela-tive to nature of substrate, angle and direction of ground slope, associated vegetation, and proximity of other colonies. I measured distance-to-nearest-neigh-bor (nest rim to nest rim) for individual nests after chicks fledged. To aid in the description of how colonies were formed, I observed individual (n = 25) and paired (n = 42) birds for 30-min periods. I supplemented these ob-servations with telephoto, time-lapse photography of localized aggregations of courting birds. I surveyed contents of pelican nests periodically us-ing a 20X spotting scope from vantage points over-looking colonies. I recorded numbers of eggs and chicks as pelicans stood to stretch. turn the eggs, or feed a chick. I observed nests in specific colonies daily to define incidence of egg and chick mortality and in-ternest synchronization of hatching. I photographed each pelican colony weekly at dis-tances of 30 to 60 m from a hill or cliff above a colony. With these photographs I prepared weekly histories of each colony and determined the timing and location of nest abandonments. I also used photographs to census numbers of young surviving the nestling period in larg-er colonies. Late in the nestling period (21-28 days of age), I banded 300 pelican chicks in 1973 and 699 chicks in 1974. Plastic leg bands of different colors were applied to 64 of those chicks to aid in recognition of individuals after the nestling period. After all chicks fledged, I searched the island for banded chicks that died during the postnestling, prefledging (4-12 weeks of age) pe-riod.