We analyzed data on the size of nesting aggregations of Sabine's Gulls (Xema sabini) observed in the Yukon–Kuskokwim Delta (YKD), Alaska, between 1986 and 2013, to determine whether the species is a “solitary low-density nester” there, as in other parts of its range. Data for 1087 nests show that 56% of Sabine's Gull nests were solitary and that <3% of aggregations exceeded 10 nests, although factors positively correlated with the evolution of colonial nesting, including exposure of nests to predators, aquatic habitat, and absence of feeding territories, apply to Sabine's Gulls nesting in the YKD. Although Sabine's Gulls in the YKD were most often solitary nesters, in 2010 we found a 102-nest aggregation on a small island in the study area, larger than any reported for the species. Among all aggregated nests, 75.4% occurred on islands, and only 13.9% along shorelines, while 39.8% of all solitary nests occurred on islands, and 39.3% along shorelines. The proportion of Sabine's Gull nests in the YKD located on islands ranged from 0.15 (in 1991) to 0.78 (in 2010) and showed no significant trend between 1986 and 2013, despite an increase in population size during the period. Although islands may provide greater protection from mammalian predators, and the majority (54.6%) of Sabine's Gull nests in our study area occurred on islands, thousands of similar, apparently suitable islands remained unoccupied by Sabine's Gulls and other nesting species every year. Given the potential benefits of nesting on islands, in colonies, or in colonies on islands, the YKD Sabine's Gull population maintains considerable behavioral variability, in terms of nest-site selection and size of nesting aggregations, perhaps to take advantage of locally abundant food resources.