The sedentary subspecies of Allen's Hummingbird (Selasphorus sasin sedentarius) was originally endemic to the Channel Islands off the coast of Southern California, but it colonized the mainland at the Palos Verdes Peninsula sometime before 1966. In the decades since, its population has expanded in Southern California. I tracked its growth using eBird checklists. The mainland range of S. s. sedentarius has grown from ∼70 km² in 1970 to ∼13,000 km² today, representing an increase of ≤23% in the total range of the species as a whole. Its main habitat within Los Angeles, Orange, San Diego, and western Riverside counties is urban parks, gardens, and campuses. The range expansion of S. s. sedentarius seems to be driven by food availability—although, given that it is found in urban habitats that the other subspecies, S. s. sasin, does not seem to utilize, a subtle change in the ecology of the 2 subspecies is also implied. Analysis of eBird data suggests that breeding S. s. sedentarius met S. s. sasin near Santa Barbara perhaps as early as 2005, raising the possibility of a new zone of intergradation of the forms. Given that S. s. sedentarius has a substantially longer breeding season and, thus, a potential fecundity advantage over S. s. sasin, it is possible that the island S. s. sedentarius will outcompete the mainland subspecies. Partners in Flight has Allen's Hummingbird on its 2016 watchlist because analysis of Breeding Bird Survey data suggest that this species has declined by 83% since 1970. This estimate is not credible for 3 reasons: it implies a 1970 population of 10 million Allen's Hummingbirds within the restricted range of this species; there are no suggestions that S. s. sasin has become extirpated anywhere throughout its historical range; and the geographic range occupied by S. s. sedentarius has grown by ∼700% in the same period. I found eBird to be a useful new source of data for monitoring urban birds such as S. s. sedentarius.