Ecological release of the Anna's Hummingbird during a northern range expansion

During range expansions species can experience rapid population growth if changes in climate or interspecific interactions remove limits on growth rates in novel habitats. Here I document a century of range expansion in the Anna's Hummingbird (Calypte anna) and investigate the causes of its recent abundance through a combination of demographic, climatic, and phenological analyses. Christmas Bird Count records show that populations have been growing within the native range since the early twentieth century. Sites across the Pacific Northwest show striking fits to simple models of exponential growth following colonization in the 1960’s and 70’s, and nest records indicate that the species now delays the start of the nesting season by at least 16 days in the north. Although the species now occurs in a much wider range of climates than it did in its native range, the fastest growing sites in the northwest are in regions with minimum breeding season temperatures similar to those occupied by the species prior to the expansion. Range expansions in the Anna’s Hummingbird thus reflect an ecological release likely caused by a mix of human facilitation, introduced plants, and phenological acclimation that allowed a California native to expand across western North America.