Time Lags and Niche Shifts in a Biological Invasion of Hummingbirds

Shifts in a species realized niche can lead to rapid population growth by increasing the carrying capacity of local habitats and allowing colonization of new areas. This process is well known in \"invasive\" species introduced to novel ranges by humans, but can also occur when native species expand their range. In these cases expansions may be driven either by a shift in the available environment or by a shift in the species use of existing niche space, but identifying the specific environmental or behavioral changes involved is often hindered by time lags in the process of colonization. Here I document a century of range shifts in the Annas Hummingbird (Calypte anna) and show that recent abundance in the Pacific Northwest is the product of a series of range and niche expansions that started in the early 20th century following the spread of garden cultivation and introduced plant species in California. Demographic trends in the northwest have tracked simple models of exponential growth since populations became established in the 1960s and 70s, and nest records suggest that the species has delayed the beginning of the breeding season by at least 18 days in the north. Niche models trained on historic climate and occurrence data fail to predict the modern range, suggesting that climate change is not the primary cause of the expansion. Range expansions in the Annas Hummingbird thus closely track the dynamics of an invasive species spreading across a novel range, and were made possible by a mix of introduced plants, phenological acclimation, and an expansion of the realized climatic niche.