American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Ecological release of the Anna’s Hummingbird during a northern range expansion”

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C. J. Battey (Sep 2019)

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Anna’s Hummingbird populations are booming in the northwest: A century of survey records shows how they got there

The Anna’s Hummingbird Boom

Anna's Hummingbird (<i>Calypte anna</i>) in Seattle.<br />(Credit: C. J. Battey)
Anna's Hummingbird (Calypte anna) in Seattle.
(Credit: C. J. Battey)

In the early twentieth century, Anna’s Hummingbirds nested in coastal California and the Baja Peninsula, but today their breeding range extends from Arizona to British Columbia. How and when did they spread across western North America? New research conducted by Dr. C. J. Battey at the University of Washington used a century of survey and museum records to track the range and population growth of Anna’s Hummingbirds over time, and found that populations in the Pacific Northwest have been growing exponentially since the region was first colonized in the middle of the twentieth century. By comparing historic and modern climate records, the study shows that climate change is unlikely to explain the initial phases of the range expansion, but may play an increasingly important role in shaping the species’ range in the future. Instead, a combination of introduced plants and supplemental feeding appear to have allowed the species to escape limits on population growth in its native range and establish new breeding populations across Arizona and the Pacific Northwest during the 1960s and 70s. In one sign that populations may already be adapting to their new ranges, nest reports suggest that northern populations now delay the beginning of the nesting season by at least 16 days. The study provides a detailed historical record of this remarkable range expansion, and is a reminder of the profound ways in which humans have shaped the ranges and populations of native species over the last century.


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 indicate that populations have been growing in California since the early twentieth century. Sites across the Pacific Northwest show striking fits to simple models of exponential growth following colonization in the 1960s and ’70s, 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 prior to the range expansion, the fastest growing populations in the northwest are in regions with minimum breeding season temperatures similar to those occupied by the species in its native range. Range expansions in the Anna’s Hummingbird thus reflect an ecological release likely caused by a mix of introduced plants, human facilitation, and phenological acclimation that allowed a California native to expand across western North America.