These Spangly Hummingbirds Are Moving North To Alaska | @GrrlScientist

It’s long been known that a common hummingbird species, Anna’s hummingbird, is expanding its range throughout the West Coast of North America, but why? Now, a newly published study provides some answers

by GrrlScientist for Forbes | @GrrlScientist

Adult male Anna’s hummingbird (Calypte anna) visits a bird feeder. (CC BY-SA 3.0)

As any west coast bird watcher will tell you: Anna’s hummingbirds, Calypte anna, are rapidly expanding their range northward. Prior to the 1930s, this colorful species never nested farther north than San Francisco Bay, and it was never seen north of the Oregon state border until 1944. Later, this diminutive bird showed up in my home town, Seattle, in 1964 (before I showed up there). Now it nests on Vancouver Island and is regularly seen as far north as southeastern Alaska.

The appearance of this flashy bird raises all sorts of questions, not the least of which is how on Earth can it survive the winters in such cold places? Although most hummingbird species are tropical, many live high up in the mountains, so they’re supremely well-adapted to surviving the cold: for example, hummingbirds reduce their energy consumption by going into torpor at night — even in summer.

But torpor is only one piece of a much larger and more complicated puzzle as to why Anna’s hummingbirds — in particular — are expanding their range. Another puzzle piece is that, unlike other North American hummingbird species, Anna’s hummingbirds are non-migratory, so they tend to stay put after they’ve finished breeding — when other hummingbirds head south for the winter. But the evidence clearly shows that Anna’s hummingbirds are really not staying put at all: these birds (or, more likely, their youngsters) are steadily moving north.

Is this northward range expansion encouraged by a warming climate? Or is it supported by the increasing numbers of people living along the west coast of North America, combined with the growing popularity of gardens crammed with a huge variety of flowers, and bird feeders? (There is some research from New Zealand to support this hypothesis.)

Adult female Anna’s hummingbird (Calypte anna), feeding. (Credit: Matthew Field / CC BY 2.5.)

To investigate which of these two hypotheses explains Anna’s hummingbird range expansion, a team of researchers based at the Cornell Laboratory of Ornithology and at California State University, Los Angeles, predicted that if this species’ range expansion is the result of warmer winters, then:

  1. Anna’s hummingbirds occupying the expanded portion of their range would live in areas with similar climactic conditions to those found their historic range
  2. these hummingbirds would be just as likely to live in urbanized areas as the hummingbirds living in their historic range; and
  3. these hummingbirds are equally dependent upon nectar feeders in both the expanded and historic portions of their range

Alternatively, if this species’ range expansion is due to an increased availability of suitable habitat or food provided by people, then:

  1. Anna’s hummingbirds would be found in areas with temperatures that are cooler than in their historic range
  2. these hummingbirds would preferentially colonize areas that are more highly urbanized; and
  3. those hummingbirds living in the expanded range would be more dependent upon nectar feeders than those living in their historic range

Anna’s hummingbirds’ range expansion is all about people

To carry out their investigations, the researchers obtained data collected by Project FeederWatch, a citizen science program with more than 20,000 participants from across the United States and Canada. Originally established as the Ontario Bird Feeder Survey in 1976, this project was greatly expanded to include all of North America in 1987, after Canada’s Long Point Bird Observatory (now known as Bird Studies Canada) joined with the Cornell Lab of Ornithology to oversee this project. These data monitor the abundance and distribution of winter bird populations that rely on bird feeders, bird baths or urban landscapes that incorporate native plants that attract birds.

Using the Project FeederWatch data, the researchers started by mapping the distribution of Anna’s hummingbirds in winter (January and February) over a time period of 24 years (Figure 1):

The researchers analyzed climate and elevation data, housing density and land cover data, and estimates of the prevalence of supplementary bird feeding from 1997–2013, and compared these to the Project FeederWatch data to distinguish between their two alternative hypotheses. First, their analyses revealed that, over the past 20 years, Anna’s hummingbirds occupied areas in their expanded range that were colder, on average, than in their historical range, suggesting that the birds were not following (and exploiting) warming winter temperatures.

Second, the researchers found that the hummingbirds were more likely to colonize urbanized areas in their expanded range than in their historic range. This suggests that an urban habitat is most beneficial to Anna’s hummingbirds in colder areas. Certainly, by their very nature, urban areas do present several advantages, including heat retention (cities tend to be warmer than surrounding non-urban areas), increased availability of hummingbird-friendly flowers that bloom throughout the winter, and the greater probability that nectar feeders are present.

The data suggest that Anna’s hummingbirds living in northern latitudes were more reliant on supplementary feeding, and thus, these birds had a higher visitation rate at nectar feeders than birds living in their historic range. Interestingly, this sets up an ostensible feedback loop where people living in the hummingbirds’ expanded range were more likely to provide nectar feeders than people living in the hummingbirds’ historic range, possibly because having these exotic tropical hummingbirds visit one’s feeder is perceived as a reward in itself. Perhaps hummingbirds are also training humans?

How may this range expansion affect other species in those areas that Anna’s hummingbirds are colonizing? In areas where other hummingbird species nest, this range expansion can lead to increased competition between newly arrived migratory hummingbirds and an army of firmly-established resident Anna’s hummingbirds that have laid claim to all the local resources. But even more interesting is this research highlights how human activities — even something as passive and seemingly innocuous as gardening and bird feeding — can affect the species composition and dynamics of the local flora and fauna.

Adult male Anna’s hummingbird (Calypte anna) shows off his colors. (Credit: nigel / CC BY-SA 2.0)


Emma I. Greig, Eric M. Wood and David N. Bonter (2017). Winter range expansion of a hummingbird is associated with urbanization and supplemental feeding, Proceedings of the Royal Society B: Biological Sciences, published online on 5 April 2017 ahead of print | doi:10.1098/rspb.2017.0256

Read more:

How do hummingbirds survive cold nights?

Birdfeeding favours exotic birds.

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Originally published at Forbes on 5 April 2017.

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