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Annual Changes in Hummingbird Migration Revealed by Birders’ Sightings

By Jennie Miller
Calliope Hummingbird
Maps revealed that North American hummingbirds, like the Calliope Hummingbird, migrate along different routes each year, possibly in response to food variability. Photo by LAP75 via Birdshare.

Imagine circling the Earth twice on foot while drinking your weight in flower nectar each day. That’s the human equivalent of what Calliope Hummingbirds do, by wing, twice a year, in their migrations between Washington and Mexico. How such small birds travel such immense distances has puzzled ornithologists for decades because hummingbirds are too tiny to wear GPS tags for tracking. Now finally, through the use of citizen science, researchers have mapped the daily movement of migrating hummingbirds, and outlined a path of hope for the species along the way.

Using data from the eBird citizen-science project, researchers patched together hummingbird sightings from more than 300,000 checklists across North America to track the central hub of migration over a five-year period. Based on the number of eBird sightings at different locations, researchers calculated the average location of hummingbird populations for each day. For example, of the estimated 2 million Calliope Hummingbirds in North America, some individuals were recorded by eBird participants during the study period from 2008 to 2013. Researchers used these sightings to then find the average location of all Calliope Hummingbirds each day and visualize overall movement of the species throughout migration.

“We could not have surveyed the entire United States 365 days a year,” explained Sarah Supp, a postdoctoral fellow at Stony Brook University and lead author of the study. “Citizen-science data gave us that.” The research team also included the Cornell Lab of Ornithology’s research scientist Frank La Sorte and was funded by NASA’s Biodiversity Program. Their study focused on 5 hummingbird species that migrate up to 2,500 miles across North and Central America: the Calliope Hummingbird, Black-chinned hummingbird, Broad-tailed Hummingbird, Ruby-throated Hummingbird, and Rufous Hummingbird.

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Published in the January issue of Ecosphere, the maps revealed that the birds hopscotched between different stopover sites each year. For instance, over the five-year study, Calliope Hummingbirds shifted their central path of migration as far as 320 miles east or west between years, switching from routes focused around Tucson, Arizona, to Albuquerque, New Mexico, and from Bakersfield, California, to Las Vegas, Nevada.

According to Supp, this year-to-year variation suggests some “wiggle room” in hummingbird decision-making. The birds could be choosing sites with high-quality food, such as flowers with nectar to fuel their fast-paced metabolism. Such versatility may enable hummingbirds to respond as the rising temperatures of climate change alter the timing and location of flowering plants.

“Under climate change, one of the things we’re concerned about is mismatch in timing for migrating species,” said Supp. “If hummingbirds come north and the plants are already done flowering at the location where they’re expecting food, then that could be a problem.”

North American hummingbirds are especially vulnerable because they are so small, weighing the equivalent of two or three paperclips, and have such high metabolic rates, the fastest of all warm-blooded animals. These limitations prevent them from packing on large quantities of body fat to last them through migration. As a result, they feed continuously as they migrate, requiring frequent access to flowering plants with nectar throughout their route. “For some birds, going for extended periods without food means they get skinny or stressed,” said Catherine Graham, a principal investigator on the study based at Stony Brook, “but for a hummingbird, it could mean death.”

As climate change alters the availability of flowering plants, bird feeders may become an increasingly important source of food for migrating hummingbirds. “Feeders serve as a supplement to natural food for birds,” explained Emma Greig, project leader of the Cornell Lab’s Project FeederWatch. “Especially in extreme climate situations, like a harsh winter, feeders could play an important role in helping hummingbirds survive.”

With 10% of all hummingbird species worldwide threatened by extinction, the study offers optimistic news on the long-term survival of these beloved birds. Says Supp, “It tells us that hummingbirds will be flexible to a certain degree because they are potentially able to sense when it’s time to move, and that time may be different from year to year. It gives us some hope.”

For more on hummingbirds and bird migration:

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American Kestrel by Blair Dudeck / Macaulay Library