In April and May, 28 American robins took flight from the Boreal Center for Bird Conservation in Alberta, Canada, each with a miniature "backpack" between its wings.
Each bird was transporting special cargo: a GPS unit that, with the help of satellites, tracked its location for two months.
Led by Natalie Boelman, an associate research professor at Columbia University's Lamont-Doherty Earth Observatory, the team hoped to use this information to understand the environmental cues some robins use as they migrate to places like Alaska that are undergoing significant climate change.
"Not all that much is known, especially when birds start to migrate, what they are responding to," said Ruth Oliver, a Ph.D. student at Columbia working with Boelman on the project.
While Boelman said that lots of studies are looking at how birds relate to the environment, the size of this study is unique.
"The specific questions that we are asking and the scales that we're asking them at haven't been addressed," Boelman said.
The information may become more important as climate change affects migratory songbirds' breeding grounds.
Changing climate, changing migrations
The start of songbird migration is triggered primarily by the changing length of days, which is caused by the tilt of the Earth, not climate. But when migratory songbirds are on their way to breeding grounds, they might be able to adjust their arrivals based on environmental cues along their routes, like temperature, scientists think.
In May, researchers led by Stephen Mayor, a postdoctoral researcher at the Florida Museum of Natural History, published an article that compared the arrival of 48 migratory bird species at breeding grounds across North America and the start of spring between 2001 and 2012.
The study found that the start of spring — indicated by "green-up," when trees unfurl leaves and draw opportunistic plant-eating insects — started earlier in the East and North, while it was delayed in much of the West.
While most of the birds were able to adjust their arrival times to the shifting start of spring, nine of the birds couldn't keep up with the changing climate — researchers observed a gap between their arrival and the onset of spring.
"For the most part there's a lot of research to suggest that birds are not going to be able to adjust really well or very quickly," Mayor said.
He pointed out "strong limitations" to how birds can adjust arrival times en route. While they can spend less time at stopovers, they have a limited capacity to do that. They can try to fly faster to make an earlier green-up — but face physiological limits on that as well.
"They are kind of stuck between a rock and a hard place," said Mayor.
While American robins have a large range in North America, the species wasn't included in the study because, as Mayor said, they can be found in the contiguous United States all winter and summer.
"We wanted a clear separation between the winter range and breeding range," Mayor said.
The study used data from eBird, a citizen science database, to estimate bird arrivals at breeding grounds.
Mayor said that GPS data, like that being captured by Boelman, could be complementary to citizen science data like that in his study.
"They give us different data that help us to better understand different aspects of bird migration, and together they lead to better conclusions," said Mayor.
In 2015, a group of scientists suggested that "a golden age of animal tracking science has begun and that the upcoming years will be a time of unprecedented exciting discoveries."
The old-school way of tracking birds relied on opportunity. Every time a bird was captured at an aviation monitoring station, Oliver said, it was tagged with a bracelet containing a tracking number. If the bird was captured again, its location could be recorded.
"You can sort of track individuals that way, but of course it has a lot of issues," Oliver said. "It is highly unlikely that you capture the same bird again and you would only be able to know that they ended up at these places."
Other researchers have tracked migratory songbirds using mini geolocators, which record light measurements at regular intervals. Scientists can roughly determine the location of the bird based on the light intensity and length of day.
But Oliver said the accuracy of those devices is poor, and in order to obtain the data, the researchers must recapture the bird.
With the increasing speed of technological development, using GPS that communicates with a satellite system on small birds like robins is now possible.
Boelman and Oliver just started their third year of a four-year grant. Including last year's work, they have received data from 42 robins.
To capture the robins, the team set up nets in trees and shrubs surrounding an open field at the Boreal Center.
"Then we wait and wait for them to land (on the nets)," said Oliver. "And of course they don't land where you want them to and don't flush up at quite the right angle … so there's a lot of waiting."
Once a bird is caught in the net, the team retrieves it and, with the help of Nicole Krikun, a certified bird bander, fits the bird with its harness and GPS unit.
Each device was programmed to record a bird's position every other day for two months, with the final GPS coordinates sent in early July. Oliver said she expects the harnesses to fall off birds within six months.
This year, many of the birds didn't travel far. While some ended up in Alaska, Oliver said, a lot of the robins stayed close to the Boreal Center, perhaps due to a snowstorm in the spring.
This "was kind of a bummer if you want to look at movement over a bigger scale," said Oliver.
But she said this finding is still interesting, to see which birds chose to stay put and which ones moved on.
"It will be interesting to see maybe those were the birds that we caught early and had the big snowstorm — it's possible that they hunkered down and never ended up migrating," Oliver said.