American robins can be found year-round almost anywhere in North America, from Canada's lower provinces and southward, but many of the birds in northern areas head south for the winter.
Recently, researchers have found that these robins have been migrating about five days earlier each decade. Findings show that the timing is likely due to a change in weather conditions.
Many robins stay where they are all year, choosing to winter in place, but many don't. They migrate home in the spring to breed and raise a family, then race back to warmer climes before temperatures drop again. For them, the lure of warmer locales like Texas and Florida isn't the temperatures, reports the American Bird Conservancy, but a lack of food in the colder climates. Once the weather warms, they quickly fly back to Canada and Alaska, often traveling up to 250 miles a day.
In a new study published in Environmental Research Letters, researchers found that robins now migrate 12 days earlier than they did in 1994.
For their work, researchers attached tiny GPS "backpacks" to individual birds, catching them at Slave Lake in Alberta, Canada, a midway pit stop for migrating robins.
"We made these little harnesses out of nylon string," lead author Ruth Oliver told Columbia University's State of the Planet. Oliver worked on the study while earning her doctorate at Columbia. "It basically goes around their neck, down their chest and through their legs, then back around to the backpack."
The backpack weighs less than a nickel, allowing the robin to fly easily. The researchers expect that the nylon string will eventually degrade and the backpacks will fall off.
The researchers placed backpacks on 55 robins, tracking their migration from April through June. Using GPS, they were able to pair the birds' movements with weather information, including temperature, snow amounts, wind speed, precipitation and other factors that might impact migration.
Their findings showed that the birds began moving north earlier when winters were dry and warm.
"The one factor that seemed the most consistent was snow conditions and when things melt. That's very new," said Oliver. "We've generally felt like birds must be responding to when food is available — when snow melts and there are insects to get at — but we've never had data like this before."
Oliver and her team say their research suggests that the robins are picking up on cues from the environment to keep pace with the changing seasons.
"The missing piece is, to what extent are they already pushing their behavioral flexibility, or how much more do they have to go?" said Oliver.