Each spring, barnacle geese migrate more than 1,800 miles from the Netherlands and northern Germany to their breeding grounds in parts of Russia above the Arctic Circle.
The journey north usually takes about a month, and the geese make multiple stops along the way to eat and fatten up before they lay their eggs, says Bart Nolet of the Netherlands Institute of Ecology and the University of Amsterdam.
But that pattern of migration is changing, as rapidly rising temperatures have led to earlier springs in the Arctic.
So Nolet's team tracked dozens of barnacle geese to figure out how they are being impacted by the earlier snowmelts. Their results were published in a new study in Current Biology.
One big thing hasn't changed: what time of year the geese start heading north. And that's a problem.
"They actually depart from the wintering areas around the same date regardless of whether it's early or late spring in the Arctic," Nolet says. That's probably because the geese "cannot predict what the weather is or what the season is up there from 3,000 kilometers distance."
Historically, the geese have arrived just after the snow melts and lay their eggs right away. That gives plants time to start growing so that the goslings can benefit from what is known as a "food peak."
These days, the weather in parts of the journey north is warmer than it used to be and the birds seem to realize that they're running late. They start to speed up — a lot.
A journey that usually takes the barnacle geese a month now takes about a week, the researchers found. It's a marathon: "They fly nearly nonstop from the wintering areas to their breeding grounds," Nolet says.
Even though they make up time on the way, the exhausted geese can't lay eggs right away because they need time to forage and recover — some 10 more days.
That means the goslings are no longer able to enjoy that tasty and nutritious "food peak," as Nolet put it. Instead, "when the eggs hatch, the food is already deteriorating in quality, and what we found is that goslings survive less well in such an early year than they do normally."
In short, rising Arctic temperatures mean that migratory barnacle geese are out of sync with some of their best food sources, which means that fewer of their chicks are surviving their early months.
So why don't the birds just set off earlier for their journey north? Nolet says they think this species likely takes its cue to leave based on how long the daylight hours last, rather than the temperature some 1,800 miles away.
But the future of the barnacle goose may depend on its ability to adapt to the new reality and set off earlier. Luckily, Nolet says, the bird is a "flexible" species that travels in groups, making it more likely that if a few start leaving earlier, others will follow.
The changing climate could be more problematic for other species such as shore birds, he says, where decreasing populations "may have to do with the mismatch that we're talking about between food peak and hatching of eggs."
Generally, climate change is likely to create this kind of mismatch for animals that migrate long distances. It's harder for them to adjust, Nolet says, when they spend part of the year in a totally different climate.