Birds depend on cues from nature to lead their lives — the length of sunlight, temperature changes, ambient scents. For thousands of years, these cues have been remarkably stable and birds have been remarkably consistent in their migration patterns.
But in this era of climate change, those cues are getting scattered and birds are getting confused — with potentially dangerous consequences.
A new study in the journal Scientific Reports from US and Canadian researchers examines the migration patterns of dozens of songbird species in North and South America between 2001 and 2012.
The researchers used satellite images to determine when birds arrived in their spring habitats.
Historically, birds arrive at the first signs of spring — when plants begin to sprout their leaves, a process known as “green-up.”
During the period the researchers studied, birds either arrived too early or too late. The gap in arrival time grew an average of half a day per year, and five days per decade.
For birds migrating in the west, spring has been coming late. For birds in the east, spring has been coming early.
This year in particular featured an especially disrupted seasonal change — the “first leaf” of spring arrived nearly three weeks earlier than expected in some places, according to a study by World Weather Attribution.
A few days might not seem like a lot, but precision is crucial to a bird’s welfare. And for nine bird species, the gap in expected arrival was double or triple the rate of other birds. Those nine birds are: great crested flycatchers, indigo buntings, scarlet tanagers, rose-breasted grosbeaks, eastern wood pewees, yellow-billed cuckoos, northern parulas, blue-winged warblers, and Townsend's warblers.
Arriving outside the window of “first leaf” can endanger a bird’s mating habits, limit and shape their diet, and, if they arrive too early, they can get blasted by the remnants of a chilly winter.
When seasons fall out of sync, there is a chain reaction of ecological patterns that get disrupted.
For instance, if plants perceive that the weather has changed earlier than migrating animals do, then they will bloom before the animals can get to them. In the Arctic, some grasses have been appearing a month before average, causing hibernating animals to miss out on an essential source of food, according to The Atlantic.
Further, early or later springs can wreak havoc on farmers who depend on stable weather patterns for planting and harvests. Essential pollinators can get deterred by the changing weather and crops can get harmed by the see-sawing temperatures that have been seen in recent years as winter gives way to spring.
Many birds also help to keep insect populations in check, protecting crops in the process. When birds arrive too late or too early, the distribution of insects can change, disrupting crops.
Other animals are also being disrupted by climate change. More than 80% of the world’s marine creatures are changing their migration patterns as water temperatures rise. In the Arctic, caribou are being affected by growing forest fires attributed to climate change, and rabbits are unable to change their coats from white to brown fast enough to account for early snow melt, leaving them exposed to predators. All told, man-made climate change threatens to eliminate 20% to 50% of all the species on Earth as ecosystems change in drastic ways.
The consequences of these shifts are unknown, but they are bound to be significant. The scientists behind the migration study, however, are optimistic about the resilience of birds.
"If anything could adapt to climate change, you'd think that birds that migrate thousands of miles could," Stephen Mayor, the lead author said in a press release.