How Global Warming in the Arctic Kills Birds in the Tropics
Climate change is presenting a bird species called red knots with a knotty problem: It’s causing them to shrink. And that’s killing them.
According to a paper published Thursday in the journal Science, a subspecies of red knot breeds in the Russian Arctic, where temperatures have slowly warmed over the past three decades. That warming trend has changed the timing of food availability in the birds’ Arctic foraging grounds. Snowmelt now can occur more than two weeks earlier than it used to, so insect populations peak before the time when parent birds need to feed their young. As a result, the young birds don’t grow as quickly or as large as they would have in the past.
This affects them later in life, often fatally. Red knots lack the ability to make up for this early nutrition deficiency, so they never grow to their traditional size. This costs them when they migrate to the tropics, where they dine on freshwater mollusks that hide deep in the mud. A normal-size red knot would be able to dig those mollusks out of the mud with its 40-millimeter bill. The smaller birds, however, have 30-millimeter bills. That puts the most nutritious food just out of reach beneath the surface.
A recent report on East Atlantic seabirds found that populations for this red knot subspecies have dropped from more than 500,000 individuals in 1980 to about 250,000 today. This climate change–inspired change in body size now appears to be one of the major factors in the decline.
Beyond the birds themselves, the shrinkage may eventually affect entire ecosystems. Jan van Gils of the Royal Netherlands Institute for Sea Research, the new paper’s lead author, notes that predators such as red knots promote biodiversity by keeping certain prey species in check, which in turn helps to keep sea grass healthy. Humans might feel that impact as populations of red knots and other migratory bird species continue to decline. A companion paper also published in Science notes that “human societies may soon miss many of the ecosystem services, such as pest controls, provided by the masses of migratory species” such as red knots.
Van Gils started his research by trying to understand why red knots’ body and bill length are related to their diet. Analysis of their blood revealed what they were eating, but that wasn’t quite enough. He then looked to a 33-year data set of red knot measurements, part of a long-term monitoring program, which revealed that the birds were becoming smaller over time.
The researchers then started looking at satellite images of the birds’ breeding ground, which provided estimated dates of when Arctic snow began to melt. “There was a very strong positive correlation between body size and date of snowmelt,” van Gils said. “It then became clear that we had a climate change story in our hands.”
The final piece of the puzzle came from van Gils’ ongoing work tagging and tracking red knots at one of their primary wintering sites in Mauritania. “This massive data set showed that the smallest birds are the poorest survivors,” he said. “Then we came full circle and started to realize that their steep population decline is very likely due to this shrinkage effect.”
The same shrinkage may be affecting other birds that breed in the Arctic. Van Gils said his lab is looking at another species, the bar-tailed godwit, which has also manifested smaller bodies and shorter bills. “That’s basically two out of two,” he said. “I am guessing that this problem is widespread for all high-Arctic breeding shorebirds. All of them winter far away in the tropics, all use their long bill to find food, and all have to cope with the rapid Arctic warming.”
That’s a problem that’s not likely to shrink anytime soon.