By Alister Doyle
OSLO (Reuters) - Red knots, a type of bird that makes one of the longest annual migrations, are shrinking because climate change in their Arctic nesting grounds makes life harder during their winters in Africa, scientists say.
Snows in Arctic Russia now melt earlier in spring and many red knot chicks hatch too late for the annual peak of insect food spurred by the thaw, according to their report on Thursday, one of the first to link the impact of warming to a single species.
That food shortage means the shorebirds, known for the males' reddish plumage, grow up smaller with shorter bills that make it harder to dig up their favored shellfish that live deep in tidal mudflats in wintering grounds in Mauritania.
Eighty percent of the birds born in Russia with long beaks survived to adulthood against just 40 percent of the short-beaked red knots, which end up eating roots of sea grasses in Africa that are less nutritious than shellfish, the study found.
"It's worrying ... we speculate that this is a very general problem" for Arctic migratory birds, lead author Jan van Gils of the Royal Netherlands Institute for Sea Research told Reuters of the findings published in the journal Science.
Thursday's study, by researchers in the Netherlands, Australia, France, Poland and Russia, drew on 33 years of satellite data of snows and observations of the size and feeding habits of thousands of birds.
Many types of shorebird fly to the Arctic to nest to avoid predators, from falcons to snakes, in the tropics. Some red knots born in Alaska fly all the way to South America.
The study suggested that red knots may evolve to have smaller bodies, with big bills. Red knots were also flying slightly earlier to the Arctic, but not soon enough. In Africa "they lack the cues of an earlier Arctic summer," van Gils said.
Red knots grow to about 25 cms long (10 inches) long. Their global population is falling, according to a Red List of endangered species compiled by experts, who reckon the species is not at risk now but may be in future.
Some other studies suggest that many creatures may shrink in a warmer world, because smaller bodies can get rid of extra heat more easily than big bodies.
(Reporting by Alister Doyle; Editing by Tom Heneghan)