Red knots, a migrating bird species able to fly about 5,000 kilometers non-stop, are becoming smaller as temperatures warm in their Arctic breeding grounds, a study said Thursday.
And these birds will find themselves in a second disadvantage when they reach their wintering grounds in the tropics: their shorter bills cannot reach their favorite shellfish food deep in the tropical sand.
This results in an evolutionary force towards smaller-sized birds with large bills, said the study published in the U.S. journal Science.
Every year in the autumn, red knots fly to winter in tropical coastal ecosystems, notably Banc d'Arguin, Mauritania, in West Africa, and in the spring returns to breed on Taimyr peninsula, the northernmost mainland of Eurasia.
An analysis of satellite images revealed that retreat of the snow at the red knot's breeding grounds and the peak of abundance of insects, their main food, moved for almost two weeks earlier in time.
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Arrival dates of the birds stayed stable, but they missed the peak of insect abundance and the lack of food caused a decrease in the size of the young birds, which is impossible be compensated later in life.
As a result, "juvenile red knots that we caught along the Baltic coast while on their way to West Africa were smaller and had shorter bills after warm Arctic summers," said first author Jan van Gils from NIOZ Royal Netherlands Institute for Sea Research.
Once they have arrived in West Africa, the smallest young birds pay the price of having a short bill: their survival was only half of that of the larger juveniles.
"The reason for this bill-length dependent mortality is quite straightforward. Only larger birds with long bill were able to reach the relatively deeply burrowed bivalves at Banc d'Arguin," van Gils explained.
"Shorter-billed birds were forced to live on seagrass, which is a poor food source for these birds. The poor survival of shrunken first-year birds clearly contributes to the current population decline seen in red knots nowadays."
Nowadays red knots are smaller, but since the short-billed small birds are selected against, the study suggested that they may evolve to have smaller bodies, with long bills.
The researchers also proposed that changes in body size and shape, and the negative population dynamical consequences, could be widespread among other High-Arctic breeding species in the future.
"This is a very serious ecological effect that requires our immediate attention," Van Gils said.
