Shorter beaks make it harder for red knots to forage for molluscs in mudflats.
Just as canaries once warned miners of the lack of oxygen, an iconic shorebird that migrates from Siberia to West Africa is sending warning signals about the impact of climate change on the planet.
In a new study published today in the journal Science, an international team details how the warming of the Arctic by climate change could be responsible for drops in the population of a sub-species of red knot bird (Calidris canutus canutus).
As the Arctic has warmed, red knots — which breed in Siberia — have grown smaller with shorter bills.
When the birds arrive at their winter feeding grounds in tropical West Africa, their bills are too short to reach the best food, resulting in higher mortality amongst the juvenile population, the researchers found.
They suggest the red knot's experience could hold the key to understanding declining populations worldwide of migratory shore birds.
Senior author Professor Marcel Klaassen, at the Centre for Integrative Ecology, School of Life and Environmental Sciences, Deakin University, said the Arctic was warming more rapidly than any place on the globe.
In response, many species were adapting through a process known as "body shrinkage", Professor Klassen said.
"We observed this in the red knot bird and found they are also starting to pay the price [of this body shrinkage] once they arrive in the tropical feeding grounds," he said.
The red knot breeds in the Taimyr Peninsula in Siberia during the Arctic summer, before flying around 9,000 kilometres to Banc d'Arguin in Mauritania, West Africa.
Using satellite data from the past 33 years, the researchers found snowmelt in the high-Arctic breeding grounds of the red knot was starting earlier, changing at a rate of about half a day each year.
Over those three decades Polish researchers on the team caught and measured 1,990 red knots as the birds stopped over in Gdansk Bay on their first migration southward.
These measurements showed the body mass and bill size of juvenile birds were smaller after Arctic summers with early snowmelts.
Professor Klaassen said the red knot breeding season was timed so that chicks would hatch when food in the form of insects was at its most abundant.
"It is a very short season where there is light for 24 hours of the day and the insect life bursts at a certain moment," he said.
The earlier onset of snowmelt meant the chicks hatched after this "peak food" period and had less food available, which then impacted on their growth.
A red knot surrounded by snow at its breeding ground in Siberia, where warming tundra is interfering with the feeding cycle of the birds' chicks.
This had an effect further downstream when the juvenile birds flew to their tropical non-breeding grounds where they used their tapered bills to detect and retrieve molluscs buried in mudflats.
An analysis of the birds' blood showed individuals with a 40-millimetre bill had access to about two-thirds of the bivalves, whereas a bird with a 30-millimetre bill was able to access only one-third.
"The short bills can't reach the good food which is the shellfish and essentially become vegetarians," he said.
As a result there was a massive increase in mortality rates.
Professor Klaassen said tagging of 2,381 red knots from 2002 to 2013 and subsequent re-sightings suggested the shorter-billed birds had a 40 per cent chance of survival.
This compared with longer-billed juveniles who had an 80 per cent chance of living.
He said the finding was unexpected as it was previously believed the drop-off in bird populations was due to changes in the migratory paths linked to increased human activity and changes to the environment.
"This is additional bad news," he said.
Two different sub-species of red knot also migrate from Siberia to Australia during their non-breeding season.
The birds were one of 49 species recently added to the endangered list.
Professor Klaassen said in Australia there was masses of data on migratory birds already collected by "legions of volunteers".
However while counts among migratory birds in Australia were also on the decline no one had looked at whether there had been changes in their body architecture.
"This study has opened our eyes. I really hadn't expected this and am keen to learn if there are similar issues here [among migratory birds]," he said.
Professor Klaassen said more and more of these migratory birds in Australia were being placed on the list of critically endangered animals.
"Doing these long distances they need places where there is plenty of food and safety so they can concentrate on growing fat for the journey," he said.
"We need to set aside areas so we can give them a chance of coping with these changes."
A formation of migrating red knots skims the ocean waters en route from Siberia to West Africa.
In an accompanying opinion piece in Science, Professor Martin Wikelski, from Germany's University of Konstanz and Dr Grigori Tertitski, from the Russian Academy of Sciences write that the results show global warming affects life on our planet in unanticipated ways.
"Changes in one habitat may have important ecological consequences in habitats halfway around the world," they write.
They said population-level changes were occurring in many migratory animals throughout the world and cited the loss of more than 400 million songbirds in Europe over the past 30 years.
"Human societies may soon miss many of the ecosystem services, such as pest control, provided by the masses of migratory species," they said.
There was potential for these population-wide changes to be used to forecast changing conditions on the planet, but more study was required.
"Once we understand the connectivity and interactions of individual animals, we can capitalise on the superior sensing of the environment that emerges from their collective behaviour," write Professor Wikelski and Dr Tertitski.
"We may then be able to rely on animals to forecast the conditions of life on the planet that we share with them."
A red knot uses its beak to forage for molluscs on a shoreline. Arctic changes are having an impact on the birds' beak sizes, making it more difficult for them to get food.