Researchers solve the case of the shrinking red knots

Many animal species become smaller or larger in recent decades, with climate change often mentioned as a cause. Red knots, shorebirds travelling 10 thousand kilometers every year between breeding grounds in Arctic Russia and wintering grounds in West Africa, are becoming smaller. Researchers have now discovered why: in the period that the chicks grow up, their most important food source is less available. They publish their findings this week in Global Change Biology.

Every winter, researchers from the Royal Netherlands Institute for Sea Research (NIOZ) visit the Banc d’Arguin in Mauritania to study red knots (Calidris canutus). In two decades, the body size of young red knots has become smaller. They also noticed that the smallest individuals were measured after summers when the snow melted early on the Arctic tundra.

Research on the tundra

In 2018 and 2019 a NIOZ team joined Russian researchers to revisit a study site at the northern tip of the Eurasian continent in north-central Siberia. From a tent encampment in the snowy tundra they followed red knots, their chicks and their insect food, and compared their findings with results from the early 90s. They found that chicks from recent years often grew slower compared to chicks growing up during the early 90s.

Food too early for the chicks

It appeared that chicks feed mostly on crane flies, which emerge from the tundra soil a month after the snow disappears. In a warming climate, the snow melts earlier and thus the crane flies also appear earlier. Too early for the chicks: they more often miss out on their favourite prey, and as a result grow slower.

Visible in the feathers

Ten thousand kilometres away, on the wintering grounds in Mauritania, there was distinctive change in the feathers of the young birds that made it there.  The composition of feathers grown a few months earlier on the Siberian tundra, showed that over the past two decades they were formed by growing chicks eating fewer and fewer crane flies.

“Earlier snowmelt, an earlier emergence of crane flies, and fewer crane flies when the chicks need them, thus cause young red knots to grow slower,” says lead researcher Tim Oortwijn from NIOZ. “This effect remains visible throughout their life, resulting in smaller red knots every generation.” With the warming being four times faster in the Arctic than in the rest of the world, body size changes are most likely to show up in Arctic-breeding birds.

The paper ‘Demand-Resource Mismatch Explains Body Shrinkage in a Migratory Shorebird’ was published on 16 April 2025 in Global Change Biology