Arctic Warming Shrinks Birds With A Price To Pay In Tropics

Red knots foraging in Mauritania. Many will find their beaks too small for the best food. Jan van de Kam

Rising arctic temperatures are causing a mismatch in migrating birds. The birds are shrinking in the heat, improving their prospects in their breeding grounds, but at least one species is paying the price when it winters in the tropics, losing one of its favorite feeds.

Allen's rule observes that animals that generate their own body heat, such as mammals and birds, minimize their surface area, relative to size, in cold climates. Getting big is one way to do this, while in hotter environments the same species, or its relatives, will be smaller.

In this context, it is expected that species will shrink in response to global warming. Studies have observed this, albeit incurring the wrath of Australian politicians in the process.

However, a study of red knot birds (Calidris canutus) has uncovered something far more alarming. Red knots breed in the Arctic, and have responded to higher temperatures by reducing in size over recent generations. During the northern winter, they fly to West Africa, where they feed on Loripes bivalves buried in the tropical sands.

Red knots flight path from Siberia to West Africa. Van Gils et al/Science

A paper in Science has shown that the red knots' beaks are also shrinking. This presents a problem, as the smaller beaks cannot reach the more deeply buried molluscs. “An individual with a 40-mm bill has access to about two-thirds of all Loripes, whereas a bird with a 30-mm bill is able to access only one-third,” the paper notes.

The paper's authors measured red knots in Poland on their way from their Siberian breeding grounds, and again in Mauritania. In years when snowmelt came early to the breeding grounds, birds were smaller and their survival rates were down.

Senior author Professor Marcel Klaassen of Deakin University, Australia, told IFLScience the year-to-year variation was probably caused by poor nutrition due to the red knots' primary Arctic food increasingly peaking before they hatch. “However, there is also a long-term trend that may be an evolutionary effect,” he added.

Klaassen also noted to IFLScience that bill size is shrinking more slowly than body size, presumably reflecting the advantage of being able to reach more bivalves, but this brings its own price. Larger bills are of no benefit in catching red knots' Siberian prey, mainly arthropods, and the additional carry-on baggage hampers these long-distance fliers.

Klaassen said: “All migratory birds, particularly long distance ones, must face competing pressures in different parts of their range. Being big is an advantage in the Arctic, particularly at the start and finish of the season, but better to be small in the tropics.” He thinks this may be the first sign of differential rates of warming causing problems for a species that had once evolved a comfortable balance, but it is unlikely to be the last with the poles warming far faster than the tropics. And Klaassen told IFLScience he is keen to sift through decades of data on Australian wading birds to see if similar patterns are evident.

An editorial in the same edition of Science described the red knots as “living sentinels for climate change effects.”

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