Vulnerability to severe weather and regulation of body mass of Icelandic and British Redshank Tringa totanus

During severe weather, Redshanks suffer the heaviest mortality amongst all the shorebird species wintering around the North Sea coasts of the British Isles. An earlier study had suggested that this resulted from a failure to accumulate sufficient body fat reserves before mid-winter. Detailed field studies in northeast England between 1993 and 1995 of seasonal changes in body mass, and in estimated lean and fat masses, of two races of Redshank, both of which winter in the same estuary, were accompanied by similar studies of small numbers held in captivity with unlimited food. After differences in body size were allowed for, there were no differences in body composition and its seasonal pattern of change in birds of the Icelandic and British races. Body mass changes in wild birds paralleled those in captives between November and Match, and mid-winter levels were not limited by food supply; indeed they were slightly higher in a winter with lower prey densities, It is concluded that Redshanks regulate body mass and, indirectly, fat reserves at levels set by a trade-off between the risks of predation and starvation. Unlike most other shorebird species, they take very small prey in relation to their body size and hence must feed for long periods during each tidal cycle to achieve their daily energy intake needs. Thus they have little scope to extend their Feeding time during severe weather, which also forces them to feed on ice-free exposed coastal habitats where wind chill cannot be avoided. Both factors lead to more rapid depletion of fat reserves than in other species which have higher energy intake rates or lower total daily requirements.