Pi. Mitchell et al., Vulnerability to severe weather and regulation of body mass of Icelandic and British Redshank Tringa totanus, J AVIAN BIO, 31(4), 2000, pp. 511-521
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 s
tudies in northeast England between 1993 and 1995 of seasonal changes in bo
dy mass, and in estimated lean and fat masses, of two races of Redshank, bo
th 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 compositio
n 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 reserv
es at levels set by a trade-off between the risks of predation and starvati
on. Unlike most other shorebird species, they take very small prey in relat
ion to their body size and hence must feed for long periods during each tid
al 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.