Energetics and nutrition of British and Icelandic redshank (Tringa totanus) during the non-breeding season
Prolonged severe winter weather increases mortality in shorebirds wintering in Britain, with Redshank suffering much more than other species. I studied eco- physiological factors which might affect the chances of survival of Redshank to determine wether British-breeding Redshank (T. t. totanus) and the larger Icelandic breeding Redshank (T. t robusta) are equally affected. I examined the seasonal changes in body mass (BM), total lean mass (TLM) and mass of fat (FM) in wild and captive birds of both races in NE England, using total body electrical conductivity. I measured mass-specific standard metabolic rates in both races at temperatures (T(_a)) between -5ºC and 25ºC using open-flow respirometry. By combining these with measurements of heat loss from heated taxidermic mounts of Redshank, I calculated the effects of air tengerature, wind speed and solar radiation on maintenance metabolism (M(_maint)) of each race. I also measured the composition of carcasses of Redshank which died on the Wash, SE England following prolonged severe weather in February 1991. I found that i) both races accumulated similar fat reserves during mid-winter as an insurance against impending bad weather and food shortages; ii) robusta had a higher mass-specific basal metabolic rate (BMR) and higher thermal conductivity (K(_es)); iii) severe weather mortality in Redshank m Britain coincided with time periods when M(_maint) in both races exceeded 2.5xBMR and daily mean temperatures were -2ºC or below continuously over a period of 5 days; iv) there were no racial differences in foraging behaviour; v) both races died during severe weather as a result of starvation after depleting almost all fat and similar amounts of protein reserves; vi) the reserves of robusta would sustain them without food for 0.5 days less than britannica exposed to the same weather conditions. The differences in BMR between the two races was discussed in terms of differences in breeding latitude and migratory lifestyle. The higher K(_es) of robusta was thought to result from the higher BMR, rather than a lower level of insulation provided by the plumage. During severe weather when food availability is reduced, I would predict that robusta, with their higher mass-specific M(_maint) and larger body size, would find it more difficult to meet their daily energy requirements through food intake alone. Given that the energy obtained by robusta from fat and protein reserves would sustain them for shorter periods than those of britannica, I would predict that during periods of prolonged severe weather, the latter would have a greater chance of survival.