WIERSMA P, BRUINZEEL L & PIERSMA T (1993) Energy savings in waders: studies on the insulation of Knots. LIMOSA 66 (2): 41-52.

The open habitats where many waders winter in the temperate zone are associated with high thermostatic costs for the birds. In this paper we examine physical and behavioural mechanisms allowing waders to save energy. This was studied primarily in the Knot Calidris canutus islandica. Photographs of Knots taken in the laboratory at temperatures ranging from -20 to +30?C showed adjustments in posture and feather position (fig. 2). At lower temperatures, feathers were ruffled up, while legs, carpal joints and back were covered by contour feathers of breast and shoulder. Infrared photos showed that the former parts of the body were least insulated under congenial conditions (fig. 3). The legs showed considerable changes in surface temperature, probably as a result of adjustments in blood flow. Thermostatic costs were estimated by measuring the standard operative temperature with heated taxidermic mounts of Knots (Copper Knots, cf Bakken et al. 1981, 1983). The standard operative temperature is an integrated measure of air temperature, wind speed and radiation. Copper Knots were placed in various habitats in the Wadden Sea wintering area (fig. 6). Open areas (mudflat, salt marsh with sparse vegetation) were the most costly places to be. On a densely vegetated salt marsh, heat loss decreased by 39% (fig. 7). In dense flocks at a nearest neighbour distance of 0.5- I bird lengths, thermostatic costs decreased by 6-9% as compared to solitary Limosa 66 (1993) 2 birds (figs. 7, 8). Standing sideways in the wind increased thermostatic costs by 9% (fig. 7). At lower standard operative temperatures, Knots occurred in denser flocks and more often moved with their heads into the wind (figs. 9, 10). The mean thermostatic cost faced by Knots in the Dutch Wadden Sea ranges from 1.5 W in summer (when few birds are present) to 2.7 W in midwinter (fig. II). If Knots would not adjust their behaviour to the cold, they would incur a 19% higher energy loss in winter (fig. 11). Given the tight foraging conditions in the Wadden Sea at the time, Knots would not survive even an average winter there without behavioural adjustments to the cold. Using the results of Copper Knot measurements, and a model describing the micro-meteorological circumstances for differently-sized birds, we tried to estimate thermostatic costs for two larger estuarine bird species, the Oystercatcher Haematopus ostralegus and the Brent Goose Branta bernic/a. Standing on a mudflat in winter, Knot, Oystercatcher and Brent would have thermostatic costs of 3.0,7.4 and 9.7 W, or 3.2,2.5 and 2.0 x BMR, respectively (fig. 12). These values are 81, 95 and 103 % higher than traditional estimates based on air temperature alone. Red Knot Calidris canutus

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