SEASONALITY IN BASAL METABOLIC-RATE AND THERMAL CONDUCTANCE IN A LONG-DISTANCE MIGRANT SHOREBIRD, THE KNOT (CALIDRIS-CANUTUS)

Knots Calidris canutus live highly seasonal lives, breeding solitarily on high arctic tundra and spending the non-breeding season in large s ocial flocks in temperate to tropical estuaries. Their reproductive ac tivities and physiological preparations for long flights are reflected in pronounced plumage and body mass changes, even in long-term captiv es of the islandica subspecies (breeding in north Greenland and northe ast Canada and wintering in western Europe) studied in outdoor aviarie s. The three to four fattening episodes in April-July in connection wi th the flights to and from the high arctic breeding grounds by free-li ving birds, are represented by a single period of high body mass, peak ing between late May and early July in a sample of ten captive islandi ca knots studied over four years. There are consistent and synchronize d annual variations in basal metabolic rate and thermal conductance in three islandica knots. Basal metabolic rate was highest during the su mmer body mass peak. Within the examined individuals, basal metabolic rate scales on body mass with an exponent of about 1.4, probably refle cting a general hypertrophy of metabolically expensive muscles and org ans. Any potential effect of moult on basal metabolic rate was obscure d by the large seasonal mass-associated variations. In breeding plumag e, insulation (the inverse of thermal conductance) was a factor of 1.3 5 lower than in winter plumage. This was paralleled by the dry mass of contour feathers being a factor of 1.17 lower. In this subspecies the breeding season is indeed the period during which the costs of thermo regulation are lowest. In captive knots seasonal changes in basal meta bolic rate and thermal conductance likely reflect an anticipatory prog ramme adaptive to the variable demands made by the environment at diff erent times of the year.