Growth and development of homeothermy in nestling European shags (Phalacrocorax aristotelis)

European Shag (Phalacrocorax aristotelis) nestlings were studied on a small island off the coast of central Norway. Increase in body mass (BM) with ag e (t, days) was described by the logistic equation: BM = 1,622 g/[1 + e(-0. 172(1-19.9))], All growth parameters measured (body mass, and length of tar sus, wing and head) showed highest relative growth rate when the nestlings were 5-10 days old, that is, before the nestlings had achieved hemeothermy. An incipient endothermic response was noted when nestlings were 9 days old , and they became homeothermic at ages of 15-18 days. Respective mass-speci fic resting metabolic rates for nestlings 0, 15, and 45 days old were 47, 2 61, and 147% of the predicted value for adult noripasserine birds of simila r body masses. Mass-specific minimal thermal conductance decreased from 366 % of predicted adult value at hatching, to 220% of that predicted when nest lings were 21 days old. For nestlings 15 days old, the factorial metabolic scope (resting metabolic rate/peak metabolic rate) was only 1.5, but that i ncreased rapidly thereafter. Rapid increase in the mass-specific RMR and de crease in minimal thermal conductance is suggested to contribute importantl y to improve homeothermic ability during the first two weeks of the develop mental period. At hatching, leg and pectoral muscles constituted 5.3 and 2. 2%, respectively, of total wet body mass. Relative leg-muscle mass increase d rapidly and had almost reached adult proportions when the nestlings were 25-30 days old. In contrast, pectoral-muscle mass increased in an almost di rect proportion to the body mass during the first 30 days of the growth per iod, and increased rapidly thereafter. At hatching, the water fraction (wat er content/ lipid-free wet mass) was significantly lower in the leg than in the pectoral muscles (0.920 vs. 0.931). The water fraction of leg muscles also remained lower during the entire growth period. Judging from the propo rtionately greater mass and higher degree of maturity of the leg compared t o pectoral muscles, the former would seem to be the main site of cold-induc ed heat production during early development of homeothermy in European Shag nestlings.