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

ABSTRACT.-European Shag (Phalacrocorax aristotelis) nestlings were studied on a small island off the coast of central Norway. Increase in body mass (BM) with age (t, days) was described by the logistic equation: BM = 1,622 g/[1 + e^sup -0.172(1-19.9)^]. All growth parameters measured (body mass, and length of tarsus, wing and head) showed highest relative growth rate when the nestlings were 5-10 days old, that is, before the nestlings had achieved homeothermy. An incipient endothermic response was noted when nestlings were 9 days old, and they became homeothermic at ages of 15-18 days. Respective mass-specific resting metabolic rates for nestlings 0, 15, and 45 days old were 47, 261, and 147% of the predicted value for adult nonpasserine birds of similar body masses. Mass-specific minimal thermal conductance decreased from 366% of predicted adult value at hatching, to 220% of that predicted when nestlings 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 increased rapidly thereafter. Rapid increase in the mass-specific RMR and decrease in minimal thermal conductance is suggested to contribute importantly to improve homeothermic ability during the first two weeks of the developmental period. At hatching, leg and pectoral muscles constituted 5.3 and 2.2%, respectively, of total wet body mass. Relative leg-muscle mass increased rapidly and had almost reached adult proportions when the nestlings were 25-30 days old. In contrast, pectoral-muscle mass increased in an almost direct proportion to the body mass during the first 30 days of the growth period, and increased rapidly thereafter. At hatching, the water fraction (water 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 proportionately greater mass and higher degree of maturity of the leg compared to pectoral muscles, the former would seem to be the main site of cold-induced heat production during early development of homeothermy in European Shag nestlings. Received 30 October 2000, accepted 24 April 2001.

NEWLY HATCHED ALTRICIAL BIRDS do not respond metabolically to low ambient temperatures and are unable to maintain a large temperature gradient between their body and surroundings (Marsh and Wickler 1982, Olson 1992, Visser 1998). Therefore, during the early part of the postnatal period, most heat required for maintenance of their body temperature must be provided by parent birds. Young altricial birds usually acquire control of their body temperature during the middle part of the nestling period, when an improved ability for shivering thermogenesis allows them to produce heat faster than they lose it when cold-exposed (Marsh and Wickler 1982, Choi et al. 1993, Olson 1994). Precocial chicks, in contrast, are able to respond metabolically to low ambient temperatures already when they hatch (Koskimies and Lahti 1964, Steen and Gabrielsen 1986, Ricklefs 1989). Although thermoregulatory capacity varies greatly among different species, precocial chicks are usually able to maintain their body temperature over a wide range of ambient temperatures. Ability of young birds to respond metabolically to cold surroundings has been found to be closely correlated with the functional maturity of skeletal muscles (Marsh and Wickler 1982, Ricklefs and Webb 1985, Grav et al. 1988, Choi et al. 1993, Eppley and Russell 1995).

Although development of homeothermy in young birds has been the subject of many studies, information on that of large altricial birds is relatively scarce (e.g. Dunn 1976a, Evans 1984, Montevecchi and Vaughan 1989). The European Shag (Phalacrocorax aristotelis) is a large altricial seabird that breeds in colonies along the coasts of western Palearctic countries to -72 deg N (Cramp and Simmons 1977). We have shown in a previous study (Ostnes and Bech 1997) that European Shag nestlings already have a well-developed ability to sense cold at hatching. Nonetheless, during the initial part of the development period, cold exposure causes a rapid decrease in their body temperature that only triggers a vocal response. The aim of the present study was to investigate timing of, and mechanisms involved in, achievement of homeothermy in European Shag nestlings.

ACKNOWLEDGEMENTS

I.E.O. was supported by a research grant from the Norwegian University of Science and Technology. The study also received partial support from the Nansen Research Foundation (grants no. 59/95 and 80 /95). We are indebted to the Norwegian lighthouse authorities for permission to use the facilities on Sklinna during our study. The Norwegian Directorate for Nature Management, Trondheim, gave us permission to work in the Shag colony. The comments of two anonymous reviewers were very helpful in revising the manuscript. We also thank Philip Tallantire for improving the written English.