Little work has been done on the cost of locomotion in seabirds: that of Eliassen (1963) on great black-backed gulls _(Larus marinus)_, Berger et al. (1970) on ring-billed gulls _(L. delawarensis)_, and Tucker (1972) on the laughing gull _(L. atricilla)_, and indirect calculations of soaring flight characteristics in albatrosses, _Diomedea_ spp. (Cone 1964), and the fulmar, _Fulmarus glacialis_ (Pennycuick 1960). Swimming has been shown to be more costly than flying in ducks (Schmidt-Nielsen 1972) and may be for seabirds as well. More energy is also probably used in underwater swimming than in flying.
The energetic costs of thermoregulation under natural conditions are not easy to estimate. Thermal energy is gained from and lost to the environment, and the degree of exchange depends not only on air temperature but also on metabolic rate, insulation, body temperature, posture, humidity, convection, and radiation. Radiation, in turn, depends on cloud cover, shade, and reflective and absorptive characteristics of the organism and of the environment (Porter and Gates 1969; Calder and King 1974). Most of these quantities are changing constantly, and insulation and metabolic rate may vary on a seasonal basis with acclimation (Dawson and Hudson 1970).
At present, no direct measurement technique exists for determining natural thermoregulatory costs, although a few estimates have been made (King 1974), including several for seabird nestlings (Dunn 1976_a_, 1976_b_ for double-crested cormorants, _Phalacrocorax auritus_, and for herring gulls, _Larus argentatus_). For most birds, the temperature environment actually faced over a year's time has never been measured, and for no bird has a full description of the complete thermal environment been made. It is clear that climate and degree of exposure are important elements in the basic cost of living, and that thermoregulatory costs average higher in small birds than in larger ones, but beyond that little information is available. Work on thermoregulatory costs of free-living chicks of two species of seabirds suggests that insulative properties can lead to marked differences in the metabolic costs of different species in an essentially identical environment (Dunn 1976_a_, 1976_b_).