Fig. 1. Energy cost of various metabolic functions in relation to body size in birds. "0° Existence" refers to total metabolic costs of caged birds held at 0°C. From Calder (1974).
The relationship between BMR and body size is paralleled by that between size and other metabolic costs, such as for thermoregulation at a given temperature and for activity (Fig. 1; Kendeigh 1970; Tucker 1970; Schmidt-Nielsen 1972; Berger and Hart 1974; Calder 1974). Basal metabolic rate can therefore be used as an index of the overall cost of living as far as metabolic functions are concerned. Small birds must allocate a greater proportion of their energy resources than larger ones to merely staying alive, and have a higher cost of living.
The suggestion in Fig. 1 that it is easy to measure activity costs in a straightforward manner is misleading, because the figure represents measures taken under standard conditions. Factors known to affect the cost of flight, for example, include anatomical adaptations (such as wing loading and wing shape), the type of flight (ascending, descending, gliding), and the speed of flight (Fig. 2; Tucker 1969, 1974; Hainsworth and Wolf 1975). The cost of a series of short flights may be higher than that for a long one because of the extra energy required for takeoff and landing. A few estimates have been made for the cost of flight, mostly in birds moving almost constantly (Lasiewski 1963; Nisbet 1963; Tucker 1972, 1974; Utter and LeFebvre 1970; Berger and Hart 1974), but the methods may be inadequate for birds that fly short distances frequently.
Fig. 2. Energy cost of flying at different speeds and angles as compared with basal metabolic rate (BMR). Solid lines and solid circle refer to flight cost and BMR for budgerigar (Melopsitticus undulatus). Dashed line and open circle refer to flight cost and BMR of the laughing gull. From Tucker (1969).
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 1976a, 1976b 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 1976a, 1976b).
Food Procurement and Processing
Gathering and processing food is another major component of the cost of living. Both the nutritional value of food and its availability (a rather vague term covering both abundance and ease of capture) are extremely diverse and variable, making estimations of foraging cost and benefit difficult (Ashmole 1971; Fisher 1972; Sealy 1975a).