Time and energy budgets can be compared among species of birds with very different ecology as a way of summarizing differences and as an approach to determining selective pressures on each species. This paper reviews time-energy use of northern seabirds. Energetic cost of maintenance (basal metabolism, thermoregulation, and procurement and processing of food) depends largely on the following factors: (1) small birds have higher metabolic costs per unit size than do larger ones; (2) body structure affects the cost of locomotion as well as of food procurement; (3) climate affects metabolic costs; and (4) food availability and nutrition vary among food types, and throughout the year within a food type. Little is known of maintenance energetics in seabirds. Time and energy allocations to items beyond basic maintenance are also compared. Patterns and costs of molt and migration are known only in a general way, and the variety of possible patterns suggests that more research would be of value. Almost nothing is known of location and daily activities of seabirds outside the breeding season. The review of breeding season activities is more comprehensive, and stresses the variety of factors known to affect timing, and the total time devoted to and the energetic costs of various aspects of reproduction. Some of these factors are weather, year, geographic location, feeding conditions, age, sex, and distance of food source from the breeding colony. Species characteristics such as clutch size, egg and yolk size, developmental type, growth rate, food type, and behavior combine with environmental variables to make seabirds a very diverse group in time and energy budgeting. Time-energy studies and determination of productive energy (energy remaining after maintenance needs have been met) can be useful in pinpointing those groups of birds and the times of year when birds are most vulnerable to environmental stress. Life history considerations suggest that most seabirds are adapted to low population turnover and would not be able to recover quickly from sudden increases in mortality.
Effective management of a population requires manipulation of the factors most critical in causing population increase or decrease. Deciding what these factors are and which are most suitable for effective manipulation is very difficult due to the complexity of life cycles and possible factors affecting demography. It takes a thorough knowledge of a species and of its relationships with the biotic and abiotic environment to make effective management decisions. The following review of seabird time and energy use is meant to emphasize the wide variation of species ecology within this avian group.
Time and energy patterning is being used as the basis of ecological comparison for the following reason. Any activity of an animal requires time and energy use; therefore, the patterning of use makes a common thread to which allocation to all activities in a bird's life cycle can be related. Time-energy patterns can be compared among birds with diverse food types, habitats, life cycles, and life expectancy, and therefore offer an opportunity for comparison not available through other kinds of analysis (King 1974).
The amounts of time and energy allocated by an organism to different aspects of survival and reproduction should be regarded as being molded by natural selection to optimize (not necessarily to maximize) lifetime reproductive output (Fisher 1958; Williams 1966; Schoener 1971). Thus, differences in time and energy use between species should reflect adaptation to different biotic and abiotic environments. By comparing time and energy use, one can gain insight into the selective pressures on each species and have a basis on which to compare complex ecology more meaningfully than if one listed other types of differences.
This review of time-energy use in northern seabirds cannot be comprehensive, largely because many of the necessary data are lacking. It stresses major areas of difference, however, and points out aspects about which little is yet known.
Cost of Living
Every animal must expend a basic amount of energy on normal maintenance, excluding activities normally allocated to a relatively narrow time span, such as reproduction. This "existence energy" expenditure consists of basal metabolism, thermoregulation, and the costs of gathering and processing food, and could also be referred to as the animal's basic "cost of living." In discussing the components of the cost of living, energy use is emphasized and time largely ignored—partly because metabolism occurs irrespective of time (it is not something the animal can turn off for a period) and partly because time use in normal maintenance and foraging has been little studied.
Metabolism
Basal metabolic rate (BMR) depends greatly on body size (Lasiewski and Dawson 1967; Zar 1968), and the costs per unit size are higher for a small bird than for a large one (Fig. 1). The BMR is somewhat lower in seabirds and other nonpasserines than in passerines of similar size (Dawson and Hudson 1970).
