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 1975_a_).
Availability of food varies throughout the year, particularly in marine invertebrates that form the diet of many seabirds (e.g., Spaans 1971; Bédard 1969_a_). High arctic oceans have a very high peak of productivity in the summer, whereas the low arctic has a lower, but longer-lasting, peak (Ashmole 1971). Fish stocks increase in summer as well (Snow 1960; Pearson 1968; Sealy 1975_a_), and decline or disperse in autumn (Potts 1968). Catch-ability may also differ widely from year to year (e.g., E. K. Dunn 1973).
Marine foods are likely to have a patchy distribution, which may make food stocks difficult to locate, even in times of abundance (Ashmole 1971; Sealy 1975_a_). Birds in localities with low food abundance frequently show alterations in time and pattern of foraging, sometimes even changing diets (Cramp 1972; Henderson 1972; Hunt 1972; Lemmetyinen 1972). The time and energy expended in finding and capturing food by different seabird species must vary widely according to the form of foraging used: plunge-diving, beach scavenging, aerial robbing, underwater pursuit, and so on. Even when different species have traveled the same distance to an identical food stock, therefore, the costs of procurement differ.
Time and energy spent foraging depends not only on abundance and ease of capture, but also on nutritional return, and on the age and size of the bird. Fig. 3 shows that the smaller species in a seabird community may spend the most time foraging. Even though this illustration is taken from the breeding season when food demands of the young must be taken into account, it suggests a difference based on cost of living according to size.
[Illustration: Fig. 3. Time spent foraging in the breeding season as a function of body size. From Pearson (1968). AT = arctic tern _(Sterna paradisaea)_, CT = common tern _(S. hirundo)_, ST = sandwich tern _(Thalasseus sandvicensis)_, K = black-legged kittiwake, P = common puffin, M = common murre, LBB = lesser black-backed gull, S = shag.]