Availability of food varies throughout the year, particularly in marine invertebrates that form the diet of many seabirds (e.g., Spaans 1971; Bédard 1969a). 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 1975a), 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 1975a). 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.
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.
Age of the bird affects time and energy commitment to foraging because younger birds are often less skilled at capturing food. This has been noted particularly in long-lived seabird species (Orians 1969; Dunn 1972; LeCroy 1972; Buckley and Buckley 1974; Barash et al. 1975). Older juveniles may be excluded from feeding areas by more experienced, territorial adults (Moyle 1966), whereas immatures are not (Drury and Smith 1968; Ingolfsson 1969).
Nutritional and energetic return obtained from food is a very important factor in foraging strategy that has not received the attention it deserves. Table 1 lists the caloric value of various foodstuffs and illustrates how little is known about foods eaten by seabirds. Although caloric content and abundance of food have often been accepted as the most important determinants of foraging strategies (Bookhout 1958; Emlen 1966; West 1967; Bryant 1973), they may frequently be less important than nutritional value and digestibility, also shown in Table 1 (Pulliam 1974).
Since fish seem to be highly digestible, most of the energy contained in them is available to the consumer. There are, unfortunately, no data on the digestibility of marine invertebrates, but those for insects suggest that digestibility, at least of crustaceans with exoskeletons, is somewhat lower than that for fish. A bird would therefore have to eat a larger biomass of invertebrates than of fish to satisfy the same energetic needs (although cost of procurement might not be as high as for fish).
| Food type | Kcal/g fresh wt. | Percent fresh wt. composed of: | Digestive efficiency | Kcal metabolizable energy/g fresh wt. | ||
|---|---|---|---|---|---|---|
| H2O | Fat | Protein | ||||
| Vegetable | 1.2-5.2 | 59-86 | 0.4-3 | 3-5 | 30-32 | 0.3-2.3 |
| Tropical fruits | 1.2 | 75 | 8 | 1 | ||
| Various seeds | 4.0-7.3 | 3-13 | 1-40 | 10-29 | 76-80 | 3.0-5.2 |
| Various insects | 1.4-5.2 | 65-75 | 1-3 | 9-18 | 66-69 | 0.9-3.5 |
| Whiting (fish) | 1.1 | 81 | 79 | 0.9 | ||
| Various freshwater fishes | 1.2 | 75 | 5 | 18 | 81 | |
| Mix of fish eaten by double-crested cormorants on NE coast | 1.1 | 74 | 1 | 16 | 82 | 0.9 |
| Fresh herring and mackerel | 1.9 | 67 | 13 | 19 | ||
| Garbage ("average" mix) | 1.5 | 67 | 8 | 19 | ||
| Crab with eggs | 1.0 | 68 | 5 | 1 | ||
| Euphausid shrimp | 0.8 | 80 | 2 | 1 | ||
| Clam (edible part only) | 0.8 | 80 | 1 | 13 | ||