Life History Strategies
The study of life history strategies is largely theoretical, and in the following discussion I do not comment on current theoretical arguments. On the other hand, life history strategies can be regarded as time and energy allocation on a grand scale, and it therefore seems appropriate to look briefly at their implications for seabird management.
Annual reproduction evidently has a negative effect on resources remaining for other functions, and may reduce the chances for an organism to reproduce again in a later season (Cody 1966, 1971; Williams 1966; Gadgil and Bossert 1970; Gadgil and Solbrig 1972; Hussell 1972; Trivers 1972; Calow 1973). If the chances of survival to another breeding season are small, the selective advantage lies with the bird putting the most effort into early reproduction, in spite of its negative effects on survival, because future chances of reproduction are small. If chances of survival are good, however, it may be more advantageous to reduce annual reproductive effort and allocate resources to other functions.
Fig. 16. Time and energy budgets of male and female red-winged (Agelaius phoeniceus) and tricolored (A. tricolor) blackbirds in the breeding season. From Orians (1961). Dotted lines show male (M) activity, dashed lines show female (F) activity, and solid lines show shared activities.
Seabirds are generally long-lived, have small clutches, and generally delay first breeding until at least the 2nd year, and usually longer (Table 6). Phalaropes seem to differ from this pattern (Hilden and Vuolanto 1972; Howe 1975). Several ecological factors (not entirely independent) are believed to contribute to the evolution of the long life and low reproductive effort pattern favored by seabirds.
First, if population size is determined largely by density-dependent mortality, individuals may be favored that allocate resources to attaining longer life (and more chances to reproduce) or insuring greater chances of survival of their offspring (Murphy 1968; Hairston et al. 1970). Density-independent mortality, on the other hand, is so unpredictable that there is no advantage in allocating resources toward protection against it (Gadgil and Solbrig 1972).
Two factors closely linked with density-dependence are high levels of competition, and perennial difficulties in obtaining food. In adapting to these difficulties, a bird may be selected which develops more efficient foraging techniques, wider dispersal, or better abilities to defend nesting territory—all of which may reduce resources available for reproduction. As mentioned earlier, marine foods tend to be patchily distributed, and a long learning period seems to be necessary before seabirds become proficient at foraging. In addition, there is evidence that food availability is low, at least in the tropics, and perhaps in the winter in other regions (Ashmole 1971). If nesting places are in short supply, long life may be favored so that the bird can live long enough for a place to become vacant. Several authors feel that competition is a serious factor in the life of seabirds, both for food (Lack 1966; Cody 1973) and for nesting space (Snow 1960; Belopol'skii 1961; Lack 1966; Manuwal 1974b). Others, however, disagree, at least for the breeding season (e.g., Pearson 1968).
| Species | Annual adult survival (%) | Age at first breeding (years) | Clutch size |
|---|---|---|---|
| Fulmar | 94 | 7+ | 1 |
| Gannet (Morus bassanus) | 94 | (4)-5+ | 1 |
| Manx shearwater | 93-96 | (4)-5+ | 1 |
| Shag | 85 (♂) | (2)-3 | 3-4 |
| 80 (♀) | |||
| Herring gull | 91-96 | 3.5 (♂) | (2)-3 |
| 5 (♀) | |||
| Black-legged kittiwake | 88 | 4-5 (♂) | 3 |
| 3-4 (♀) | |||
| Arctic tern | 89-91 | (2)-3+ | 2 |
| 75 | |||
| 82[45] | |||
| Common murre | 87 | 3+? | 1 |
| Black guillemot | 88+[46] | 3?[46] | 2[46] |
| Cassin's auklet | 83[47] | 3[47] | 1 |
There is some evidence of density-dependent population size control in seabirds, although much of it is circumstantial. For example, there are large nonbreeding populations in such diverse species as shags, herring gulls, and Cassin's auklets, which move into a breeding area when established adults are removed or colonize new breeding areas (J. C. Coulson, personal communication; Kadlec and Drury 1968; Drury and Nisbet 1972; Manuwal 1974b). Lack (1966) and Ashmole (1971) presented other arguments for density-dependence. Density-dependent mortality is difficult to demonstrate, at best, and may be obscured by interpopulation movements (Drury and Nisbet 1972).