160 



E. H. DUNN 



Table 6. Life history data for certain northern seabirds. a 



a Data from Lack (1968) and Ashmole (1971) unless otherwise noted. 



bCullen(1957). 



c Birkhead(1974). 



d Speich and Manuwal (1974). 



ties to defend nesting territory all of which 

 may reduce resources available for reproduc- 

 tion. 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 win- 

 ter 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 

 19746). Others, however, disagree, at least for 

 the breeding season (e.g., Pearson 1968). 



There is some evidence of density-de- 

 pendent population size control in seabirds, 

 although much of it is circumstantial. For 

 example, there are large nonbreeding popula- 

 tions in such diverse species as shags, herring 

 gulls, and Cassin's auklets, which move into a 

 breeding area when established adults are re- 

 moved or colonize new breeding areas (J. C. 

 Coulson, personal communication; Kadlec and 

 Drury 1968; Drury and Nisbet 1972; Manuwal 



19746). Lack (1966) and Ashmole (1971) pre- 

 sented other arguments for density-de- 

 pendence. Density-dependent mortality is dif- 

 ficult to demonstrate, at best, and may be ob- 

 scured by interpopulation movements (Drury 

 and Nisbet 1972). 



If long life is a life history option, a low 

 annual reproductive effort could be favored in 

 several ways. First, it may be necessary for in- 

 suring long life, if breeding has a serious nega- 

 tive feedback on life expectancy (Calow 1973). 

 Second, if survival of offspring is more unpre- 

 dictable than that of adults, low annual effort 

 may be selected so that reproductive effort 

 will not be wasted in years when young have 

 poor chances of survival. Unpredictable and 

 variable first-year survival in seabirds has 

 been documented (Potts 1968; Drury and Nis- 

 bet 1972). In addition, some seabirds show 

 adaptations that allow high reproductive suc- 

 cess in any given year but which do not drain 

 off resources if the season turns out to be poor 

 (e.g., small last eggs in the clutch or asynchro- 

 nous hatching, both of which lead to elimina- 

 tion of the smallest chicks when conditions 

 are poor [Parsons 1970; E. H. Dunn 1973]). 



It should be emphasized that the factors in- 

 volved in the evolution of life histories are 



