146 



E. H. DUNN 



BREEDING MIGRATION MOLT 



SABINE'S GULL 

 ARCTIC TERN 



SHORT-DISTANCE 



MIGRANTS 



(GULLS, PETRELS, 



CORMORANTS) 



SHORT- DISTANCE 



MIGRANTS 

 (MANY ALCIDS) 



GLAUCOUS GULL 

 CASSIN T S AUKLET 



IVORY GULL 



WINTER 



SPRING 



SUMMER 



FALL 



Fig. 4. Typical patterns of generalized annual cycles in reproduction, migration, and wing molt in northern 

 seabirds. Solid line shows reproductive season, dotted line the period of migration or dispersion, and 

 dashed line the period of annual primary molt. Data from Dorst (1961), Stresemann and Stresemann 

 (1966), and Ashmole (1971). 



Migration 



Among northwestern North American sea- 

 birds, most coastal feeders, such as gulls, cor- 

 morants, and many alcids and petrels, have 

 only a short southward migratory movement, 

 and many others are more or less resident 

 (Dorst 1961; Ashmole 1971). Terns, on the 

 other hand, migrate long distances in a short 

 time to places where small fish are available 

 near shore in the winter. Other long-distance 

 migrants Sabine's gull (Xema sabini), 

 jaegers (Stercorarius spp.), pelagic phalaropes 

 (Phalaropus, Lobipes), and kittiwakes (Rissa 

 spp.) tend to scatter widely over the south- 

 ern ocean, concentrating near areas of upwell- 

 ing (Dorst 1961; Ashmole 1971). Groups such 

 as murres (Uria spp.), eiders (Somateria spp.), 

 and grebes (Podiceps spp.) may move con- 

 siderable distances by swimming (Dorst 1961; 

 Tuck 1960). True migration tends to take 

 place directly before and after reproduction, 



whereas dispersal or nomadism takes place 

 over a long period of the winter (Fig. 4). 



Among species remaining in the northern 

 hemisphere, younger birds frequently dis- 

 perse greater distances than do breeding 

 adults (Coulson 1961; Kadlec and Drury 1968; 

 Southern 1967), and the degree of dispersal 

 can vary among colonies of the same species 

 (Coulson and Brazendale 1968). 



Energy costs of migration must vary ac- 

 cording to distances covered and amount of 

 time allocated to migration. Aside from the 

 references to cost of flight mentioned earlier, 

 however, migratory costs have scarcely been 

 studied. Dolnik (1971) has estimated that 

 chaffinches (Fringilla coelebs) expend about 

 as much energy migrating south as they 

 would on thermoregulation if they overwin- 

 tered on their breeding grounds. Long-dis- 

 tance migration is presumably selected be- 

 cause the birds are able to collect food more 

 efficiently, because the risks of death or in- 



