SOURCES OF VARIATION IN MORTALITY FROM OIL POLLUTION 



A large number of factors are involved in determining the magnitude of det- 

 rimental effects of oil pollution on marine birds. Birds oiled in cold weather 

 and cold waters have a much higher fatality than do those in warm weather and 

 warm waters. Even minimal amounts of oil may lead quickly to death under the 

 stress of a cold environmental regime (Levy 1980), but birds in warmer areas may 

 survive the same degree of oiling (R. Clapp, pers. observ.; C. Harrison, pers. 

 comra.). Reports from Europe (Bourne and Bibby 1975, Riisgard 1979) indicate 

 that mortality from oiling is greater during the winter months than during the 

 summer. 



Oil spilled in cold water remains liquid longer than in warmer water and 

 is likely to cause more damage as a result. It first forms a "chocolate mousse" 

 water-in-oil emulsion and then becomes tar-balls. Although these forms of oil 

 may present some hazard to birds (Bourne and Bibby 1975), the hazard of fresh 

 oil is apparently much greater. 



Bourne (1976) summarized some of the changes in daily, annual, and life 

 cycles of marine birds that may increase their vulnerability to oil pollution. 

 Local currents and winds may bring drifting slicks into rafts of birds roosting 

 on the water. Bourne and Devlin (1969) suggested that most mortality from oil- 

 ing occurs when roosting or feeding birds are trapped by drifting slicks. 



Breeding populations are particularly susceptible to oil. The loss of one 

 member of a pair may mean complete loss of their reproductive potential for that 

 year. Depending on the number of offspring usually produced, this could mean 

 that every breeding bird killed by oil represents a theoretical loss to the pop- 

 ulation of two birds or more. Although this loss may be recouped in future gen- 

 erations, most marine birds have relatively low productivity and their popula- 

 tions may take many years to recover from one severe oiling incident. Oil in 

 the vicinity of breeding colonies may also diminish reproductive success in 

 other ways, by causing a decrease in the hatching success of contaminated eggs, 

 and by disturbance to the colony resulting from attempts to control pollution 

 (Bourne 1976). 



Bourne (1976) also pointed out that marine birds are particularly suscep- 

 tible to damage from oil when they are molting. When birds lack their usual 

 insulation, smaller than usual amounts of oil may lead to death from chilling, 

 shock, and starvation. Some waterfowl perform a molt-migration in which large 

 numbers gather away from the breeding ground to renew feathers prior to continu- 

 ing migration. Some molt in late summer, others in the spring just prior to 

 their migration north. Birds in such concentrations are more likely to die in 

 large numbers than those of normal mobility. 



Few observations on the behavior of birds encountering oil have been re- 

 ported. Information available indicates that differences in behavior between 

 species may increase or decrease their vulnerability. According to the Inter- 

 national Council for Bird Protection (1960), Long-tailed Ducks (Oldsquaw) will 

 choose to land on oil slicks. If true, this may account for some of the very 

 high oil-related mortalities that have been reported for this diving duck. 

 On the other hand, Guillemots (Common Murres) dive to escape floating oil but 



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