The greatest loss of eggs and nestlings in a wild 
population is through predation. Tinbergen (1960) showed that 
as predation increased, gulls spread their colony over larger 
areas, which because of the criptic coloration of the eggs and 
chicks, and the harrassment of the adults, made it more dife 
ficult for the predator to locate the nests. 
Physical discomforts are not so apt to be lethal to 
nestlings as might be supposed. Barth (1951) found that gulls 
could withstand temperature ranges of 16°C. to 449°C. If the 
temperature was held between 419°C. and 43°0., foureday=-old birds 
died in 38 minutes. An eighteday-old bird reached a temperature 
of 44,2996, in 10 minutes, but did not die. 
Weather plays a role in the survival of the young 
but survival can usually be related to food. Lack and Arn (1947) 
and Lack (1951), as reported by Lack (1954), found that the 
Alpine Swift had greater success in dry years than in wet, foggy 
years because in wet years the insect food was lower, and much 
more difficult to obtain. | 
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