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The importance of bird predation as a selective pressure on the eggs, larvae and pupae of the cabbage butterflies, Pieris rapae L. and P. brassicae L. has been investigated in a rural garden and an allotment. Birds are the main predators of all stages in a well-tended garden whereas in a field the eggs and young larvae are mainly preyed upon by arthropods. The species of avian predators vary according to the stage of development and species of prey. Consequently the relative survival rate of P. rupae and P. brussicue varies with stage of development. This changing pattern of survival has in turn acted as a selective pressure on their two main braconid parasites. Many aspects of the size, shape, colour and behaviour of the larvae and pupae of both species of Pieris appear to be adaptations to the selective pressures exerted by bird predation.