DURING the spring of 1948 George Edwards, Eric Hosking and stuart Smith carried out experiments to test the reactions of certain of the smaller birds to presence of stuffed cuckoo and other animals in their nesting territories (British Birds, 42, No. 1 ; January 1949). Experiments were of an exploratory nature, but enough evidence has been collected to show that certain birds appear able to determine one type of dummy from another. Willow-warblers will attack a stuffed cuckoo but will not go near a stuffed sparrowhawk. Nightingales will attack both hawk and cuckoo, but fear a stuffed stoat. Blackbirds disregard a cuckoo but attack a jay. Tree-pipits attack a cuckoo violently, but will not approach a jay ; whinchats attack both cuckoo and sparrow-hawk. The article contains a detailed account of these reactions as well as many excellent close-up photographs which were taken during the experiments.