Appleby suggests that in studying animal behaviour, we tend ‘to concentrate on discrete rather than variable stimuli’ such as the robin’s breast and other sign stimuli. He proposes that the behavioural literature leaves ‘the impression that most stimuli which matter to a.nimals occur in ready-categorized form’, and he argues that the importance of continuous: variation (as opposed to discrete variation) is not adequately realized or acknowledged. I assume that these comments are directed mainly at applied animal behaviour; and Appleby, as co-editor of the major journal in the field, is well positioned to make such a judgement. If he is correct, then this indicates an area where applied workers have much to gain from our non-applied colleagues who have long ago made the transition to analyzing animal behaviour in terms of continuous variation. Flipping through texts such as Huntingford (1984); McFarland (1985); and Krebs and Davies (19841, we are bombarded with examples of continuous variation. On the x-axes of the graphs we see continuous variation in environmental features such as prey density, resource quality, light intensity, territory size, group size, risk of predation, and degree of food deficit. On the y-axes we see continuous variation in behavioural responses: food intake, distance travelled, parental investment, time in patch, and attack rate. Furthermore, non-applied animal behaviour researchers, from behavioural ecologists to motivation theorists, provide many well developed theories and models to deal with continuous variation, often based on combining cost and benefit curves. Given this, I am surprised that the main conceptual model used by Appleby in his own analysis of continuous variation is the ‘supernormal stimulus’, a concept borrowed from an earlier era of ethology and greatly extended in some of Appleby’s examples. I am not convinced, however, that this concept provides the best, or even an adequate, model for understanding animals’ use of space, food, and other commodities. A supernormal stimulus is one for which the animal’s preference or responsiveness is ‘open-ended’ (Anonymous, 1981). For geese retrieving eggs into the nest, the larger the egg, the more strongly the birds respond, to the point that they may ignore their own