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Summary In the neonatal life of young nidifugous birds, there are certain crucial periods determining the development of the following response. The sensitive period is the period during which it is possible to elicit the first following response in a naive young bird. The unconditioned stimuli eliciting this response are both visual (perception of movement) and acoustic (in ducklings and domestic chicks, perception of short and intermittent sounds). With increasing age the following tendency decreases, whereas there is an increasing tendency to respond to moving objects by escape, which apparently is one of the factors that limit the sensitive period. Eliciting the following response initiates a learning process, called imprinting, which gradually results in a strong preference for the object first followed and in a persistence beyond the sensitive period of a following response to that object. Although some imprinting seems to be possible throughout the whole sensitive period, the length of which may exceed one week, there is a restricted subperiod of maximum imprintability within this period. The strongest and most complete imprinting can only be attained during this critical period, the length of which rarely exceeds some few hours. The strength of the imprinting produced during the critical period is dependent on several factors, the most important of which seems to be the effort expended by the young bird in following. Like the sensitive period, the critical period seems mainly to be limited by the development of fear responses, and the onset of both periods is apparently determined by the development of a motor ability sufficient to make the following response possible. Under experimental conditions, the length of these crucial periods can be changed by several factors, such as deprivation, social facilitation and the use of drugs reducing fear or slowing down metabolism. In naive mallard ducklings, the following tendency is maintained at a fairly high level during the major part of the sensitive period, and the susceptibility to auditory stimuli eliciting following even reaches its peak at an age far beyond the critical period. This is apparently a consequence of the conditions in nature, where the departure of the young from the nest may often not occur until after what under experimental conditions appears as the critical period, and where a successful accomplishment of the subsequent journey to the nearest water is dependent on the maintenance of a high responsiveness to stimuli eliciting following and particularly to the acoustic stimuli, on which they rely whenever visibility is restricted by dense vegetation. Besides the critical period for maximum imprintability, there may also be other important subperiods within the sensitive period. Thus, there are observations indicating the existence of a period of maximum effect on the persistence and strength of the following response disregarding the development of discrimination, and it seems possible that other elements of the following response and impri...

Electrical stimulation within restricted parts of the hypothalamus and the preoptic area of the pigeon was found to elicit polypneic panting, hyperphagia and polydipsia. The spatial arrangement in the brain giving rise to the different effects corresponded well to that found in mammals. It may therefore be assumed that central regulation of body temperature, food and water intake is principally the same in birds and mammals.

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