THE LEARNING ABILITIES OF BIRDS.

SUMMARY 1. The object of the present paper is to survey our knowledge of the learning abilities of birds and consider them in relation to the theory of instinctive behaviour. It is pointed out that the concepts of instinct and of learning are intimately related as complementary aspects of the total behaviour and that one cannot be understood without the other. Besides the definitions necessary for the description of instinctive behaviour and of learning itself, definitions are also given of certain other terms–such as reflex, kinesis, taxis–which are necessary for the full description of behaviour. 2. Habituation is considered to be, in many respects, the simplest type of learning, and isone which is well nigh universal in animals. It is shown to be of vitalimportance in the process of adapting behaviour to generalized environmental stimuli (e.g. sudden sounds, shadows and movements) which may be precursors of danger but which experience has in fact shown to be harmless or of no significance. Habituation is thus caused by some stimulus or situation which puts the animal into a given mood but fails to lead to the release of a particular consummatory act. The more closely linked a stimulus situation (sign stimulus) is with a specific instinctive act, the less likely it is to be the subject of habituation. 3. Examples of the classical conditioned reflex are described. It is pointed out that this is an artificially isolated part of a whole learning process and is only intelligible biologically as part of such a process. As with habituation, it is primarily one of the methods by which innate receptory correlates of the less specific kinds are elaborated to fit the environmental situation more exactly. 4. Trial‐and‐error learning is next defined; it incorporates an active appetitive behaviour which constitutes the process of trial and is characterized by the retroactive action of the influence exerted by the reward. Trial‐and‐error learning is shown to be of immense importance in the development of bird behaviour, particularly on the effector side, in the acquisition of skill involved in perfection of elaborate behaviour patterns, such as flight, nest‐building and escaping from enemies. On the receptor side of such behaviour the conditioning element in trial‐and‐error learning leads to modification of the configuration of stimuli to which the animal is responding. Trial‐and‐error learning also serves to account for many examples of learning with puzzle‐boxes and mazes. The learning function of play in improving skill is also considered as a phase of trial‐and‐error learning. It is shown that “play” is very common in adult birds and is more prevalent in young birds than is usually thought. The danger of confusing maturation of an innate behaviour pattern with improvement due to trial‐and‐error learning is stressed. 5. The nature of insight learning is discussed. Under this heading is considered the type of exploratory learning now known as latent learning, and its significance in the establishment of territory is discussed. Tool using and detour behaviour are also considered a9 possible examples of insight learning. Recent work on the number concept in birds is, for convenience, also dealt with here. Kohler's conclusions that certain birds show two pre‐linguistic faculties, (1) simultaneous, and (2) successive, unnamed number sense, little, if at all, inferior to that of man, are accepted as proved. The essential difference, therefore, between the number concept of animals and men is that only the latter have in addition the ability to deal with “named” numbers. The significance of visual illusion experiments for the theory of insight learning is also considered. Some other learning abilities suggestive of insight are also described. 6. The imitative abilities of birds are considered. A distinction is made between social facilitation, local enhancement and true imitation. The first two play an important part in bird life, but the evidence for true imitation is at present very slender. For reasons which are given, vocal “imitation” is not considered necessarily to involve imitation in the true sense. 7. Imprinting is discussed as an innate disposition to learn in a particular direction and in a particular restricted period. Once accomplished, this type of learning is particularly difficult to eradicate. Its importance for the theory of instinctive behaviour, and especially of the innate releasive mechanism, is stressed. The important evidence supplied by pathological imperfections in imprinting is discussed. The perceptory side of song learning is considered as a special case of imprinting, and the possibility that the learning of territory also involves something of the same nature is considered. 8. Some learning abilities which are at present of doubtful significance are then described. The most important of these is the ability to recognize individuals. The subject of tameness is also discussed, and it is pointed out that genetic tameness and wildness are undoubted realities. No evidence is found for an inherited recognition of man as an enemy, but it is shown that a tendency to general wariness readily becomes attached to man but can be overcome by a variety of learning processes. 9. The memory powers of birds are briefly discussed. Instances of memory for individuals extending for periods of a year or more are well authenticated, and it is possible that memory for territory is often still more enduring. The need for more carefully documented information is stressed. 10. The very scanty information at present available about the anatomical site of learning and the relation of individual variation in learning ability to endocrinological changes is briefly summarized.