Observations on the Comparative Anatomy of the Avian Brain

OBSERVATIONS ON THE COMPARATIVE ANATOMYOF THE AVIANBRAIN STANLEY COBB, M.D., D.Sc. (Hon.)* Birds have two conspicuous characteristics: one is their specialization in form and function, the other is the great extent to which instinct governs their behavior. Within a given species birds are much alike; similarity in anatomy and behavior is the rule. Some birds have remarkable learning ability and develop considerable individuality, but this is more conspicuous in mammals. It is generally accepted that the cerebral cortex ofman is the organ essential for the discriminative adaptations that make intelligent behavior possible. (Throughout this paper "intelligence" will be defined as the ability to adapt effectively to new and varying environmental situations .) There is no satisfactory evidence indicating which structure in a bird's brain subserves this function. The simple methods ofcomparative anatomy, when used in connection with knowledge ofa bird's behavior, in the field or laboratory, may give clues concerning the function ofdifferent parts ofthe bird's brain. Special adaptations in the general anatomy ofbirds are so common and striking that it seemed important to compare the brains ofmany durèrent birds in the hope offinding behavioral correlations. Much careful work has been done on a few species, but a broader survey ofthe brains ofthe whole class Aves has never been systematically carried out. This general paper describes some preliminary studies and outlines some general problems. I. Instinctive Behavior When one compares the behavior ofbirds and mammals, it is obvious that birds are usually more stereotyped. One is inclined to agree with Mrs. * Bullard Professor of Neuropathology, Emeritus, Harvard University. The work reported in this paper was aided by a grant from the Foundations' Fund for Research in Psychiatry. 383 Scherman (?) that "we cannot envy the birds for the set and narrow paths oftheir uncompromisingly efficient lives." Lower forms such as reptiles, amphibians, and fish are even more fixed in their actions than birds, because more oftheir patterns ofbehavior are inborn and modified but Utile by experience. This is not the place to go into a long discussion of "instinct " as a biological concept. The thesis that all behavior is either inherited (unlearned, instinctive) or acquired (learned) is too simple to meet the facts. It is a question ofgenetically determined processes being influenced by environmental stimuli. Morphology and behavior are the result ofan interaction ofboth heredity and environment. Concerning birds, we can say that their behavior is more instinctive than that of most mammals, and much more so than the behavior of man. Much has been written recently, especially in England, about the behavior ofbirds. The ethologists have given careful descriptions ofthe life histories ofdifferent single species with emphasis on how much one individual acts like another. They bring out the importance ofinherited patterns ofbehavior and ofrituals characteristic ofa species. Tinbergen in his Herring Gull's World (2) has produced a beautiful example ofthis kind of detailed observation. He leaves little doubt in thereader's mind that a great part ofthe behavior ofa gull is inherited, stereotyped, and ritualized. Certain acts or series ofacts are performed in response to certain stimuli. The mating, nesting, and parental performances are specialized, highly developed , and so uniform for the species that one is convinced that these intricate patterns ofbehavior are almost entirely instinctive. Lorenz (3, 4) has made similar studies ofducks and geese and lays stress on the important part played by heredity. Marshall (5) in his description ofthe bower-birds of Australia and the neighboring islands gives the most astonishing examples ofthe building ofavenues, bowers, and stages by these crow-like birds. Their constructions are often truly artistic productions. Some ofthe bowers are so characteristic that they are said to be better taxonomic guides than the skins of the birds themselves. The riddle ofmigration and the "homing" ability of birds is far from solved. But experiments on free and caged birds are beginning to give evidence that their remarkable feats ofnavigation are based on visual cues from the sun and even the stars. Starlings have returned to their nesting places in Germany from as far away as 1,250 miles. Sauer (6) in Freiburg 384 Stanley Cobb · Avian Brain Perspectives in Biology and Medicine · Spring i960 has made a...