ANIMAL BEHAVIOR — WELLS 425 



on moving paper whenever the pigeon hopped on the floor of its cage, 

 or the wren passed through the opening of her nest. Most bird ac- 

 tivities would be difficult to register in this way. There are, however, 

 plenty of hints to be found in the literature that inherent rhythms of 

 quite long period are widespread in birds. Among the beautiful de- 

 scriptions of the herring gull's behavior in Dr. Tinbergen's "The 

 Herring Gull's World" there are several — such as the way in which an 

 incubating bird will get up, as he says "spontaneously," at intervals 

 of one or more hours, shift the eggs a little with its bill, and then sit 

 down again. 



These facts and hints encourage us to believe that a large part of a 

 bird's behavior springs from within — not only in reflex response to 

 physiological urgencies, but also in obedience to timing mechanisms 

 that are essentially arbitrary. As in a polychaete worm, an activity 

 may begin without an immediate need and subside without any satis- 

 faction. This arbitrariness is evident in the detailed structure of the 

 song. The song phrase, if it is to fulfill its biological purpose, must 

 be audible to other birds, it must be sufficiently directional to inform 

 them where the singer is located, and it must be distinct enough from 

 that of other species (and perhaps of other individuals) to identify 

 him. Beyond these requirements there is no reason why it should have 

 one pattern rather than another; the great variety of songs that de- 

 light and sometimes irritate us in spring is largely the expression of a 

 variety of types of innate organization. If we turn our attention to 

 the other notes and calls, we can hear that an individual bird is 

 equipped with a repertoire of alternative rhythmic patterns that 

 modulate or replace each other as its mood changes. All of this is 

 pretty obvious ; the role of the more slowly moving rhythms is less so ; 

 and to myself, at least, it is an exciting idea that changes of activity 

 or mood, occurring at intervals even of several hours, may often be 

 due to the action of arbitrary internal "clocks." It may be that the 

 further investigation of this aspect of behavior, by the continuous 

 study over long periods of the way in which a bird's various acts are 

 distributed in time, might help very materially to bridge the gulf be- 

 tween the province of the field observer on the one hand and that of 

 the physiologist on the other. 



Perhaps the average bird watcher is too much inclined to suppose 

 that every act a bird performs is biologically useful in itself. There 

 are, of course, a number of things the bird must do — eating, breeding, 

 and so on; but on the other hand, like any living animal, it is in a 

 state of organized instability, and can never be completely at rest or 

 even completely steady. A motionless bird, sleeping, for example, 

 while perched on a twig, breathes and stays upright — the latter per- 

 formance involving feats of equilibration which are no less consider- 



