424 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



Dr. Hinde has recently described fluctuations in the frequency of 

 various activities, of various birds, and he remarks : "It is character- 

 istic of instinctive activities that they do not occur regularly but in 

 complex patterns of bursts." Nest-building in the great tit, for 

 example "occurs not in more-or-less evenly spaced visits, but in rather 

 irregular bursts of visits with occasional visits between." Of out- 

 standing interest are his observations on the food-begging calls of 

 juvenile blue tits, which are accompanied by particular movements 

 and attitudes. The birds were fed to capacity and then observed for 

 more than half an hour as they gradually became hungry again. 



The record shows that the phrases come in bursts. Increased hunger results 

 not in a gradual increase in the rate of calling, but in an increased frequency of 

 bursts and an increase in the length of bursts. . . . The observations of begging 

 . . . were made under controlled conditions which were very nearly constant 

 throughout each experiment. It is thus certain that the fluctuations do not 

 depend on environmental changes. 



The next two examples are of rather longer cycles. Nearly 30 years 

 ago, Professor Curt Richter described the behavior of a pigeon in a 

 cage which automatically recorded its movements. Every 20 minutes 

 or so, the bird left its perch, hopped around on the floor for a few 

 minutes, and then returned to the perch again. If corn was available, 

 the bird ate every time it jumped down, but the very regular rhythm 

 persisted in the absence of food. The outbursts of restlessness ap- 

 parently coincide with outbursts of contraction in the emptying crop ; 

 and the crop may in fact play a part comparable to that of the 

 esophagus of Arenicola, and set the rhythm of the whole performance. 



More recently Dr. Whitehouse and Mr. Armstrong, using an auto- 

 matic recording device, studied how a wren divided her time between 

 sitting on her eggs and other activities. Her day consisted of "ses- 

 sions" and "recesses." The lower the temperature, the more time she 

 spent on her eggs, as one might expect ; but the sum of a session and 

 the following recess was not affected. The two together occupied a 

 time which fell steadily during the day (quite irrespective of tem- 

 perature) from about 19 to 27 minutes; cold weather simply length- 

 ened the sessions and correspondingly shortened the recesses. This 

 looks like a clear case of an inherent rhythm with environmental tem- 

 perature playing a secondary, modifying role. Every 20 minutes or 

 so, some kind of "physiological alarm clock" goes off inside the wren, 

 and she gets off her eggs ; the colder it is, the sooner she comes back. 

 Later in the season, when she is feeding her young, another rhythm 

 appears; the frequency with which she visits the nest shows a well- 

 marked peak every four or five hours. 



The demonstration of these slow cycles would hardly have been pos- 

 sible without the use of the automatic recording devices, which wrote 



