6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']2 



of high and low temperature in individuals belonging- to the same 

 species, as shown in the table on page 5, are probably to be explained 

 on the grounds of individual temperament in the birds under experi- 

 ment. Thus a bird that soon became accustomed to handling would 

 give a slightly different series of readings from one that remained 

 wild and that struggled violently whenever approached.) 



In the case of those species normally active during the hours of day- 

 light a constant diurnal rhythm was indicated, with a gradual rise 

 until late in the afternoon and then a corresponding decrease until 

 early in the morning. In owls, species of nocturnal habit, the tem- 

 perature curves were reversed, the highest point being observed late 

 at night and the lowest during the day. It was found on the whole 

 that the temperature curves of daylight-loving species were similar 

 to those of diurnal mammals save that the point of highest tempera- 

 ture in the birds came earlier in the afternoon and that of low tem- 

 perature earlier in the morning. 



Hilden and Stenback ' record a series of experiments in which birds 

 were confined in a dark room and their activities regulated by means 

 of artificial light. Light was turned on from 6 p. m. to 6 a. m., with 

 a more brilliant illumination from 9 p. m. to 3 a. m. to correspond to 

 the brighter portion of the middle of the normal day. After the 

 second day the diurnal birds studied in general adapted themselves 

 to this change in condition in such a way that the temperature rhythm 

 was reversed, the highest point in the record for each twenty-four 

 hours coming after midnight instead of afternoon. When the experi- 

 ments were terminated and the birds again led a normal life in relation 

 to daylight the temperature curves at once adjusted to the normal 

 rhythm. 



In studies made by the present writer the range of diurnal tempera- 

 ture is well shown by records for certain of the owls. Thus a male 

 barn owl (Ahico pratincola) killed at 3.00 p. m., as it flew from a 

 perch in a cottonwood tree had a temperature of 101.9°. The day 

 was bright and clear and the bird had in all probability been at rest 

 since early morning. Another male, shot at 8.30 p. m., while quarter- 

 ing back and forth across a level flat near the Gila River in Arizona, 

 showed a body heat of 105.0°. This bird was seen coursing about for 

 several minutes and had evidently been hunting for food for some 

 time. The variation in the screech owls (Otus a^io) also is instruc- 

 tive in connection with the same points. A male and a female taken 

 by hand at 3.30 p. m., from a low pine at the edge of a swamp in 



Skandinavisches Arcli. fiir Phys. Bd. 34, 1916, pp. 382-413. 



