NO. 12 BODY TEMPERATURE OE BIRDS WET MORE 9 



channels there is a gradual accumulation that warms the tissues and 

 thus is held. This accumulation reaches its height near the close of 

 the day's activities. At night the body is inactive so that at once the 

 production of heat is lessened. The stored up heat energy of the 

 day therefore is slowly dissipated and is replaced only in part, so 

 that there is a steady lessening of the body temperature until the 

 beginning of a new period of activity. ^Xkh this there is a sudden 

 jump again in the production of heat and a corresponding increase 

 in body temperature. In the case of our small birds that migrate by 

 night the increased activity induced must of necessity make a decided 

 break in the daily temperature rhythm. No data on this point is avail- 

 able, but it must be supposed that the prolonged flights that are made 

 tend to bring the temperature above the normal for individuals at rest. 

 A part of the accumulated heat secured during the day may be attrib- 

 uted to the continual ingestion and digestion of food, as the nutriment 

 thus secured would be provocative of renewed energy. As digestion 

 is rapid the stomach is soon empty when no food is being taken in 

 so that the temperature of night-flying migrants may be held down to 

 some extent by a decrease without constant renewal in the stored 

 nutriment in the tissues. 



Although in birds there seems at present no indication of a m?.rked 

 seasonal variation in degree of temperature yet we may suppose that 

 the total amount of heat produced by the body may be slightly more in 

 summer than in winter. As the period of daily activity is one of high 

 temperature (considered on the basis of diurnal species) it will be 

 readily seen that that period of activity is much longer, in species 

 living north or south of the tropics, during a day in June than during 

 a corresponding period in December. There is therefore a contrast 

 in many birds in the total amount of heat produced in the seasons of 

 summer and winter even though no birds are known to hibernate. 

 Although it may be supposed that the increased activity in simimer 

 among our small birds may be ofiFset in part by brief siestas taken in 

 the heat of the day, still it would not seem that the diflFerence between 

 the two seasons would be am'where near compensated. The quantity 

 of body heat produced during the summer period may therefore be 

 considered greater than for the winter. 



VARIATION IX TEMPERATURE IX RELATION TO SEX 

 Variation in body temperature correlated with sex has been well 

 established in mammals, where the temperature of the female is stated 

 to average slightly higher than in the male. Thus Roger* records a 



^Richet's Diet, de Phys., Vol. Ill, 189^, p. g6. 



