to predict that differing bird species should exhibit, under 

 normal conditions of health, differing body temperatures, 

 and perhaps, in a given individual, this normal temperature 

 should be found to vary according to the stage of incuba- 

 tion, since the parent's body heat is that which develops its 

 embryo in practically all of the myriads hatched each year. 



It then becomes self-evident that the optimum incuba- 

 tion temperature for any species is the temperature of the 

 incubating parent (true or foster). 



The physiology of a bird's temperature is not nearly 

 so well known as that of man and other warm-blooded ani- 

 mals, but there is, nevertheless, enough information on the 

 subject to enable one to get a fairly comprehensive view of 

 its physiologic characteristics. 



Birds are homoiothermic ; they have in health a body 

 temperature which is relatively characteristic and of a con- 

 stant curve, one peculiar to the family, the genus, or pos- 

 sibly even to the species. This temperature, with all species 

 so far studied, has a daily swing, being highest, with diurnal 

 birds, between noon and six in the evening, and lowest be- 

 tween midnight and six in the morning, these extremes 

 being reversed with nocturnal birds (167). The amplitude 

 of this daily temperature swing varies with different species, 

 and seems to be correlated more or less closely with the 

 bird's size, since Simpson (167) found it to be widest with 

 small birds and narrowest in large birds, there being 7.68 F. 

 between the extremes with a thrush, and 1.65 F. for a duck, 

 and with birds of intermediate size this swing was inter- 

 mediate in amplitude. This daily swing of body tempera- 

 ture corresponds fairly closely with the bird's activity, being 

 lowest when it is at rest, i. <?.* the hen's temperature is lower 

 while incubating than when active; it remains nearly con- 

 stant during most of the period of incubation, and rises only 

 when the hen becomes more active and "excited" at the 

 hatching of the eggs. It is said (164), however, that the 

 temperature of the nest is lowest in the first week of incu- 

 bation and highest at the end, a change possibly not due to 

 the hen's temperature alone, but also to the eggs themselves 

 producing heat as the embryos develop. This twenty-four 

 hour rise and fall of temperature also obtains with man, 

 and is also slightly correlated with his condition of activity 

 or rest; the curve can be reversed if his daily mode of life 

 be reversed, a fact throwing light on the reversed curve in 

 nocturnal birds. There is some evidence at hand showing 

 that the female carries a higher temperature than the male 

 (165-167), since with cormorants, guillemots, razorbills and 

 ducks the females have the higher temperature, the excess 

 varying between .07 F. and .5 F. This difference may 

 seem negligible, but it must be noted and borne in mind 



49 



