1917-] Recently published Ornithological Works. 619 



results of an immense number of observations on the incu- 

 bation period of birds as well as on the temperature, body- 

 weight and egg-weight of many species, all of which are 

 set out in a series of tables. He has also discussed at length 

 these data and the theories which have been proposed to 

 explain the varying periods of incubation among birds. 



The conclusion he has himself reached is that the incu- 

 bation period depends on the body-temperature of the bird, 

 and that this again is closely related to the " taxonomic 

 lowness or highness of the species. '^ That is, that those 

 birds which are furthest removed or modified from the 

 proto-avian ancestor are those in which the body-temperature 

 is highest and the incubation period the shortest. 



He also finds, as has already been pointed out by many 

 writers, that there is some relation between the size of the 

 bird or size of the egg and the period of incubation, larger 

 birds and larger eggs naturally having a longer incubation 

 period. On the other hand, there is little or no relation 

 between the incubation period and precocity, nor does the 

 age of the female or the size of the yolk have any effect on 

 incubation. 



According to Dr. Bergtold's table the body-temperature 

 of birds varies from 100° F. to 111° F., the Penguins and 

 Halite birds being nearer the lower figure, the more highly- 

 developed Passerine birds being at the other end of the 

 scale. In correspondence with this we find that the incu- 

 bation period of Ratites and Penguins varies from a month to 

 six weeks, while that of the Passerine birds averages twelve 

 to sixteen days. We must always remember, however, as 

 Dr. Bergtold himself admits, that many of the observations 

 on which he relies are far from accurate, more especially 

 those for body-temperature, as this is a matter which has 

 not received much attention. As regards the incubation 

 period, too, errors of observation are probably far from 

 uncommon, as unless each egg in a nest is marked — and in 

 doing so the parent bird is unavoidably disturbed — the exact 

 duration of incubation cannot be discovered. 



In this matter Dr. Bergtold draws a great distinction 



