58 On the Periods occupied by Birds in Incubation. 



that I watched their progress till the 31st, when I had to 

 leave home. Before leaving I tested the eggs at light and in 

 water, and was satisfied that five of them not only contained 

 living embryos, bnt that they were several days from hatch- 

 ing. In my absence Mr. Eagle Clarke kindly continued the 

 daily observations, and was able to report that early on the 

 33rd day two of the chicks had chipped their shells. Only 

 one of them, however, lived to hatch out. This took place 

 three days later, or after an incubation of fully 35 days, in a 

 temperature of from 102° to 103° Fahr. Considering the size 

 of the egg, and still more of the bird itself, this is a remark- 

 ably long period of incubation, but, though fully a week 

 more than the imperfect observation made for me in Shetland 

 in 1889 led me to expect, I see no reason to suppose that it 

 was abnormally prolonged. The fact that, although only one 

 egg was hatched, two were chipped &i precisely the same time, 

 is of considerable importance, and practically doubles the 

 weight of the observation. Possibly under purely natural 

 conditions the interval between chipping and hatching may, 

 as a rule, be less than three days, and therefore from 33 to 

 35 days would perhaps be a very fair general conclusion to draw 

 from these observations regarding the duration of incubation 

 in this small Petrel. Reliable information regarding the 

 period of incubation in allied species, such as the Manx 

 Shearwater and the Fulmar, will now be looked forward to 

 with more interest than ever. 



We have yet a great deal to learn before we can dogmatize 

 on this subject, but I think my observations warrant the 

 general conclusions which I indicated at the close of my 

 paper, namely (1) that each well-defined natural group of 

 birds has a characteristic period of incubation ; and (2) that 

 within each group the larger the egg the longer the period. 

 Within each group, however, the range of duration is appa- 

 rently slight, as compared with the differences in the size 

 of the eggs. 



