68 The Wilson Bulletin. 



INCUBATION PERIOD OF THE MOCKING BIRD. 



BY JOHN W. DANIELS, JR. 



Little has appeared concerning the exact time required by 

 various species of birds for the incubation of their eggs. 

 The period of time necessary for the development of the 

 egg contents, from the fresh state to the hatching point, 

 must needs vary with species and be subj'ect to the many 

 diversified conditions under which birds breed. Features of 

 climate, as sunshine, atmospheric humidity, precipitation and 

 temperature enter, along with locality, time of year, nesting 

 site, nesting material, and nesting habits in general, as potent 

 factors of control. In order to determine the period of in- 

 cubation of any species the closest observation is necessary, 

 and the observer should visit the nest daily, from the time 

 of the laying of the first egg until the young leave the nest; 

 aad better still, if his observations commence with the lay- 

 ing of the foundation of the nest and are carried on until 

 the young are able to fly. Exact data, as to the time nec- 

 essary for the incubation of the eggs of even our commonest 

 birds, is very meager — so here is a little-worked and profit- 

 able field for research; and as the securing of this informa- 

 tion involves no destruction of bird life, it may be had with- 

 out sacrifice of any sort save that of time and patience. 



The following data regarding the period of incubation of 

 the Mocking Bird (Mimus polyglottos) was secured in Camp- 

 bell county, Virginia, April 29th to June 3rd, 1901, inclusive. 

 Upon the former date the half-completed nest was found, 

 and upon the latter date the young were seen to leave the 

 nest. The nest was five feet up among the lower branches 

 of a Norway spruce, which grew within twelve feet of the 

 veranda of our home, in our front yard. Thus, the nest 

 could not have been more favorably located for my purposes 

 of observation, as its nearness to my home allowed me to 

 visit it daily, regardless of weather, and at short intervals: 



April 29. Found the half-completed nest. 



