40 JOURNAL OF MAINE ORNITHOLOGICAL SOCIETY. 



What Can We Call the Incubation Period? 



By Ora W. Knight, Bangor, Me. 



While observing the incubation period of birds I have ahnost 

 invariably noticed that the egg last laid hatches in a far shorter 

 ])eriod of time than the egg first laid, and the intermediate laid eggs 

 of the litter also hatch in a proportionately shorter period. For ex- 

 ample : A certain Robin laid an egg each day until the full comple- 

 ment of four eggs was deposited. The bird was on the nest the 

 greater part of the day when the first egg was laid, and likewise on 

 each following day, leaving the eggs alone only a few miimtes at a 

 time until the eleventh day, when one parent or the other was on the 

 nest almost continuously until the fourteenth day, when the first egg 

 hatched. All four eggs were hatched by the evening of the fifteenth 

 day. Now I should report the incubation period of the Rol)in as 

 fourteen days, Init assuming that the last egg laid was the last to 

 hatch on the evening of the fifteenth day, then the incubation period 

 of this egg, which was laid the morning of the fourth day, was only 

 a little over eleven days. 



Again, in the case of the Myrtle Warbler's eggs mentioned on 

 page 74 of the December number of the Journal : The first egg 

 hatched within 295 hours, plus or minus an hour and forty minutes 

 of the time when it was laid. The last of the four eggs in the nest 

 was hatched within eighteen hours afterward. Now in this case an 

 egg was laid the morning of each consecutive day, and consequently 

 the incubation period of the last egg laid was of lesser duration than 

 the first laid egg. 



I could give many other instances along this same line, but 

 would like to hear what the experience of others has been. 



To me it seems that the incubation period must be taken to 

 include the time elapsing between beginning incubation and the 

 hatching of the first egg laid. 



