72 BRITISH BIRDS. [vol.xiv. 



which the Meadow-Pipits about to be victimized behaved immediately 

 preceding and at the moment of tlie laying and deposition of the 

 Cuckoo's egg ; and (e) charts showing, with dates, the number of nests 

 built by each of the nine pairs of Pipits, and the number of times each 

 pair of Pipits were victimized by the Cuckoo. 



I want to make it clear that I lay claim to no exceptional knowledge 

 of Cuckoos in general, but trust that what I am able to record about 

 the doings of this particular Cuckoo this year as closely observed by 

 myself and at different times also by ten or more of my friends, may 

 explain why one and all of us found the study of absorbing interest. 



Edgar Chance. 

 9, Hay Hill, Berkeley Square, W., July i^th, 1920. 



[We shall look forward to receiving Mr. Chance's detailed report of 

 his most interestins; observations. — Eds.1 



ARTIFICIAL INCUBATION. 



To the Editors of British Birds. 



Sirs, — With regard to the Rev. F. C. R. Jourdain's editorial note 

 to my paragraph on the incubation period of the Storm-Petrel, to 

 the effect that Mr. W. Evans succeeded in hatching apparently fresh 

 eggs of this species on the 36th day, in an incubator, I am afraid that 

 such artificial incubation is no guide to the true incubation period 

 of the bird itself. 



The eggs of Terns, for instance, hatch in an incubator on the igth 

 or 20th day, yet in a natural state take 23 to 26 days to hatch. With 

 greater heat they might even be hatched earlier than this, although 

 the young would not survive. 



Such artificial incubation of Storm-Petrel's eggs in 36 days, goes 

 to prove that their natural incubation is at least a week longer, i.e., 

 43 or 44 days. H. W. Robinson. 



[Mr. Robinson raises some interesting points as to the value of 

 results obtained by hatching in incubators, but there seems to be no 

 justification for the suggested formula that (roughly) results obtained 

 from incubators only represent four-fifths of the natural period. A 

 study of Mr. W. Evans's paper will show that in many cases the results 

 obtained artificially coincide very closely with those obtained by 

 watching nests. Mr. Robinson states that in an incubator, Tern's 

 eggs hatch out on the 19th or 20th day, while Mr. Evans's period, 

 obtained by the same means ranges from late on the 22nd to the 23rd 

 day ! It would be interesting to know what temperature was employed 

 by Mr. Robinson, as it seems probable that his eggs were kept too hot. 

 Some discrepancy in the period might be expected in the case of birds 

 like the Common or Arctic Tern, which often leave their eggs exposed 

 for considerable periods during the day, but there is no reason why it 

 should also occur in the Storm-Petrcl, any more than it does in the 

 case of the Wild Duck, Partridge, Domestic Fowl, Cuckoo or many 

 other species. In the preface to Mr. Evans's paper it is distinctly 

 stated that the eggs were not hatched at unnaturally iiigh temperatures, 

 but that great care was taken that the conditions should as nearly as 

 possible approach those of nature. — F. C, R. Jourdain.] 



