138 BRITISH BIRDS. 



species which normally lay the full clutch before com- 

 mencing to incubate we occasionally find individuals that 

 prove the exception to the rule. Also there are a few 

 species which habitually commence incubation with the 

 laying of the first egg. In these cases there is always some 

 factor which causes the habit to be beneficial to the 

 species, any disadvantages being outweighed. It is 

 with the latter class that this paper chiefly deals. 



It may be well to give a few examples of the more general 

 type {i.e., in which incubation commences with the laying 

 of the last egg) for the sake of comparison with those 

 species which commence to incubate as soon as the first 

 egg has been laid. 



A Jackdaw (C. monedula) laid five eggs, the last on April 26th ; on 

 that date incubation commenced. Four eggs hatched on May 14th ; 

 the fifth was not fertile. 



A Song-Thrush (T. musiciis) which laid four eggs on consecutive 

 days, the last on April 18th ; commenced incubation on that date. All 

 four eggs hatched on May 2nd. 



A Dipper (C. aquaticus) laid five eggs on consecutive days, the last 

 on April 30th, when incubation commenced. On May 16th four of the 

 eggs hatched, the fifth proving to be infertile. 



The three foregoing cases are typical of their kind. 



The following observations made on the nesting of a Great Tit (P. 

 ■major) are exceptional : — The nest was found on May 23rd. It con- 

 tained seven eggs. The bird laid regularly one egg a day until she 

 had the large clutch of thii'teen. She, however, commenced to incubate 

 when tlie tenth egg had been laid, on May 26th. Six of the eggs proved 

 to be infertile ; six hatched on June 9th, and the last one on June 11th. 



The Magpie (P. rustica) and Jackdaw are two species which normally 

 do not commence incubation until all the eggs have been laid, but I 

 have known an instance of each sjDecies commencing incubation with 

 the first egg. 



We now come to those species which habitually incubate 

 on the laying of the first egg. From observations made 

 in previous years I suspected that the Rook (C. frugilegus) 

 was one of the number, I therefore kept some nests under 

 close supervision during the past spring. The result 

 was that my suspicions were confirmed, for each bird 

 commenced to sit as soon as the first egg had been 

 deposited. The following are some of the particulars 

 obtained : — 



