52 Mr. W. Evans on the Periods 



X. — On the Periods occupied by Birds in the Incubation of 

 their Eggs. By William Evans, E.R.S.E. 



Probably no point in tlie life-histories of the different species 

 of birds has received less attention than the period of incu- 

 bation, a circumstance all the more surprising -vvhen we 

 consider the importance of the subject and the great activity 

 which has been manifested du.ring recent years in the eluci- 

 dation of ornithological questions. 



In the spring of 1887 I had occasion to seek information 

 regarding the periods of incubation in the case of a number 

 of our native birds, and was surprised to find how little that 

 was reliable I was able to cull from the standard works on 

 British ornithology. It then occurred to me that some 

 useful experiments might readily be made by means of an 

 incubator ; and as a preliminary step in this direction I had 

 a few eggs placed in a home-made machine belonging to a 

 friend. The results, though disappointing as regards the 

 number of eggs hatched, clearly foreshadowed a series of 

 most interesting facts from experiments made under better 

 conditions. Accordingly, at the commencement of the 

 breeding-season of 1888, I purchased one of Hearson^s self- 

 regulating incubators, and kept it in operation as long as 

 fresh eggs could be procured. It was again kept going 

 during the season of 1889 and for a short time in 1890. 

 Eggs of all sizes, from those of the Goldcrest to that of the 

 Solan Goose, were experimented on. Taken all over, about 50 

 per cent, of them were hatched, the number of failures being 

 greatest among the Passeres and least among the Gallinae. 



In the selection of eggs one must be guided greatly by 

 experience and a knowledge of the nesting-habits of the dif- 

 ferent groups, as many birds commence to incubate before 

 the full complement has been laid ; but a good general rule 

 is to make use only of incomplete clutches. 



Having only one incubator, new eggs were continually 

 being introduced as others hatched or became addled, and on 

 this account it was deemed best to fix upon a temperature at 

 the beginning of each season and maintain it as steadily as 

 possible throughout. Guided by the usual practice among 



