VOL. vn.] ON INCUBATION. 109 



period will be extended when the last egg comes to 

 be deposited, and this is no doubt caused by the 

 partial exhaustion of the reproductive powers. The 

 result of this is very noticeable in many instances when 

 a nest of young is examined, for the older birds are 

 graduated in size more or less regularly, whilst the 

 youngest of the brood exhibits a great inferiority in 

 bulk to the nestling hatched previously to it. It is 

 quite clear that this must result in the youngest chick 

 being still more severely penahzed in its struggle for 

 existence, than if it had been hatched out after an 

 interval similar to that intervening between the other 

 members of the brood. 



Additional species in which I have known the youngest 

 members of broods of young birds to disappear, are the 

 Kestrel, the Long-eared Owl, and the Tawny Owl. 



Another bird which rears few young compared to 

 the number of eggs it lays, is the Great Crested Grebe. 

 Four or five eggs are very frequently deposited, but it 

 is not usual for more than two young to be brought to 

 maturity. Mr. Edward Tristram suggests, in the Field 

 of October 14th, 1911, that as incubation commences 

 as soon as the first egg is laid, when the first one or two 

 chicks are hatched the parents at once leave the nest 

 with them, regardless of the remaining eggs which have 

 not hatched out. This occurred in the case of a pair 

 which he kept under observation. 



Some facts connected with another family of birds, 

 the Gruidae, are worthy of attention with regard to the 

 subject of this paper. Mr. F. E. Blaauw notes of a pair 

 of Japanese Cranes which he had in captivity, that an 

 egg was deposited on May 12th, 1906, when incubation 

 began, a second being laid two days later. The first 

 chick emerged on June 14th, and the other on June 

 16th. Again, of the Japanese Cranes at Lilford Hall, 

 it has been recorded* that the first egg is laid two days 

 in advance of the second, and the male begins to incubate 



* Avicultural Magazine, Third Series, Vol. II., p. 147. 



