VOL. VII.] ON INCUBATION. Ill 



eggs were infertile. For example, of thirteen nests 

 containing two eggs, in three nests both were bad, in 

 ten both were good, but with every good pair there 

 was about a week's difference in the age of the em- 

 bryo. In six nests, each containing one young and one 

 egg, five of the eggs were decomposed. 



Walter K. Fisher, writing of the Blue-faced Booby 

 (S, cyanops) on Laysan, has the following passage : — 



We found young and eggs in about equal numbers, and most of the 

 eggs were far advanced in incubation. The young varied from about 

 a week old down to newly hatched individuals. It is a curious fact 

 that although there are two eggs, only one young is reared. Often 

 all signs of the second egg were removed, as if the yoimg had been 

 hatched and had been devoiu"ed by a parent or some marauding 

 Fregata. But more frequently there would be one nestling and one 

 egg. Sometimes this egg was spoiled, sometimes contained an embryo. 

 In one case I found two newly hatched yovmg, one of which had already 

 been trampled to death. Professor Nutting saw one large nestling 

 and one small, still alive, but I doubt if it lived long. The presence 

 of only one young bird has been noted in the eastern Pacific, at Clipper- 

 ton Island, by R. H. Beck, and Rothschild mentions the same 

 fact for Laysan. The voracity of the bu'd first hatched is probably 

 responsible for the death of the second. 



The observations recorded concerning the Rook 

 may be taken in order to illustrate the fact that a habit 

 can be evolved which saves a species from extinction or 

 a near approach to it, and yet this very habit destroys 

 approximately one half of the young which are brought 

 into existence. 



As has been shown in a previous paper, the habit 

 of incubating from the deposition of the first egg is 

 undoubtedly of inestimable value to this species. It 

 is a matter of everyday observation in spring that 

 Rooks will steal their neighbour's nesting-material at 

 every possible opportunity. No sooner is a nest left 

 unattended than one or two birds also engaged in nidifi- 

 cation make haste to appropriate the materials which 

 have been built into the unguarded nest. Kleptomania 

 is one of the Rook's worst failings. 



Again, his taste for eggs is very strongly developed, 

 and he gratifies it whenever he has a chance. That the 

 Rook would rob his neighbour's nest of material and not 



