ON INCUBATION. 



139 



I.— April 4th, 

 April 5th, 

 April 6th, 

 April 7th, 

 Aiiril 8th, 



II.— April 7th, 

 April 8th, 

 April 9th, 



III. 



IV. 



-April 22nd, 

 April 23rd, 

 April 24th, 

 April 25th, 



-Four eggs. 

 April 8th, 

 April 9th, 

 April 10th, 

 April 11th, 



-Three eggs. 

 April 12th, 

 April 13th, 

 April 14th, 

 April 15th, 



First egg laid. 

 Second egg laid. 

 No change. 

 Third egg laid. 

 Fourth egg laid. 



First egg laid. 

 Second egg laid. 

 Third egg laid. 



First egg laid. 

 Second egg laid. 

 No change. 

 Third egg laid. 



First egg hatched. 

 Second egg hatched. 

 Third egg hatched. 

 Fourth egg hatched. 



First egg hatched. 

 Second egg hatched. 

 No change. 

 Thii'd egg hatched. 



The nest was visited at various 

 times of day, the bii'd being 

 on in every instance. 



At whatever time the nest was 

 seen the bird was on. 



The bird was always on the 

 nest when it was visited. 



Why does the Rook incubate as soon as the first egg 

 has been laid ? The following explanation occurs to 

 me. Everybody knows what arrant thieves Rooks are 

 whilst nest-building is in progress ; they help themselves 

 to their neighbours' property whenever the opportunity 

 presents itself. Also they are well known to be egg- 

 thieves. We can hardly suppose that birds that help 

 themselves to the nesting-materials would leave the eggs 

 untouched if a chance of appropriating them occurred. 



We have already seen in the case of species which 

 normally do not incubate until the full clutch has been 

 laid, that there are occasional exceptions — that individual 

 birds give way to the desire to brood as soon as the first 

 egg has been laid. At one time the Rook as a species 

 probably did not incubate until the whole clutch had 

 been laid, and no doubt the birds suffered severely from 

 the depredations of their own species. Any bird which 

 incubated with the laying of the first egg would reduce the 

 risk of losing her eggs to a 7ninimum, and thus gain a 

 great advantage over other Rooks ; birds hatched 



