110 BRITISH BIRDS. [vol. x. 



one note of a bird following a six with a six ; I have 

 only twice known second sets contain five. Usually 

 four, and often only three are laid. The fertility of the 

 first set is at least 95 per cent. ; my notes bring it out 

 at about 97. In the second sets more than 25 per cent, 

 are as a rule infertile, while in one case three out of five 

 eggs were infertile. 



The incubation period is five weeks. This is based 

 on a very considerable amount of careful watching and 

 calculation ; at the same time, there is a variation in 

 individual eggs of two days above and below this estimate. 

 As eggs sometimes take as long as four days to hatch 

 after they are chipped, this can easily be understood. 



Elsewhere I have commented on the additions to the 

 nest during the incubation and the nestling periods. 

 In July 1916 I watched a hen arranging the twigs in the 

 well of the nest, and generally shaking up the middle 

 of the nest, a week after the young had left. The con- 

 clusion I arrived at was that the birds were continuing 

 to roost on the nest, and probably this is the case for as 

 long as the nest is used as a dining table. 



I have also stated that the hen eats the egg-shells 

 when the young hatch. This may be rather a rash 

 generalization ; it is based on the fact that I have 

 watched two broods hatch and the egg-shells were eaten 

 in each case, for I saw them eaten. The bird places 

 them on the rim of the well and then mmiches them up 

 at leisure. One shell was blown over to the ground by 

 a gust of wind ; perhaps this accounts for sheUs found 

 sometimes just below nests. I have also found very 

 fuie fragments of egg-shell in the wells of other nests 

 which we have not watched from a hut ; these cases 

 support the eating theory. It was not until late in July 

 1916 that I got certain evidence that the egg-shells are 

 sometimes carried away and dropped. Yet this may 

 be the usual thing after all, and the eating of the egg- 

 shells which I witnessed may have been one of the 

 effects of the hut. 



