26 THE ZOOLOGIST. 



Though Pigeons only lay two eggs, they produce several 

 broods in the year, 



But the number of eggs in a clutch does not only vary in 

 different families or different species, but in different individuals 

 of the same species. This is clearly shown in books on birds, 

 where a varying number of eggs is nearly always given in the 

 account of a species. I take this variation to be the result of — 

 (1) the abundance or otherwise of the food-supply; (2) the age of 

 the hen. But there are curious local conditions which are difficult 

 to explain. For instance, Mr. Howard Saunders, in his ' Manual,' 

 gives the number of a Jackdaw's eggs as four to six. But years 

 ago I was birdsnesting in East Yorkshire and found two Jack- 

 daws' nests each containing seven eggs. Whereas in North 

 Derbyshire I have examined numbers of their nests, and have 

 never found more than four eggs or young birds in any one of 

 them. Also in the same district, with one exception, I have 

 always found four eggs as the clutch of the Dabchick ; but in the 

 ' Manual ' the clutch is given as four to six. 



A most interesting example of the effect of food-supply upon 

 the number of eggs of individuals is be found in the official 

 " Keport on the Vole Plague in Scotland in 1889-1892." At 

 that time the Short-eared Owl, which had hitherto been a rare 

 breeding species there, became a common one, many of these 

 birds laying ten to thirteen eggs ; whereas six is the ordinary 

 clutch. Moreover, in some cases there were second broods.* 



Should Mr. Davies or others wish for another interesting 

 study in connection with birds and their eggs, I am sure that 

 they would find the meaning of colours an engrossing subject. 



>!< No attempt is here made to cHfcuss the relation of fertility to length of 

 life. "We are at present considering what are those factors which tend to 

 limit or increase productiveness in birds. But length of life does not affect 

 their egg-bearing powers ; though the converse of this is probably true. 

 Koughly, it may be said that the number of eggs laid by a species corresponds 

 to the amount of destruction to which it is subjected. But it must be re- 

 membered that such destruction — by starvation, epidemics, or enemies — is 

 more or less a fixed quantity, and therefore is not accidental so far as the 

 species is concerned, though with regard to the individual it may seem to be 

 so (cf. Weismann's ' Essay on the Duration of Life,' p. 11). If for a time 

 more than the average numbers of a species are destroyed by enemies, the 

 quantity of food per head will necessarily increase, and the birds of that 

 species will become temporarily more fertile, as a result of more liberal 

 feeding. But, should such additional destruction become a normal and per- 

 manent condition, it may be essential that the lives of the individuals of the 

 species be prolonged, in order that the species may avoid extinction. 



