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 Jackdaws' 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 "Report 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.[1]
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 discuss 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. Roughly, 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 remembered 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 permanent condition, it may be essential that the lives of the individuals of the species be prolonged, in order that the species may avoid extinction.
