The Zoologist/4th series, vol 3 (1899)/Issue 691/Varying Fecundity in Birds, Storrs Fox

 

VARYING FECUNDITY IN BIRDS.

By W. Storrs Fox, M.A.

 

In a very interesting article in the December number of 'The Zoologist,' Mr. Basil Davies attempts to explain why some species of birds lay more eggs than others. Personally I feel grateful to him for suggesting this enquiry, and for the reasons he assigns for the remarkable diversity in the number of eggs laid by different species. If, therefore, I criticise to some extent the theory which he propounds, I hope that it will be understood that I do so in no unfriendly spirit.

Mr. Davies compares the reproduction of birds and mammals. He says: "Birds feel it their duty not only to produce a certain number of offspring each year, but also to bring a certain number to maturity." To illustrate this he compares the Cat and the Nightingale. The former breeds at stated periods whether you destroy her offspring or not; but the latter at once prepares to produce a second brood if the first is destroyed. The truth is that the main object of every organism is to reproduce itself. Each species has its own method of bringing this about. The Cat provides for the peopling of the world by future Cats as thoroughly as the Nightingale provides against the extermination of its kind. These facts are familiar to us, but it is not easy to explain them. Under natural conditions the Indian Elephant does not become exterminated, nor the Brown Rat exceed certain limits. On the one hand, with the former the period of gestation is about nineteen months, and rarely is more than one produced at a birth (Roy. Nat. Hist. vol. ii. p. 536; Darwin estimated that though a pair might live to be one hundred years old, their offspring would probably average only six, 'Origin of Species,' 6th edit. p. 51); whereas the Rat bears "four or five times in the year from four to ten blind and naked young, which are in their turn able to breed at an age of about six months, the time of gestation being about twenty days" (Flower and Lydekker's 'Mammals,' p. 475). The immense number of eggs laid by some fish, and the amazingly rapid increase of some lowly animals, are well-known facts. Each species has its own place in nature, and produces sufficient offspring to keep that place filled. But how this is regulated is another matter. We are sure that individuals are quite unconscious and regardless of the requirements of their species. Probably the food-supply itself is the chief factor, increasing fertility in times of plenty, and checking it in times of scarcity.

With birds is it not mainly the food-supply which confines the breeding to a certain season? Can it be supposed that our insectivorous summer visitants usually nest only once in the season because they feel that the time for migration is approaching, and a second nest is therefore useless? I understand Mr. Davies to suggest this. These birds leave us partly because the supply of insect-food is running short, and partly because a mighty impulse drives them to go. But they cannot be conscious weeks beforehand that the time for their departure is drawing near. If Finches as a rule go in for a second family, I would suggest two possible reasons, though I do so with diffidence, for I feel that I have not sufficient data as evidence for them.(1) Do not our resident Finches as a rule begin to nest earlier than the migratory Warblers, and so get the start of them?(2) If the particular food needed for feeding young birds is decreasing, the parent Finches can provide their own sustenance in the form of seeds, and so they will not need to draw upon the insect-food to such an extent as Warblers. Moreover, young Finches soon become capable of digesting seed. Nature as a whole keeps those numbers under control.

I take the rules which Mr. Davies gives to amount to this:—Every individual does what it can to produce offspring, and to increase the number of its species. We can only suppose that it is quite unconscious of what it is doing.

Now, as to the number of eggs laid by Finches and Warblers. Mr. Davies gives five as the average clutch; and then proceeds to show why this is the only suitable number. I cannot agree with him that a hen of small size could not well lay more than five. As he himself states, Tits may lay very many more. It seems probable, however, that the number may be limited by the catering powers of the parents, and certainly by the covering capabilities of the sitting hen. Mr. Davies allows that the food-supply may affect the parents, for he says that the number of eggs is often less when insect food is not abundant. And, again, he gives as a reason for the two broods of Finches, &c, that "it is necessary for them to produce eight or ten of their kind in a season to aid in killing off from the cultivated lands the vast swarms of insects to which the summer has given birth;" which means that where the supply of insects is great there will be plenty of birds to prey upon them. But this ought to apply equally to the Warblers, &c.

Mr. Davies proceeds to give reasons why in one family of birds the usual number of eggs laid by the species of that family is large; whereas in another family the reverse is true. With regard to Game Birds, he suggests that the large number of eggs is to meet a large amount of destruction. It seems to me that not only with Game Birds, but with all birds, this is the secret of a larger or smaller number of eggs. Darwin wrote: "The Fulmar Petrel lays but one egg, yet it is believed to be the most numerous bird in the world" ('Origin,' p. 52).[1] And I should suppose that the causes which controlled the average numbers of eggs of different species were—(1) the supply of food; (2) the number of enemies; (3) the power of self-defence or escape.

It is not possible to accept some of Mr. Davies' reasons. For instance, he supposes that the Nightjar lays two eggs, because several gaping young birds would be a conspicuous object. As they only gape after dusk, no number of them would be conspicuous. I know no object less conspicuous than a Nightjar covering its young or eggs.

Again, is not the reason for the single egg of the Guillemot to be looked for in the special defences of this bird rather than in the shape of the egg? No doubt this shape is a protection. If Guillemots' eggs were shaped like those of most birds, very few would be hatched. But the one egg is laid in a place of comparative safety, and the bird itself is quick on the wing and an apt diver, and for part of the year lives far from land, and so is probably less subject than most birds to attacks of foes.

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.[2]

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.


  1. Mr. A.R. Wallace has thus modified this statement:—"The Fulmar Petrel exists in myriads at St. Kilda and other haunts of the species, yet it lays only one egg." ('Darwinism,' p. 30).
  2. 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.