The Zoologist/4th series, vol 2 (1898)/Issue 690/Varying Fecundity in Birds

by Basil Davies

 

VARYING FECUNDITY IN BIRDS.

By Basil Davies.

 

Mr. Warde Fowler, in one of his bird stories, describes the uneasiness of a youthful hen Wagtail when she began to ponder on the question, "Why do we wag our tails?"—the moral being, I suppose, that facts come before reasons for facts, and ought to suffice for most of us. Checked somewhat by this allegory, my ideas on the above subject received a fresh impetus when, in 'Summer Studies of Birds and Books,' I read that Mr. Warde Fowler actually felt it his duty to ask and to attempt to answer that very same question which used to trouble his little hen Wagtail. I have no apology of duty to offer for my poor attempts at explanation: I can only say that the subject is one to which very little attention has been given, and that it is one in which a really skilful ornithologist could probably make most successful researches.

The ordinary birdsnesting naturalist regards an abnormal clutch, whether large or small, only with a view of its suitability for his collection. He robs a Nightingale of five eggs and a Partridge of fifteen without attempting to explain why the offspring of the one species is numerically so superior. Some years ago, reviewing my season's "take" of eggs, I felt myself somewhat of a monster when I imagined the table on which my cases lay peopled with those birds whose embryos I had removed from every shell—six Nightingales, a dozen Bullfinches, and so on—though I never took more than one egg from a nest. Consequently, in abandoning collection, I sought for a new interest in eggs to take its place; and this chapter is an endeavour to explain the interest of a different sort that I now take in the nests I find.

There are certain general principles which it is well to keep in mind in this particular branch of bird-study. Such is the rule, that birds do not merely breed so many times a year in the course of nature, but that they feel it their duty not only to produce a certain number of offspring each year, but also to bring a certain number to maturity. Take the case of a cat. A female may be perpetually running with a male. You drown her kittens; vet she does not again kitten for six months or so. Compare her with a Nightingale. Harry a Nightingale's nest when the fledglings are nearly ready to fly. The bird does not sit down and ejaculate "Kismet," and feebly await the period of migration. She feels desolate without her young ones around her; she knows she has a duty to fulfil, and that the time is short. She begins to bustle about, and in a week she will have started laying again in a safer spot. In a dell at Clifton there were two pairs of Nightingales. Some deadly person of the rabid collector type took each clutch as it was laid, and again he did the same with the second clutches; but the faithful birds each nested a third time, and met with success at last.

There are further a few rules which are useful, and which I must endeavour to state more briefly: —

(1). The object of the breeding season is to maintain the numbers of each species at an equable level (not necessarily to increase them, though this is sometimes the case).

(2). By August the numbers of each species are probably treble what they were in April.

(3). These numbers are subsequently curtailed: —

(a). In the case of migratory species, many succumb to the hardships and dangers of the passage.

(b). In the case of resident species, many succumb to cold and lack of suitable food during the winter months.

(c). Every species alike is liable to losses through accident, from carnivorous birds, and at the hands of the collector, gamekeeper, and other misguided people. These losses, however, cannot compare with (a) and (b).

I will now attempt to treat of the various species more or less in detail.

1. Finches, Pipits, Buntings, and Larger Warblers (such as the Nightingale, Blackcap, &c).—Throughout the country five eggs is the usual number for all these birds to lay in a clutch. The migratory species in the majority of instances probably confine themselves to one brood, while nearly all the Finches regularly have a second nest. It is not, I think, difficult to see why they respectively lay their five and ten eggs a season, and neither more nor less. These birds, resident and migratory alike, feed their young on various forms of insect-life—flies, grubs, aphides, the smaller kinds of caterpillars, and the ova of these insects. The two parent birds would be unequal to catering for the wants of a larger brood than five; neither could a hen of this size well produce more than five eggs. Indeed, four is not an uncommon clutch by any means in districts where insect-food is not specially abundant. On the other hand, a Blackcap Warbler must produce five young in a season to prevent her species diminishing; and as the breeding season is curtailed by migration, which the young must be old enough to undergo when the time arrives, we see that a smaller clutch would not be convenient. The resident small birds, however—Finches, Buntings, &c.—are not hampered by the approach of the period of migration, and they indulge in a second brood. 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, and which the efforts of the parents when feeding have proved utterly inadequate to cope with. Although the Finches thus produce four or five times their own number, yet by the next spring each family of Finches will usually have dwindled down to a pair once more; for what the birdcatcher spares, God and the winter take.

2. The Tits and the Wren.—These birds during the season feed on a very similar diet to those described under 1, and they lay from six to twelve eggs in each nest, though one cannot say definitely how often they have a second brood. Still, taking into consideration the number of Finches' nests that the small boys destroy, I should be inclined to say that the Tits rear more young than the Finches. They are not the prey of the birdcatcher, who annually robs our woods and fields of tens of thousands of Finches. Why then are they so prolific? Simply because they feed mainly on an insect diet all the year round, and in the depth of winter insect-food is scarce and difficult to obtain. I have found a score of Tits lying dead on the snow in a single walk in a winter that was not specially severe, All were dead from starvation, not from cold; their bodies were thin and emaciated, the breast-bone often protruding almost through the skin. By training a terrier to find the dead bodies, one gets some slight idea what an ordeal a hard winter is to our birds. Another point is that eight young Tits would hardly require more food than five greedy little Robins, and so the labours of the parents in the two species would not differ appreciably.

3. Smaller Warblers (Chiffchaffs, Willow Warbler, &c). — Here again it is no more difficult to feed eight small Warblers than five large ones. A Wood Wren usually lays six or seven eggs; she can rear her family as easily as a Redstart can rear five; and these species succumb in greater numbers during migration than their more stalwart relations.

4. The Nightjar lays but two eggs, probably because a huddled mass of half a dozen gaping youngsters could hardly fail to be distinguished, seeing that she incubates on the bare ground.

5. The Wryneck lays nine eggs as a rule. This bird has a great advantage over the other insectivorous birds, because it feeds largely on ants. It is structurally adapted for searching treetrunks, and if it finds the supply on the trees run short it has only to preserve a few ant-hills to obtain an unbounded quantity. I observed one pair very carefully when feeding their young, and they seemed to rely almost wholly on some neighbouring anthills. When I cut one open for them they had a joyous quarter of an hour, and did great execution.

6. Doves and Pigeons.—I have only the old hackneyed explanation for the unvarying pair of eggs laid by these birds, i.e. that they are conspicuous among birds for their tender affection to their mates, and that the eggs always hatch out male and female in the same nest. I have had no opportunities of verifying this theory among the wild kinds, but it is undoubtedly true in most instances of the domestic Pigeon.

7. Plovers and certain other Waders.—These are peculiarly interesting birds. They build in a very dangerous situation—on the ground in tolerably open and exposed places. This occasions three difficulties; for, to balance these dangers and the probable resulting losses,

(1). The number of young must be passably large.

(2). The young must be able to run when hatched. (3). If (2) be necessary, the egg must be abnormally large for the size of the bird.

Everyone knows how wonderfully these three difficulties are surmounted.

8. Crakes and Rails.—These birds lay from seven to nine eggs in well-concealed situations amongst the stems of standing grass or grain. Owing to the cover afforded by the stems, the young need not be so large when hatched as the young of the Plover; consequently the eggs are much smaller, and the hen can incubate a greater number. At the same time it is imperative that she should produce a good clutch, for very many nests are destroyed when the grass comes to be mown. The birds are also migratory, and encounter the usual dangers during passage.

9. Game Birds lay a good many eggs, as the situation of their nests lays them open to many enemies—Stoats, Crows, &c. Further, I should not be surprised to learn that they were originally less prolific before they were persecuted under the name of sport. At any rate, the least persecuted species, the Ptarmigan, as a rule lays the fewest eggs.

10. Coming to the order of Natatores, I plead guilty to a very small experience of these birds. It is obvious enough why Razorbills and Guillemots lay but one egg. It is well known that their single egg is of such a tapering form that a gust of wind, instead of sweeping it from the ledge of rock on which it is laid, merely causes it to twist round in a circle with the thin end as centre. If there were more than one egg in a clutch, these gyrations would result in disaster, and a Guillemot's breeding station in a high wind would indeed be a curious spectacle.

It may further be noticed that the largest clutches in this order are those laid by the Teal and Wild Duck, whose nests are accessible to many enemies, and who are not altogether free from the molestation of man.