The Zoologist/4th series, vol 3 (1899)/Issue 694/Fecundity in Each Avine Species, Varying According to Accidents of Locality, Davies
FECUNDITY IN EACH AVINE SPECIES, VARYING
ACCORDING TO ACCIDENTS OF LOCALITY.
By Basil Davies.
In a former article I attempted to describe how the peculiarities of any species might cause its reproduction to differ from that of another: it is now my purpose to describe, if in a somewhat partial and incomplete manner, how the members of a single species may differ inter se as regards this function, because of the more or less favourable circumstances under which they may happen to breed. The possibility, and, later, the certainty of these differences and their origin, was early brought under my notice, seeing that for several years I alternately resided in the bleak and smoky outskirts of a northern manufacturing town, and on the edge of some of the richest land in the fertile western counties. I have also to thank Mr. W. Storrs Fox for supplying a little evidence upon my present subject in his kindly criticism of last month.
My ornithological books early informed me that a Hedge-Sparrow laid from four to six eggs, yet near my northern home I never found a clutch to exceed two; and so scant was the insect-life of the neighbourhood that a year would occasionally pass without my finding a single nest of the species. I personally have notes of many completed clutches of two, and a friend's voluminous diary can only furnish three clutches exceeding that number during a continuous residence of several years in the same district. In Gloucestershire, however, five was the usual number, and a nest of six occasioned no remark. In Lancashire the lingering winter, combined with a foul and smoke-polluted atmosphere, rendered insect-life nowhere abundant. In most English localities you may rely on retaining a pair of "resident" birds to breed with you during the summer, if you mark them frequenting your fields and hedgerows in the latter end of March; but at R the birds would weary of waiting for the tardy spring. By means of some agency in the bird-world, corresponding, I suppose, to our daily press, they would hear of lovely nesting weather in Derbyshire; and to me March's promise brought but regrets in May. Even when insectivorous birds were few and far between, nature's providence forbade the laying of a full clutch, clearly evidencing the sparseness of the food-supply. Near Clifton I have often found six Hedge-Sparrows' nests containing the full clutch within the bounds of a single field, without regard to Chiffchaffs and Whitethroats catering for hungry families on very similar lines.
My favourite authorities would further inform me that the Sand-Martin is accustomed to lay from four to six eggs in its solitary clutch year by year. My notes of expeditions in the south and west confirm this rule, giving five as the common number, and four as the minimum. There rises before my vision a northern colony of this river-haunting bird. I see a miniature amphitheatre of oozing clay, its lofty sides dotted with Irishmen wielding spades and encroaching yet farther on the plateau-like meadow-land above; where we expect the arena is a loathsome clay-pool, slimy brown and forbidding, destitute of reed or flag. One side of the encircling banks has ended abruptly in a sandwall, and here the Martins have found a home. The birds are flitting over the clay-pool, actually struggling for each rising fly. The meadows they will resort to towards sunset. The land is too poor to breed the humble fly; there are on it only the tiny moths which sleep by day among the blades and grass roots. On Aug. 10th, 1896, I examined seventeen nests in such a place as this, and no nest contained more than three eggs or young.
If we transport ourselves to some shelving sand-bank on some southern stream, we see the Martins flitting about careless of each other's prey. A warmer temperature and the vegetation plenteous in the stream-bed render insect-food abundant, and every tunnel in the wall's face will give to light five or six young Martins before September comes.
It is a great help in bird study to acquaint oneself with gamekeepers. One vacation I was trespassing, countenanced by the head keeper, and I found two Sparrow-hawks' nests in woods three or four miles apart. Each contained the magnificent clutch of seven eggs, forming a picture none the less delightful because I had no desire to "collect" them. I resolved to tell the keeper of the unusual discovery, although I expected him to grumble because I had not destroyed them. To my surprise he was well pleased. He told me how his master had caused all the Hawks on his estate to be slain as far as was practicable, with the exception of an occasional pair in woods lying remote from each other. He desired to protect his coverts, but, like a true sportsman, he could admire a stately bird in mid-air; consequently a pair was suffered to nest here and there undisturbed. These orders, the keeper continued, had been in force some ten years, and the clutches of surviving pairs had each year increased from the time when he had received orders to destroy as many as possible. There were now remaining some three or four pairs of Sparrow-Hawks on the whole estate. The Kestrels had been exterminated. He had frequently found clutches of six of late years, and on rare occasions the larger number of seven. This certainly appears to point to the conclusion that increased scope for foraging results in increased fecundity.
The Yellowhammer is an excellent example of my point. After a long correspondence in the 'Feathered World,' Mr. John Craig, of Beith, and one or two others began to collect statistics regarding the usual number of eggs deposited by this Bunting in one nest. Mr. Craig himself showed that in Ayrshire a clutch of three was normal; this county consists largely of sheep-farming land, and alternates between rather thin close-cropped grazing-ground and furze-clad moorland, foliage and herbage being nowhere luxuriant. In a western English county I obtained sufficient evidence to show that five was there the usual clutch; while a Cheshire friend stated that four was usual in his neighbourhood, five and three being of less common occurrence. Cheshire, as regards fertility, comes about half-way between the two extreme instances previously cited. It possesses a tolerably productive soil, bearing a reasonable proportion of woodland and thick ground herbage.
To speak on broader lines, I everywhere found large clutches in the west and small clutches in the north. I well remember one afternoon with the birds of Somersetshire. The ground we traversed was a large plain, moist, loamy, and dark-soiled, intersected by numerous rhines, fences, and hedgerows. Nests were everywhere abundant, everywhere cramful of eggs, and all species seemed to be adequately represented. Nearly every nest we examined contained the maximum clutch permitted by bookwriting authorities, and in some cases the legitimate number was exceeded, the most notable instance, perhaps, being that of a Whinchat incubating seven eggs. Indeed, the wit of the party remarked that the prescribed maximum had been passed in the case of every nest we had found, save that of the miserable Cushat-Dove, which had merely deposited the regulation couple.