MORTALITY AMONG NESTING BIRDS.
yEAss with long spells of drought in spring and early summer entail a death-roll among certain species of birds which it is difficult to compute, but which must undoubtedly be heavy. Not all birds suffer in the same way. Birds of prey, and the egg-stealers, like the jays and magpies, probably find not much difference in dry and wet seasons; the supply of voles. rats, mice, clutches of eggs, and nestlings varies, no doubt, but it is never wholly absent. Birds which depend on a supply of insect food, on the other hand, are seriously affected as soon as the drought has really taken hold of the soil. Worms ars no longer near the surface, and the hosts of flies and larvae which swarm among grasses and low herbage in warm, damp weather, and which appear in email quantities even after • slight shower, seem to vanish altogether from the dusty roots and benta of the hayfields. The summer of 1913 afforded a curious example of the effect which an absence of food supply of this kind may have in the case of partridges and wild pheasants. The young broods of both birds died off in thousands. Wherever the fields became thoroughly baked the story was the same ; the partridges had hatched off their eggs well, but in a little time the broods began to dwindle; eighteen and sixteen dropped to six and three, and in some cases every young bird died. Wild pheasants perished in the same way from lack of insect food, and when they were picked up their crops were empty, or full of unsuitable food which dried without being digested. Mortality of this kind is always noticed and commented upon in the case of game birds, since so many interests are affected by good and bad game seasons, apart from the interest of the problem of what kills the birds. But if statistics were kept in regard to mortality among other birds, the results would probably be found to be very much the same. Drought is one of Use chief causes of the failure to increase in numbers of oat-labs species of birds which we protect and encourage.
Last spring, to judge from certain areas over which observation is usually fairly close, was one of the worst nesting seasons of recent years. The number of nests was smaller than usual, and out of those found and watched the percentage which came safely through to the flying of the young birds was extraordinarily small. As regards the nests built, it is only possible to compare numbers with those which were found in the preceding year in the smaller defined areas. Take for example a typical enclosure as to which fairly precise figures are available—a space of about twelve acres of garden, wood, and field. It is not in every way an ideal piece of ground for birds for a good deal of the wood is larch, but everything possible is done to pre. tect and encourage the birds with food in the winter, water in the summer, and nesting-boxes. In 1913 there were two nuthatches' nests, one of them that of a pair of birds which became tame enough to call to the writer on his going out in the garden, and to fly down and pick up nuts thrown le them close at hand. This pair of nuthatches disappeared in December of the same year, and no nuthatch was seen or heard near the house until twelve months later, when another pair made their appearance, and soon became almost as tame as their predecessors. In 1913 a wryneck, whose neat was never discovered, was continually being caught in the nets over a strawberry bed ; the next year there were no wrynecka Only just over half of the nesting-boxes, which were all taken in 1913, were occupied, and in these nest after nest failed. A marsh tit's was pulled out by jays. A great tit's and a blue ties were destroyed, possibly by a squirrel or a cat. But the reason for the end of other broods, Wetly great tits, was more obscure. One brood flew safely from the box and then died one by one during the next few days. Aboxcontaining another brood, which had unaceountably become silent, was examined, and all the young birds were found dead in the nest. Another brood flew and disappeared, and though as a rule in the first week in July the garden is full of the " wizza-wuzza-wizza " of young tits being fed, during that July there was scarcely a tit to be seen. Other birds which do not nest in holes or boxes fared equally badly.
The jays, which during recent seaaone have increased to • disconcerting extent in numbers, while they certainly have not decreased in cunning, accounted for rasa] nests of robins, wrens, hedgesparro we, chaffinches, black- birds, and. thrushes Two nightingales were singing in the early part of May, and then both stopped; it may have been the cold wind, and it may have been jays, or possibly a poaching cat, which belonged to nobody, lived wild in the woods, and was fortunately at last shot. But the real problem in such seasons as these is not the destruction of nests by the birds' natural enemies, but the death or disappearance of young and old birds alike. The only reason which suggests itself is the failure of the supply of insect food. The birds picked tip S eem to be what the gamekeepers call "going light," and though that may he due to some wasting disease, that very disease may well have been due to the bird's constitution being weakened by insufficient food.
It is possible that in these seasons, when the mortality among the smaller nesting birds seems to be higher than usual, we may be witnessing a parallel to the ravages of 61.103 an epidemic as grouse disease. The problems of groans disease, of course, have been fully investigated by a Com- mittee, and it is known that the actual cause of death is the presence in the caeca of overwhelming numbers of a small tbreadworm, Trichostrongylus pergracitia, which is a parasite in healthy grouse, but does not kill unless the bird's constitu- tion is weakened by starvation owing to the lack of a proper manly of the young heather which is the grouse's staple food. Grouse disease nearly always occurs after a year in which the birds have become too many for the available heather supply, and though not everything is known as to the reasons which govern the outbreak or absence of disease in places apparently similarly situated, and though there are many obscure ques- tions waiting to be answered in regard to the migration of grouse to and from moors where disease is rife, still, the curve of mortality among grouse can be observed, and up to a point is regular. So, probably, would be theenrve of mortality among such birds as woodpigeons, which are subject to deci- mating outbreaks of a form of diphtheritic roup; but no one has thought it worth while to obtain statistics on this point on any scale large enough to be valuable. In the ease of both these birds disease follows overcrowding, and it is con- ceivable that as regards other and less easily observed birds the level of population over large areas is maintained by some similar decimation when the curve reaches too high a point. It may be that when birds seem for some reason to absent themselves wholly from their usual haunts, what we are really witnessing is the dip in the curve of popnlation, and that in a few years' time we shall find the curve at the highest point again. It is also possible that by the very process of protection and encouragement we may be altering or hastening the waves in the curve. Up to a point we can certainly add to the numbers of particular species, and by protecting them can induce theni to nest and multiply in particular places, as has been proved in the case of Kentish plovers on the South Coast. On the other hand, in another area, also protected by a watcher, it happened lest year, for no visible reason, that numbers of the young terns died soon after they were batched. That, possibly, may have been a case of over-population; and perhaps in the same way it may happen that by inducing birds to nest in places where they would not otherwise be numerous we may be upsetting in some way the balance which Nature desires to maintain. Only prolonged observation over different areas of ground could decide such a point, but a comparieon of individual experiences during the past few years, when so many nesting boxes have been put up in gardens which formerly had none, might provide some useful information. One point seems to be pretty clear, which is that some species are freer from epidemics than others. Sparrows do not decrease in numbers, nor do starlings, nor, so far as can be seen, do rooks. But do rooks increase in numbers ? It would be extremely interesting to know, and it would he easy to ascertain, if any one in authority thought it worth while. But a census of rookeries Las more than once been suggested as a valuable basis for deter- mining the services or disservices of the rook to the farmer, and no Board of Agriculture hitherto has considered a census necessary.
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