The Effects of Fluctuations in the Number of Birds.—It is now fully recognized that wherever the insect-feeding birds of 4 district or of districts are destroyed, either wantonly or through climatic and other causes, there is an accompanying insect oscil lation which is not reduced until the balance of bird life is re stored. In the case of many insects whose numbers remain rela tively constant, the controlling influence is largely, if not entirely, due to the uniformity of the bird life which prevails from year to year. Any factor, therefore, that tends to modify or upset the restraining influences of wild birds in their relationship to injurious insects is distinctly detrimental. In a like manner, where raptorial birds and owls are destroyed, farm vermin, such as rats, voles and mice, rapidly increase in numbers.
Some species of wild birds are injurious because they are too numerous, and as a result there are too many birds feeding upon the same kind of food in a given area, in consequence of which certain species supplement their diet by feeding upon cultivated crops. No better instance of this can be found than the European house sparrow. It has been estimated that in Great Britain alone the losses due to this species reach the incredible figure of .I8,000,000 per annum. Wherever it has been introduced it has in creased and spread with startling rapidity, and proved an enemy to the cultivator. Injury to crops invariably proceeds from an excessive number of individuals rather than from the natural habits of the species. Once any species exceeds what may be described as its high-water mark of abundance, its food habits change and it becomes a source of danger to mankind. The failure of a normal source of food supply occasionally leads birds to in jure crops which under ordinary conditions they seldom attack. Many fluctuations in number are either only local or temporary, and if not interfered with will adjust themselves.
The contention that insectivorous birds do more harm than good by attacking beneficial insects and parasitized caterpillars does not appear to be well founded. Similarly those species which feed largely upon the seeds of weeds have been regarded as bene ficial, but it is now known that many of these species act as distrib utors of the seeds. In any economic consideration of such birds it is perhaps best to regard such activities as neutral, but on this point considerable diversity of opinion exists.
status of wild birds can be complete without a reference to the food of the young birds in the nest. During the first few days of life nestlings consume daily considerably more than their own weight of food and add 20 to 50% to their weight. From sunrise to sunset feeding continues, 200 to 300 visits being paid to the nest by the parent birds. With the exception of doves and pigeons, and aquatic and raptorial species, the food brought to the young consists of caterpillars, soft-bodied insects, spiders, worms and slugs. Moreover, during the whole of the nesting period the parent birds are feeding upon food similar to that fed to the young. "Few people," states H. C. Bryant, "have any realization of the great quantities consumed by birds. For instance, if we consider that there is an average of one meadow lark to every two acres of available land for cultivation (I t,000,000ac.) in the Sacramento and San Joaquin valleys, and that each pair of birds raises an average of four young, each one of which averages one ounce in weight while in the nest, and consumes half of its own weight of food each day, it takes over 3431 tons of insect food each day to feed the young birds in the great valleys alone." It is difficult to bring home to the mind by an expression of figures the millions of caterpillars, grubs, flies, beetles, etc., that birds consume. Careful investigations have shown that the British song thrush during the breeding season, April to June, consumes on an average io,o8o caterpillars, flies, grubs, etc., per month; as suming that there are zoo,000 of these birds in the British Isles, they would account for 3,024,000,00o insects, etc., weighing up wards of 52o tons. Such a number are capable of destroying in three months upwards of 65,000 tons of produce, and assuming that this was worth 15 per ton, the activities of the thrushes would result in a saving of £325,000. Other species are equally beneficial, so that these figures might be multiplied by 20 or 30, thus showing a saving of produce every spring approaching ten millions sterling.
To summarize :—"A careful examination of the circumstances in which birds have done harm leads to the belief that the damage is usually caused by an abnormal abundance of a species within a limited territory. In such cases so great is the demand for food that the natural supply is exhausted and the birds attack some of the products of garden or orchard.