Having established the principal facts regarding the pe riods of migration, and'the circumstances by which it is accompanied, it now remains for us to ascertain those proxi mate causes to which these movements are to be referred. Powerful indeed must be the causes which prompt those animals to forsake the woods in which they were reared, or the rocks on which they were hatched, and undertake a perilous journey to distant countries. They must be inti mately connected with the first laws of life, otherwise the movements to which they give bit th would not be so con stant and uniform. The procuring of a supply of Food, a suitable temperature, or a safe breeding place, are probably all the proximate causes which have any concern irr such migrations.
If we attend to the food of many of our summer visitants, we may easily perceive, that it can only be procured during those months in which they remain with us. Subsisting chiefly on insects, they are compelled to shift their quar ters, and retire to warmer districts at the end of our sum mer, in order to procure support. Montagu, when speak ing of the cuckoo, makes the following pertinent observa tions. " Few birds but the titmice will devour the larva of the cabbage butterflies ; and none that we have noticed make a repast on the hairy species of caterpillars but the cuckoo, who is a general devourer of all kinds of Lepidop terous larva, more especially the rough sort. It is there fore probable, that the early remigration of this bird is the defect of this favourite food, the greater part having by that time enclosed themselves, preparatory to a change. Of the many cuckoos we have dissected in the months of May and June, the stomach has always been found to con tain more or less of the hairs of caterpillars, and sometimes quite lull of them." If insects are thus the favourite food of many of our sum mer birds of passage, it must frequently happen that their food will be scarce, even after their arrival in this country, owing to the variableness of our climate, and the depen dence of the movements of insects on the temperature of the weather. hence it happens, that some birds disappear again, retiring to other districts where insects are to be ob tained. Montagu mentions a curious fact of this kind with regard to the chimney swallow. " It makes its first ap pearance with us in April, sometimes_as early as the first week, if the weather is mild ; and it sometimes happens, that after their arrival a long easterly wind prevails, which so benumbs the insect tithe, that thousands die for want of food. We recollect, as late as the ninth of May, the swal lows on a sudden disappeared from all the neighbouring ,villages around. The thermometer was at 42°, and we were at a loss to conceive what was become of these birds, which a clay or two before were seen in abundance. But by chance we discovered hundreds collected together in a valley close to the sea side, at a large pool which was well sheltered. Here they seem to have found some species of fly, though scarce sufficient to support them ; for many were so exhausted, that after a short time on wing, they were obliged to pitch on the sandy shore." In the case of the waders, which obtain their food in the neighbourhood of springs and marshes, they are compelled to leave the re gions of the north, where, during winter, these are all fro zen, and the extent of their migration southwards depends on the severity of the weather.
A supply of food is certainly one of the proximate causes of migration, since we can support many of our summer visitants during the winter, as the nightingale for example, by giving them a regular supply of food. But powerful as this principle may appear, it is certainly not the only one in operation ; as we observe one or two species of a genus migrating, while the others are stationary ; and this taking place among granivorous as well as insectivorous birds. Equally powerful as the desire to 'obtain food, seems to be the love of a suitable temperature.
If we attend to the motions of the snow bunting, which is D granivorous bird, we find, that on its first arrival in this country it is only to be met with on the high grounds. As the temperature sinks at the approach of winter, it descends to a lower level, while it occupies the higher grounds in more southern districts. Its migrations to the south, there fore, depend entirely on the state of the winter. It has been attempted to preserve these birds during the summer season in this country, but, although liberally supplied with food, they have not survived. The experiment has succeeded, however, in America, with General Davies, who informs us, (Linn. Trans. vol. iv. p. 157.) that the snow bird of that country always expires in a few days, (after be ing caught, although it feeds perfectly well,) if exposed to the heat of a room with a fire or stove ; but being nourish ed with snow, and kept in a cold room or passage, will live to the middle of summer ; a temperature much lower than our summer heat proving destructive to these birds. The swallow, on the other hand, seems to delight in the tem perature of our summer, and at that heat to be able to per form the higher operations of nature. When attempted to be kept during our winter, besides a regular supply of food, care must be taken to prevent it from being benumbed with Cold. It is probably owing to some constitutional dif ference with respect to cold, that the female chaffinches in Sweden are migratory during winter, while the males are stationary. Eckmark, when speaking of the migrations of this bird, informs us, 44 Mares inter prirnas sent avictilas, qua sonuin suum hieme usitatum in cantum vertunt jucun dissimum : vere primo, sub initium mensis regelationis, ar boribus ad pagos insidentes garruli, faminis adhue absenti bus, ver indicant adstans. Redeuntibus denique turmis maximis, qua ccelum fere abscondunt, jaminis, omnes con juges requirunt, quibus conjuncti sylvas petunt, ibi tit nidu los construant et multiplicentur. Initio mensis defoliationis mares suos, apud nos remanentes,famina deserunt muta biles, sola regiones petentes peregrinas." The same cause, namely temperature, renders some birds migratory in one country, while they are stationary in another. No separa ration of this kind takes place between the sexes of the chaffinch in this country. The linnet, which is a summer bird of passage in Greenland, is always stationary with us.