394 



HYBERNATION. 



Animals 



which 

 Migrate. 



Tempera- 

 ture. 



tricts at the end of our summer, in order to procure 

 support. Montagu, when speaking of the cuckoo, 

 makes the following pertinent observations. " Few 

 birds but the titmice will devour the larvae of the cab- 

 bage 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 I.e- 

 pidopterous larvce, more especially the rough sort. It 

 is therefore probable, that the early remigration of this 

 bird is the defect of this favourite food, the greater 

 part huving by that time enclosed themselves, prepa- 

 ratory to a change. Of the many cuckoos we have dis- 

 sected in the months of May and June, the stomach 

 has always been found to contain more or less of the 

 hairs of caterpillars, and sometimes quite full of them." 



If insects are thus the favourite food of many of our 

 Bummer 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 dependence 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 obtained. Montagu mentions 

 a curious fact of this kind with regard to the chimney 

 swallow. " It makes its first appearance with us in 

 April, sometimes as early as the first week, if the wea- 

 ther is mild ; and it sometimes happens, that after their 

 arrival a long easterly wind prevails, which so benumbs 

 the insect tribe 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 neighbour- 

 ing villages around. The thermometer was at 42, 

 and we were at a loss to conceive what was become of 

 these birds, which a day 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 suffi- 

 cient 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 regions 

 of the north, where, during winter, these are all fro- 

 zen, and the extent of their migration southwards de- 

 pends 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 cer- 

 tainly 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 granivo- 

 rous 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 a 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 ap- 

 proach of winter, it descends to a lower level, while it 

 occupies the higher grounds in more southern districts. 

 Its migrations to the south, therefore, 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 succeed- 

 ed, however, in America, with General Diivies, who in* 



Animals 

 which 



forms us, (Linn. Trans, vol. iv. p. 157.) that the snow 

 bird of that country always expires in a few days, (af- 

 ter being caught, although it feeds perfectly well,) if ^S^ 

 exposed to the heat of a room with a fire or stove ; but 

 being nourished with snow, and kept in a cold room or 

 passage, will live to the middle of summer : a tempera- 

 ture much lower than our summer heat proving de- 

 structive to these birds. The swallow, on the other 

 hand, seems to delight in the temperature of eur sum- 

 mer, and at that heat to be able to perform the higher 

 operations of nature. When attempted to be kept du- 

 ring 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, " Mares inter pri- 

 mas sunt aviculas, quas sonum suum hieme usitatum 

 in cantum vertunt jucundissimum : vere primo, sub ini- 

 tium mensis regelationis, arboribus ad pagos insidentes 

 ga.rru\i, fceminis adhuc absentibus, ver indicant adstans. 

 Kedeuntibus denique turmis maximis, qua? coelum fere 

 abscondunt, foeminis, omnes conjuges requirunt, quibus 

 conjuncti sylvas petunt, ibi ut nidulos construant et 

 multiplicentur. Initio mensis defolialionis mares suos, 

 apud nos remanentes, fcemina: deserunt mutabiles, solae 

 regiones petentes peregrinas." The same cause, name- 

 ly temperature, renders some birds migratory in one 

 country, while they are stationary in another. No se- 

 paration 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 sta- 

 tionary with us. 



But, independent of these two causes, we presume, Breeding, 

 that the desire of obtaining a safe breeding place is like- 

 wise intimately connected with the movements of many 

 species. " Of the vast variety of water fowl," says 

 Pennant, " that frequent Great Britain, it is amazing to 

 reflect how few are known to breed here : the cause 

 that principally urges them to leave this country, seems 

 to be not merely the want of food, but the desire of a 

 secure retreat. Our country is too populous for birds 

 so shy and timid as the bulk of these are. When great 

 part of our island was a mere waste, a tract of woods 

 and fens, doubtless many species of birds (wliich at this 

 time migrate) remained in security throughout the 

 year. Egrets, a species of heron, now scarce known in 

 this island, were in former times in prodigious plenty 

 and the Crane, that has totally forsaken this country, 

 bred familiarly in our marshes, their place of incu- 

 bation, as well as of allother cloven-footed mater-fowl (the 

 heron excepted) being on the ground, and exposed to 

 every one. As rural economy increased in this coun- 

 try, these animals were more and more disturbed ; at 

 length, by a series of alarms, they were necessitated to 

 seek, during the summer, some lonely safe habitation. 

 On the contrary, those that build or lay in the most 

 inaccessible rocks that impend over the British seas, 

 breed there still in vast numbers, having little to fear 

 from the approach of mankind ; the only disturbance 

 they meet with, in general, being from the desperate 

 attempts of some few to get their eggs." It happens, 

 in consequence of this desire of safety during incuba- 

 tion, that the same species may be stationary at one 

 place> while it is migratory at another. Thus, in the 

 Western Islands, the common plover is stationary, 

 while on the Grampians it is migratory. Herons 

 may be met with along all the British shores during 



