94 The Scottish Naturalist. 



affirm that the failure of food necessary to different kinds of 

 birds is the great stimulant to migration, and upon which it 

 solely depends ; others again, that it is the want of light, and 

 that the sun alone governs their motions. True, but it is the 

 combination of these ; or in other words, the seasonal changes 

 from the earth's position, in relation to the sun, either increas- 

 ing or diminishing the peculiar food on which they subsist, as 

 well as those constitutional causes above alluded to, which 

 compel them, according to the season of the year, to seek the 

 climate most suited to their wants, not only for themselves but 

 for their tender progeny, requiring a temperate climate and the 

 necessary caterpillar food, without which they could not subsist, 

 and which, had the parents remained in their winter quarters, 

 could not have been obtained in sufficient supply, as most grubs 

 and caterpillars would by that time have changed into a perfect 

 state. Besides which, as is the case in many of our waterfowl, 

 the impulse implanted in them of reaching a place of refuge 

 far from the haunts of man, where they can in safety rear 

 their young, urges them on with irresistible power to the 

 extreme wilds of the North, from whence they return in count- 

 less myriads to replenish the earth. 



Birds are especially affected by the extremes of heat and cold, 

 and any one who has lived in warm countries may have noticed 

 the total absence of bird-life during the great heat of the day; 

 not a note, not a sound is to be heard, but that of the 

 monotonous and never ceasing chirp of the cicada insect, 

 which seems to glory in the heat. The cool of the evening and 

 sun-rise, however, tells a very different story ; the woods and 

 groves then resound with life. 



On the other hand, many of our summer birds of passage 

 are most susceptible of cold, and Mr. J. H. Gurney gives an 

 instance of this in the case of the Swallow * even so far south 

 as Algeria. "On the 26th and 27th of March," he states, ''we 

 experienced at Laghouat a very cold wind; so benumbed were 

 the poor swallows, that hundreds might have been killed with 

 stones. The poor birds were to be seen sitting about in all 

 directions." And he goes on to say " I am inclined to think 

 they were suffering from cold, rather than from the want of insect 

 food. Insect life seemed to me to be always abundant." This 

 susceptibility of cold, is frec[uently to be noticed in our winter 



* Ibis. 1 87 1, p. 74. 



