Dr. Jenuer on the Migration of Birds. 59 



sects which are to afford them food. If the numbers which 

 flock in upon us in May, were to arrive in April, when only 

 part of them appear, all must be insufficiently supplied, and 

 many of course perish from a want of the needful succours ; 

 but by the middle of May, jnyriads of insects have produced 

 eggs, and great numbers have either brought forth or ma- 

 tured their progeny; and it may be remarked there is still a 

 greater increase of insect food by the time the young birds 

 begin to require it. Swallows, on their first coming, feed 

 principally upon gnats. These insects are called forth from 

 their wintry retreats when the air is but moderately heated, 

 48 degrees of Fahrenheit's thermometer being sufficient to 

 put them on the wing. It is in pursuit of them that we see, 

 in cool weather, the swallow incessantly skimming over the 

 surface of ponds and brooks; and their thus early hovering 

 over water has strengthened the idea of their having lately 

 emerged from their watery abode, where they are supposed 

 to have lain dormant during the winter. But they are driven 

 by necessity to feed on the gnat. Like the swift and mar- 

 tin, their more favourite food is a small beetle of the sca- 

 rabasus kind, which, on dissection, I have found in far greater 

 abundance in their stomachs than any other insects. 



The tumid state of the testes and ovaria sometimes comes 

 on prematurely, and in the same manner sometimes subsides. 

 When this happens, swallows and martins desert their nest- 

 lings, and leave them to perish in the nest. The economy of 

 the animal seems to be regulated by some external impulse, 

 which leads to a train of consequences. When this change 

 in the testes and ovaria takes place, the bird becomes impelled 

 by a stronger principle, that is, the desire of self-preservation. 

 This sometimes happens when they produce a very late hatch. 

 A pair of martins hatched four broods of young ones in the 

 house of a tradesman in this place in the year 1786. The 

 latter brood was hatched in the early part of October. About 

 the middle of the month the old birds went off, and left their 

 young ones, about half fledged, to perish. The pair returned 

 to the nest the 17th of May 1787, and threw the skeletons 

 out. 



Thus scarcely a winter passes but we hear of a nest of 

 robins, hedge-sparrows, and some others of the smaller birds. 

 We have been informed by Pennant, and it has been noticed 

 also by others, that the cuckoo has been heard to give his 

 song early as so the middle of February, two months sooner 

 than the usual time. The same deviation from the ordinary 

 course of nature, which prematurely occasions the pairing of 



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