56 Dr. Jenner on the Migration of' Birds. 



attachment which the male begins to show to the female, 

 plainly indicate some new agency acting upon the constitu- 

 tion *. This newly excited influence, which so conspicuously 

 alters the habits of our birds at home, is, at the same time, 

 exerting itself abroad upon those which are destined to resort 

 hither. It is the -preparation which nature is maki?ig for the 

 production of an offspring by a new arrangement in the struc- 

 ture of the sexual organs, {viz.) the enlargement of the testes in 

 the male, and the ovaria in the female. 



No sooner is the impulse arising from this change suffi- 

 ciently felt, than the birds are directed to seek a country 

 where they can for a while be better accommodated with suc- 

 cours for their infant brood, than in that from which they de- 

 part f. 



It is not at the commencement of this enlargement, nor until 

 it is considerably advanced, that the birds are prompted to 

 migrate ; and this is very wisely ordered ; for were they to set 

 off when first the testes and ovaria begin to grow tumid, they 

 must waste much time here unnecessarily, and indeed arrive 

 at too early a period to find a supply of food. Very little time 

 is lost after their arrival, before they form their connubial al- 

 liances J. The business of nesting then begins ; and as a con- 

 vincing 



* The rook, among many others, exhibits a familiar instance of the 

 change of voice. 



f Birds of the same species that are commonly stationary in this island 

 throughout the year (1 say commonly, for all, I believe, occasionally migrate) 

 are migrators in other countries. The adult bird might, perhaps, find <i 

 subsistence for itself in the country it quits during the incubating season ; 

 but the nestling is probably the object nature chiefly holds in view, both 

 with respect to food, and to the temperature of the air in which it is first 

 to feel existence. The one may be unfit or too scanty, and the other too 

 hot or too cold. It is wonderful to see with what peculiar care the parent 

 birds select the food for their young until they are four or five days old. 

 For the most part it is purely animal ; but not an atom even of that is suffered 

 to go into the nestling's stomach, that is not perfectly adapted to the tender 

 state of its digestive powers. While the swift is feeding on small beetles that 

 have hard crustaceous wings, and whose habitations are the air, its nestlings 

 are fed in their early state with gnats. The sparrow, a granivorous bird, 

 feeds its young for several days after they are hatched, with the softest in- 

 sects only, now and then introducing a little coarse sand, smooth on the 

 surface, to inure the stomach, as I suppose, to bear the same kind of sub- 

 stances in a more rugged state, which will shortly be required. 



X Should a fatal accident befall either the male or female bird after this 

 alliance is newly formed, no time is lost in unavailing sorrow, nor any great 

 nicety shown in forming a new connexion, as the following little history 

 will evince. A pair of magpies began to build their nest in a gentleman's 

 garden at Burbage, in Wiltshire. Disliking their familiarity, he shot one of 

 them from an ambush made for the purpose. The next day there were 

 again a pair going on with the work. One of these was also shot. The 

 loss was not long in repairing j for the day following the pair were again 



complete, 



