Dr. Jenner on the Migration of Birds. 99 



the blackbird, will occasionally afford us a transient song; 

 but it may be observed, that the notes of these birds are rather 

 to be considered as plaintive, than lively. The lark, too, will 

 sometimes mount in the air, beguiled, as it were, by the faint 

 rays of a wintry sun, but his notes are then as poor and feeble 

 as the beams that call him forth. The robin indeed cheers 

 us with his song during the whole of the winter, unless driven 

 off by intense frost, and is the only bird I know, whose notes 

 at this time would fully accord with our feelings, so perfectly 

 do they mingle with the surrounding order of things. The 

 goldfinch, were he now to open his full song upon us, would 

 be as appalling as tones of the owl in the midst of a fine sum- 

 mer's day. 



III. 



Mr. John Hunter, my late valued friend and honoured 

 preceptor, under whose roof I first caught a gleam of that 

 light which so successfully conducted him through the obscure 

 paths of nature, first demonstrated the different sizes of the 

 testes of birds at different seasons of the year. On a further 

 investigation of this subject, a fact presented itself to me, which 

 may not be unworthy of the attention of this Society, and, as 

 it is in some measure connected with the preceding observa- 

 tions, I have taken the liberty of annexing it. 



In those birds that remain but a short time paired with the 

 female, there appears a vast disproportion in the size of the 

 testes, compared with those that live in the connubial state 

 much longer. The cuckoo and the swift point out the fact 

 most obviously. The common brown wren, which remains 

 united with its female from the early part of spring until the 

 autumn, exhibits testes very far exceeding in size either those 

 of the cuckoo or the swift. The cuckoo, although a polyga- 

 rnist, may here be considered in the same point of view as the 

 birds that pair. The time which he devotes to the female 

 being so very short, more so indeed by some weeks than even 

 that of the swift, the testes are formed extremely small in pro- 

 portion to the size of the bird. I never saw them exceed in 

 size the common vetch, while those of the wren were full as 

 huge as a common sized garden pea. The medium weight 

 of the cuckoo is about four ounces and a half, that of the wren 

 but little more than three drachms*. The testes of the swift, 



* Ornithologists might easily have given us the weight of a bird with 

 greater precision, by divesting the stomach of its contents previous to the 

 bird being weighed. For example : how very different must the weight 

 of the owl be, which, in its nocturnal (lights, had the luck to pick up a 

 mole or two, compared with that which had met with opposite fortune; 

 ■ >i of the falcon, that had picked the bones of a leveret, or of the one that 

 •\a. killed with an cni|it\ stomach ! 



N 2 which 



