Dr. Jenner on the Migration of Birds. 91 



its origin from this source, as neither the winter mew nor 

 any of its congeners ever make any stop, and are rarely seen 

 in his neighbourhood. 



Carr End. , WlLLIAM FoTHERGILL. 



XV. Some Observations on the Migration of Birds. By the 

 late Edward Jenner, M.D. F.R.S. ,- with an Introductory 

 Letter- to Sir Humphry Davy, Bart. Pres. R.S., by the 

 Rev. G. C. Jenner. 



[Concluded from p. 60.] 



I PROCEED now to make some observations on another 

 -■■ kind of migration, directly opposite to the foregoing, namely, 

 the return of the spring migrators to their respective homes. 



The great disproportion in numbers between those species 

 of birds which quit the country in summer, and those that 

 leave it at the autumnal season, has led naturalists to lose sight 

 of the early migrators, and to confine their reflections on the 

 subject to the late ones only. Hence the common observa- 

 tion, that they are all driven off through a failure of food or 

 a cold temperature of the air. But seeing that many of them 

 disappear in the summer season, when food is placed before 

 them in the greatest plenty, we must seek for some other 

 cause. If we examine what is now going forward in the ani- 

 mal economy, dissection will point out a change in the testes 

 and ovaria, the very opposite to that which took place in the 

 spring. These parts now begin to shrink*, the disposition 

 for raising a further progeny ceases, and the nuptial knot is 

 dissolved. What inducement have they to stay longer in that 

 country where, I think, it clearly appears their chief object is 

 to multiply their species? This being now effected, they re- 

 tire to different parts of the globe, doubtless better suited to 

 their general dispositions and wants, when disengaged from 

 parental duties. In many of the migrating species, indeed in 

 the far greater number, the disposition for further incubation, 

 and the season for their procuring a further supply of insect 

 food, cease at the same time. It is pretty evident from the 

 habits of the cuckoo and the swift, that quit us in the summer 

 as soon as their nesting is at an end, that swallows, martins, 

 and those birds that disappear in the autumn, would depart 

 at an earlier season, even though their supplies were to con- 

 tinue, if the rearing of their young were perfected. Indeed, 

 as lias been before observed, so strong does this propensity 



* I examined a female cuckoo the first week in July, and found the 

 oviduct shrivelled, and all the cji^s disposed of. 



M 2 now 



