16 
The late Dr. Jenner 
This is no very rare exhibition. Necessity here, as in migra- 
tion, becomes the parent of exertion, which when thus called 
forth, cannot be shown in a much greater degree by any of 
the feathered tribe. The moor-hen (which winters with us) 
gives another instance of what a bird, which appears so much 
to want activity in its ordinary flights, is capable of perform- 
ing when exertion is actually required. When pursued by a 
hawk, and self preservation calls up all its powers, it may be 
seen to rush up into the air with amazing velocity, almost as 
high as the eye can reach, then darting down with an equal 
pace, it often, by such rapid manoeuvres, escapes the destruc- 
tive talons of its swift pursuer. 
It is a remarkable fact that the swallow tribe, and proba- 
bly many other birds which absent themselves at stated 
periods, should return annually to the same spot to build 
their nests. The swift, which for nine months has some dis- 
tant region to roam in, was selected for the purpose of an 
experiment to ascertain this with precision. At a farm-house 
in this neighbourhood I procured several swifts, and by tak- 
ing off two claws from the foot of twelve, I fixed upon them 
an indelible mark. The year following their nesting places 
were examined in an evening when they had retired to roost, 
and there I found several of the marked birds. The second 
and third year a similar search was made, and did not fail to 
produce some of those which were marked. 1 now ceased 
to make an annual search, but at the expiration of seven 
years, a cat was seen to bring a bird into the farmer's kitchen, 
and this also proved to be one of those marked for the expe- 
riment. 
That the bird, when the stimulus for migration is given, 
