BIRDS OF PASSAGE. 
£ 0 ? 
attention of the naturalist, because the circumstance, when pro- Local circnm- 
perly understood, shews how attentive the bird is to the local stance ’ 
causes, which retard the spring in certain districts. The 
swallow appears upon an average, six days earlier at Stratton 
in lat. 52° 45-, than at Catsheld in lat.51°. There is little 
or no doubt that this apparent exception to the present theory 
arises from some circumstances which retard the increase of 
the vernal temperature at Catsield ; and make the spring ad- 
vance more quickly at Stratton. As I am unacquainted with 
the situations of both places, it will 'be proper to state a few 
facts, which shew how powerfully causes of this sort influence 
the excursions of migrating birds. 1 st. The bapk martin, greatly in- 
(Hirundo ripnria) is commonly seen at the mouth of the river -fluence the 
Kent six or seven days before it arrives at Kendal, though the ^ rc j JS< 
distance does not exceed five or six miles. But the town lies 
near the mountains ; and the air is colder in that part of the 
valley than at the head of the estuary. 2 d. I have frequently 
heard the red-start, the yellow-wren, and the white-throat, 
singing in the gardens at Kendal, two or three days prior to 
their arrival at Middleshaw. I attribute this difference to the 
same cause 5 for Middleshaw lies 200 feet higher than the 
town, being distant from it three miles to the south east. 
Lastly, the chimney-swallow w^s seen at Kendal on the 24th 
of April, A. D. 1808 5 but did not make its appearance at 
Settle, before the first of May. The latter town lies south of 
east thirty miles from the former, in a mountainous district not 
far from the source of the Kibble. 
The preceding instances, with other facts of a similar nature, p eca pitula~ 
shew how absolutely the motions of the birds under considera- tion. The 
lion, are regulated in the vernal months by local causes affect- JIrdsfs seen 
ing local temperature ; and the principal object of the present upon scales] of 
essay may be called an attempt to demonstrate, that the same ma ^ 
leading cause, naturally connected with the article of food, 
compels them to traverse the temperate zone, wholly or in part, 
twice in the course of the year. When the phenomena of 
migration are considered in this way, winter and summer birds 
of passage become relative terms, belonging to the place of 
observation. For instance, the twite inhabits the southern 
parts of Britain during the cold months, but returns to the 
hills of Yorkshire in spring $ and if we may judge from the 
opposite 
