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fiich Infects as Birds, that feed upon the Wing, and par- 
ticularly the Swallow Kinds fubfift on,, muft make them 
pcrifh inevitably, were they not to change their Quar- 
ters for thofe more favourable Climes, where a Con- 
tinuance of warm Weather affords their natural and 
proper Food. This their Sagacity dilates to them, 
and is the apparent Caufe of their periodical leaving 
us at the Approach of Winter, before Flies are fo 
diffipated by Cold and Winds as to be found no 
longer in the Air 5 tho* they may with other Infetts 
be met with in Holes and hidden Receffes, and ferve 
to fubfift other Birds of Paffage. 
What 1 infer from hence is, that as Swallows can- 
not continue and fubfift fo long in cold Seafons as 
other Birds of Paffage, they are neceffitated to vifit 
us fomewhat later, and to depart fooner : For tho' 
Nightingales, and other Birds of Paffage, are not often 
feen or obferved after they ceafe finging, yet I have 
frequently taken notice of them in their folitary 
Coverts a Month after the Departure of Swallows. 
From thefe Reafons I therefore conclude, that Birds 
of Paffage, particularly Swallows, are neceffitated to 
pafs the Tropic of Cancer s but how far more South, 
or to what Part of the Southern Hemifphere they 
go, remains unknown. 
The Manner of their journeying to their Southern 
Abode may vary, as the different Structure of their 
Bodies enables them to fupport themfelves in the 
Air: Thofe Birds with fhort Wings, fuch as the 
Red-ftart, Blackcap, &c. tho' they are incapab e of 
fuch long Flights, and with fo much Celerity, yet I 
can't fee why they may not pafs in the like manner, 
but by gradual and flower Movements. Swallows 
L 1 1 2 and 
