136 Observations on Periodical Birds. 
are occasionally to be seen at the customary 
time in spring, even when the weather is 
frosty, the increments of temperature by no 
means corresponding withthe sun’s increasing 
northern declination, but they generally 
seem to withdraw again. On the 9th of 
April, 1821, several sand martins were ob- 
served at a sand-pit in the township of Cheet- 
ham, but the weather becoming cold and 
stormy, they quickly disappeared: they were, 
however, soon after discovered in greatly 
increased numbers, at asheltered bend of the 
river Irwell, in the adjoining township of 
Broughton. This circumstance proves, that 
if the weather is severe and boisterous when 
the summer birds are first seen in spring, 
they do not retire to their winter retreats, as 
has been supposed, but merely seek sheltered 
situations where they can procure a supply 
of food.’ 
Inquiries into the temperature of the sup- 
posed winter retreats of the periodical sum- 
mer birds, may now be looked upon, it is 
presumed, as quite superfluous, since it is 
sufficiently apparent from the preceding ta- 
bles, that even that of the atmosphere is much 
higher at the time they disappear than it is 
covered the lesser pettychaps (Motacilla hippolais) in the south of 
Devonshire, in mild winters. 
NE 
