Observations on Periodical Birds. 131 
Remarks tending to establish the opinion that 
the Periodical Birds migrate. 
THe gradual increase of temperature in 
spring, and its decrease in autumn, are cir- 
cumstances that seem to be so closely con- 
nected with the appearance and disappear- 
ance of the periodical birds, that they have 
long been considered as the primary causes 
of these phenomena. In reflecting on this 
very generally received opinion, it occurred 
to me, that [ had never met with any 
attempt to ascertain how nearly the tem- 
perature at the time of the appearance of 
these. birds coincides with the temperature 
at the time of their departure; and as this is 
a consideration of no slight importance, I 
resolved, at least in some measure, to supply 
the deficiency. For this purpose, I took 
from my journal the dates at which several 
of the more remarkable species of periodical 
birds have appeared and disappeared in this 
neighbourhood during the last five years; 
and having arranged them in the order of 
their occurrence, I attached to each the 
mean temperature for that day, as in the an- 
nexed table, (see table.) 
According to this table, it seems that the 
temperature is, in general, considerably 
higher when the sand martin, willow wren, 
