2 ON THE MIGRATION OF BIRDS. 
numerous species of Periodical Birds is still very im- 
perfect; and we are almost entirely ignorant of the 
places of their retreat, and of the mode of their exist- 
ence in those retreats. Whether, when they with- 
draw, they depart from those districts and countries 
in which they cease to appear, or whether they con- 
ceal themselves and remain in a state of torpidity, 
has not yet been positively determined ; and opinions 
must continue to be divided on the subject so long 
as authors indulge in fanciful speculations, instead 
of collecting and arranging well-authenticated facts, 
from which alone legitimate conclusions can be de- 
duced. 
The accumulation of facts, then, appears to be the 
most important object to be attained at present; and 
my principal motive for giving publicity to the fol- 
lowing Tables and remarks is the hope that they may 
be found to contribute, in some degree, to increase 
our scanty stock of information on this obscure branch 
of natural history. 
It is remarkable that almost all the catalogues of 
Periodical Birds with which I am acquainted have 
been formed from observations made in the south of 
England. This circumstance is certainly calculated 
to give additional interest to the following Tables, 
made in so northern a county as Lancashire. In 
forming them I have ventured to deviate a little from 
the usual mode of arrangement, having separated 
those birds which are irregular in the times of their 
