Observations on Periodical Birds. 149 
establishes the fact, that some species of pe- 
riodical birds perform their migrations in the 
night; and it is probable that this is the case 
with most of them; as I have frequently 
looked through the woods and plantations in 
Crumpsall with great care in April, the month 
in which most of the summer birds appear, 
without perceiving a single individual of any 
of the migratory tribes; yet early in the 
morning of the day following that on which 
the search was made, I have been surprised 
to hear the notes of the redstart and willow 
wren, and to find that the latter species had 
arrived in considerable numbers. 
Having endeavoured, in the foregoing 
remarks, to prove the migration of the pe- 
riodical summer and winter birds; I shall 
here briefly observe, that our irregular visit- 
ors also must migrate; as it is equally im- 
possible that they should lie torpid during a 
period of several years, or that they should 
escape the notice of observers for so great a 
length of time. Thus, the migration of 
every description of periodical birds, whose 
disappearance it has been difficult to account 
for, seems to be established; and I shall con- 
clude this paper with observing, that if swal- 
lows and martins do become torpid under 
some circumstances, they are probably en- 
