x PREFACE. 
ably other causes, as climatic, have been operating to effect 
a gradual northward migration, in certain species at least. 
These changes are of great interest, not only generally, but in 
a scientific point of view, and we shall be able to trace them 
and their causes only by comparing, from time to time, ex- 
haustive faunal records of the same localities. 
‘Tn a district so little diversified as that portion of Massa- 
chusetts lying east of the Connecticut River, it is perhaps a 
little unexpected that marked discrepancies should occur in 
the observations made at adjoining localities by equally compe- 
tent naturalists, in respect to the relative abundance of certain 
species. As every experienced observer must have noticed 
‘that the birds of passage, as many of the Warblers especially, 
vary greatly in numbers in different years, and in the time 
occupied by them in passing a given locality, it is less surpris- 
ing that at different points they should vary in abundance the 
same year. Among the birds that regularly breed in the dis- 
trict in question, there are some that are not equally common 
at all points.” 
‘‘ Birds, as probably other animals, are not quite so invari- 
able in their habits as has been commonly supposed, nor in the 
precise character of their notes and songs, or the situation and 
materials of which they compose their nests. Hence one should 
not rashly question the accounts given by usually reliable au- 
thorities, because in particular instances they do not accord 
with their own observations. Neither should differences in 
habits, in song, etc., be taken as infallible evidence of a differ- 
ence of species.” 
‘‘How universally the Chipping Sparrow (Spizella socialis) 
breeds in trees, and generally at an elevation of several feet, 
is well known, but several authentic instances of this bird’s 
nesting on the ground have come to my knowledge, one of 
which I myself discovered. Variations of this character in 
other species are of occasional occurrence, examples of which 
have doubtless been met with by every experienced collector. 
‘‘The materials which birds select in the construction of 
their nests are well known to vary in different localities ; the 
