390 THE IRREGULAR MIGRATIONS OF BIRDS. 
inces where they were before unknown, and a disappearance from 
their former range. Similar movements take place and, indeed, 
are constantly going on, among all ranks of the animal and vege- 
table kingdoms, though owing to their preéminent mobility, birds 
afford the most conspicuous examples, excepting, perhaps, the 
class of insects. The slow but sure progress of the Norway rat 
from the east is well known, it having gradually spread itself in 
the course of one hundred and fifty years, from Persia to the Pa- 
cific Ocean. The steady eastward march of the Colorado potato 
bug is another example, while among plants, Leucanthemum vul- 
gare and Rudbeckia hirta afford familiar instances. 
Audubon speaks of the chestnut-sided warbler as one of the 
rarest Sylvias of his day. In his “ Ornithological Biography,” he 
tells us that he searched for it for years in vain; and finally on ob- 
taining five specimens in the same spring, considered himself 
extremely fortunate. At the present day it is, in the very regions 
where Audubon spent years in collecting, one of the commonest 
warblers ; and the most inexperienced collector could shoot, not five, 
but five hundred in one season; indeed I have seen it far outnum- 
bering all the other species together, and literally swarming in the 
woods. At the same time, the mourning warbler, rare in the time 
of Wilson and Audubon, remains quite as much so still; only in 
certain other localities it has been found very abundant. Now it 
is not to be supposed that the former species could have been com- 
mon in the eastern states, and yet have eluded the observation of 
Audubon ; and it is not at all probable that their present abundance 
is owing to the natural increase of the species. Plainly there must 
have been a migration or extension of range from some other re- 
gion where it was at that time abundant; and in the same manner 
the next fifty years may see the mourning warbler extending its 
limits further and further eastward from Minnesota, where it is now 
common, until it is as abundant in the Atlantic States as the 
chestnut-sided warbler. 
A somewhat similar case, but occurring in a much more limited 
space of time, happened in my own experience. In a series of sev- 
eral years’ close observation at Orange, New Jersey, I searched for 
the great-crested flycatcher (Myiarchus crinitus), year after year, 
but all in vain; and what made the fact very singular was, that 
twelve or fifteen miles off, I had seen the bird sufficiently often to 
Convince me that, if not common, it was by no means rare. Yet 
