19 0 Henshaw on Causes affecting the Decrease of Birds . 
in their nature and the number of observers too small for exact 
inferences to be drawn. Evidences of a general stability in the 
course of migration are. however, numerous, and it is certainly 
true that given species may confidently be expected to occur and 
recur in abundance along a certain route, while the uniformity 
with which a species known to be rare will present itself each 
year in just about the same numbers is still more remarkable. 
Nevertheless, most observers will agree that no two seasons 
yield precisely similar notes on migrations, and not infrequently 
the differences are very great. It is evident that it needs but a 
slight deflection to one side or the other to carry the bulk of a 
certain species quite outside the range of observation of the 
limited number of observers, and this may readily happen from 
a number of causes. The effect of a severe storm is frequently 
to retard the migration, and in the long delay that sometimes 
follows, birds become more or less scattered and may resume 
their journey on somewhat different lines. 
Possibly, also, and especially in fall, the scarcity of food along 
an accustomed line of flight, and its abundance elsewhere, may 
be to some extent instrumental in influencing the course taken. 
But the most potent agent in deflecting birds from their course 
is adverse winds, and to them doubtless are due most of the 
variations noted in the number of migrants which are not caused 
by actual mortality. In connection with the effect of winds in 
turning migrating birds aside from their usual paths, the inter- 
esting question arises whether large numbers are not forced to 
such remote distances that they are unable to regain the lost path 
leading to their old homes, and are thus compelled to settle in 
new quarters. That this does occasionally happen there can, I 
think, be no question, and doubtless these accidental dispersions 
form an important means for the spread of species. 
Doubtless many of the “ accidentals” that so frequently figure 
in our local lists are to be thus accounted for. But these wan- 
derers are almost always reported singly, and I can recall no 
instance where a neighborhood has been invaded by a large 
number of breeders of a species hitherto rare. The home in- 
stinct in birds is so strong and enduring that it seems certain that 
nothing but the most adverse circumstances will cause them to 
relinquish their efforts to reach their old homes. Were it not 
indeed so our Avifaunae would become sadly unstable. 
