19° 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 hulk 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 patli 

 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 k ' 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 Avifauna? would become sadly unstable. 



