DISTRIBUTION OF BIRDS. 109 



may be favourable to their so doing. In some 

 instances, they may have to traverse long distances 

 before they can effect this object ; or they may be 

 compelled to leave the country altogether, and resort 

 to another ; and perhaps this supposition may secye 

 to explain the circumstance of their sudden appear- 

 ance in some localities, where they had not been 

 previously observed.* The well-known fact, too, of 

 old birds driving away their young may, in the case 

 of certain rare species, account for their being young 

 or immature individuals, which alone occur, perhaps, 

 at irregular intervals, in a given country. These 

 individuals are such as have wandered to near the 

 extreme geographical limits assigned to their species. 

 If, from a general excess of numbers in other places, 

 they are compelled to travel so far from central 

 quarters as to overstep these limits, existence is 

 with difficulty maintained, or may be no longer 

 possible. They then perish ; and it may be in this 

 way that the numbers of some species are constantly 

 held in check, or at least reduced, in certain seasons, 

 when particular circumstances have led to an undue 

 increase. 



It seems further deserving of inquiry, whether the 

 nocking of birds in some instances, as well as the 



* An instance of this kind is mentioned in the volume of Re- 

 ports on the Progress of Zoology and Botany, lately published by 

 the Ray Society. " In April, 1838, a flight of rooks" is said to 

 have " entered into the city of Danzig, and settling upon all the 

 larger trees, in gardens as well as in the most crowded streets, 

 built their nests there and brooded." Prof. Wagner's Report on 

 Birds, p. 71. 



