152 Birds combining for Defence. 



these are so numerous that they cannot find room in 

 another part of the quarry, they emigrate in small 

 parties. 



I think the same thing happens with rooks. The 

 older rooks will only permit a few of their last year's 

 offspring to build near them. If a gentleman has an 

 avenue of fine elm trees in which he desires to have 

 a rookery, but cannot contrive to attract them, though 

 perhaps now and then a nest is partly built and then 

 deserted, an experiment founded on this idea might 

 be tried. It would be necessaiy to ask the assistance 

 of the proprietors of the nearest rookeries, and beg 

 them for one year to refrain from shooting the young 

 rooks, after the well-known custom. An unusual 

 proportion of young birds would then survive, and 

 next building season the larger part of these would 

 return to the old trees and be immediately met in 

 battle by their older relatives. Being driven away 

 from the hereditary group of trees, they would resort 

 to the next nearest avenue or grove ; if the}' attempted 

 to mix with a strange tribe, they would encounter a 

 still fiercer resistance. In this way possibly the 

 avenue in question might become stocked with rooks. 



One reason, I fancy, why nests begun in such dis- 

 tant trees are so often deserted before completion is 

 that a solitary nest exposes both the building birds 

 and their prospective offspring to grave danger from 

 hawks. No hawk will attempt to approach a rookery 

 the rooks would attack him en masse and easily put 

 him to flight. Chickens are safer under or near a 

 rookery from this cause : a hawk approaching them 

 would alarm the rooks and be beaten away. The 

 comparative safety afforded by numbers is perhaps 



