: Birds combining for Defence. 171 
old birds drive the young ones away, when the young 
return to this country with their parents after the 
annual migration. This is particularly the case after 
a very favourable breeding season, when more than 
the usual proportion of young birds survive. After 
such a season, upon returning next year to the sand 
quarry, the older birds drive off the younger; and if 
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. Ifa 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 necessary 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 they attempted 
to mix with a strange tribe, they would encounter a 
still fiercer resistance. In this way possibly the 
