THE CAWING OF ROOKS 35 



possession of old nests or of new sites; and up to a 

 certain point there is stealing of materials. This 

 necessitates one bird mounting guard while the mate 

 collects, the division of labor alternating. Very 

 remarkable is the frequent destruction of nests that 

 are built on trees in the outskirts of the main colony, 

 as if the sense of the community insisted on main- 

 taining a close gregariousness. One tree may have 

 to carry as many as thirty nests, and it is usual 

 to see about a dozen. The nest is built of both 

 dry and supple twigs, with the addition of earth 

 and clay, and the inside of the cup is made soft 

 with grass and leaves, hair and wool. Among the 

 favorite trees are ash, elm, beech, Scots fir, and 

 sycamore; and it has been observed over and over 

 again that trees which betray insecurity are 

 abandoned by the rooks even after the nests have 

 been built. A forsaken tree is doomed, and this 

 may be one of the facts that have given basis to 

 some of the superstitions about rooks. 



The eggs are interesting biologically because of 

 their great variability in coloring that is to say, 

 in relatively safe nesting-places, where inconspicu- 

 ousness or the opposite is of little moment, natural 

 selection has imposed no limits on variation. After 

 the laying, towards the end of March, the mother 

 bird sits close, the male occasionally relieving her. 

 There is no more returning at night to the 

 communal roosting-place, which is usually quite 

 apart from the rookery; all the rooks keep vigil 

 by their nests. " Relatively safe," we said, for 



