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 
