132 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
animated scene the old rookery is for the next month, until the 
eggs are laid in the big massive nests; then there is comparative 
quietness until the young are hatched, when the noisy clamour 
begins again with greater volume until nestlings and parents get 
on to the adjoining fields. They return in many cases to the 
nest trees to roost, and then each evening the din is deafening 
as troop after troop of tired birds come straggling in from all 
directions and caw themselves hoarse before dropping off to 
sleep in the tall trees.” The Social Grosbeak has been spoken 
of in an earlier section as a remarkable example of social nesting- 
habits (see vol. iii, p. 463). 
The social habit is not infrequently conducive to successful 
defensive measures against enemies. Birds of the swallow kind 
will unite together to ‘““mob” a hawk that ventures too near 
their nests, and other instances might easily be given. 
Recognition Marks in Birds.—\t is obviously advantageous. 
for the members of a community of animals to be able to recognize 
one another with ease and certainty. They are safer, on the 
whole, when they keep together, for approaching danger is pretty 
sure to be perceived by some of them, and these individuals are 
able to communicate the fact to their fellows. Co-operation in 
the pursuit of food is also promoted, and, in migrant species, long 
journeys are more likely to be successfully made in fairly close 
order. 
Wallace has given the name of “recognition marks” to certain 
colour-arrangements which appear to be of importance in the 
present connection, and he speaks as follows about the matter 
(in Darwinism).—* Among birds, these recognition marks are 
especially numerous and suggestive. Species which inhabit open 
districts are usually protectively coloured; but they generally 
possess some distinctive markings for the purpose of being easily 
recognized by their kind, both when at rest and during flight. 
Such are, the white bands or patches on the breast or belly of 
many birds, but more especially the head and neck markings 
in the form of white or black caps, collars, eye-marks, or frontal 
patches .. . (see fig. 1104). Recognition marks during flight 
are very important for all birds which congregate in flocks or 
which migrate together; and it is essential that, while being as 
conspicuous as possible, the marks shall not interfere with the 
general protective tints of the species when at rest. Hence they 
