1 32 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. It 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 



