The instinct of self-preservation, which includes the preservation of the body from injury, is strongly developed in all organisms. Natural selection tends to develop this instinct, because the individuals in which the instinct is strongly developed are less likely to be injured by fighting than those which are pugnacious. In other words, it does not pay to fight in nature. Injured individuals are seriously handicapped in the struggle for existence. Thus natural selection tends to produce cowards.
At the breeding season an instinct, which is ordinarily dormant in birds, suddenly becomes active—the instinct of preserving the nest and its contents.
This instinct, when aroused, frequently overmasters the instinct of self-preservation, with the result that shy birds become bold, timid ones grow aggressive, little birds which usually are terrified at the close proximity of a human being allow themselves to be handled rather than leave their eggs or young.
At the breeding season the desire to protect the nest leads many birds to attack, or to make as if to attack, all intruders.
No sight is commoner in India than that of a pair of little drongos (Dicrurus ater) chasing a kite or a crow.
Similarly I have witnessed doves chase and put to flight a tree-pie (Dendrocitta rufa), and fantail flycatchers mob a corby (Corvus macrorhynchus).
Nor are such cases confined to India.
In England Mr. A. H. Bryden states that he has seen sea-gulls mob and put to flight so formidable a creature as a peregrine falcon.
In each of the above instances the bird pursued could, if it wished, turn round and rend its puny adversaries. Why does it not do so? Because the instinct of self-preservation is implanted in it so firmly.
This instinct teaches it never to resist an attack, no matter how feeble the attacker be.