but happily this is not the case. No harm need befall either old or young. The former nesting site is soon forgotten, and the new quickly adopted and defended with all the boldness and persistence of which the birds are capable.
This method of studying birds thus depends mainly upon the strength of the parental instincts, and upon the readiness with which a bird learns to adapt itself to new conditions. Upon more complete analysis we recognize the following psychological principles, of which the following are the most important: (a) The strength of an instinct increases through its exercise, and may be reinforced by habit; (b) An
instinctive impulse may be blocked or suppressed by any contrary impulse; (c) The instinct of fear is often quickly suppressed by repetition, or the formation of new habits. One might also add that: (d) New habits are readily formed and may replace the old ones; (e) Abstract ideas, if they form any part of the furniture of the average bird-mind, are extremely hazy and fleeting; (f) Still further we must recall the physiological fact that birds are guided in most of their operations by sight and hearing, not by scent. Their olfactory organ is very rudimentary at best, and avails them neither in finding food nor in avoiding enemies.
