Let us see how these principles are applied to the problem of approaching wild birds in the way described. The parental instincts begin to control the life of the adult with the periodic revival of the reproductive functions, and may vary greatly in their scope and intensity. As a rule this instinct, reinforced by habit, gradually increases until the young are reared. It is, therefore, safest to change the nesting surroundings when the parental instincts are approaching their culmination.
The general feeling of fear is gradually or quickly suppressed
according to the value of the different factors in the equation, by the parental instinct, which impels a bird at all hazards to go to its young wherever placed.
After a bird once visits the nest in its new position it returns again and again, and in proportion as its visits to the old nesting place diminish and finally cease, its approaches to the new position become more frequent, until a new habit has been formed, or, if you will, until the old habit is reinstated.
When the birds approach the nest any strange object, like the stakes which support the bough or the tent which is pitched beside it,
