Page:Popular Science Monthly Volume 77.djvu/89

Rh clearly of ancient origin, it is by no means universal; it seems in every case to be related to the needs of the animal, and to be a refinement of more simple means of securing both concealment and protection.

The causes of the disturbances, which we have to describe, are wholly obscure. We can only surmise that they may have their origin in changes in the central nervous system, which, as one result, bring about disturbances in nutrition, leading now to a premature, now to a belated development of the reproductive cells. At all events there arises what may be crudely described as an "overlap" or "blending" of instincts. Or, we may say that in the struggle of conflicting impulses victory now goes to one side, now to the other. The only facts that are really known are that the egg sometimes anticipates the nest, instead of the nest the egg, or that the migratory impulse may emerge too soon, and nip the proper parental instincts in the bud, before they have run their course. We do not doubt that the sifting process of selection would soon curb any tendency, like the last, in every species which was destined to survive.

The eccentricities of behavior, which we attribute to disturbances in the breeding cycle, will be examined under the following heads: (1) Beginning a new cycle, and scamping an old; (2) multiple and superimposed nests; (3) eccentric behavior due to conflicting instincts; and (4) premature laying of eggs, omission of nest-building and parasitism.

When the cyclical instincts rise and wane in their proper order, they may be represented by a series of circles tangent to each other, or with but little overlap (Fig. 15). Beginning at term 3 or 4, a cycle is

completed up to term 7 for each brood successively reared. Most wild birds in this part of the world have but a single brood in the season. The success of any individual pair depends upon circumstances. Storms and predaceous animals of all kinds break down the nests or destroy the eggs, when a fresh start is usually taken.

In very timid birds like the cedar waxwing, the cycle is often