INSTINCT AND INTELLIGENCE IN BIRDS 83 



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 in- 

 stincts. 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 para- 

 sitism. 



II. Beginning a New Cycle oe Scamping the Old 



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 1 is 



CYinN. C.YouTN. 



Fig. 15. Diagram to illustrate the serial instincts of the Reproductive Cycle, 

 with types of activities expressed in eight terms. 



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 

 1 That is, when it is limited to the direct reproductive activities, the rela- 

 tion of which to migration is not important in this discussion. 



