536 THE POPULAR SCIENCE MONTHLY 



For convenience the breeding cycle may be described as made up of 

 a series of terms as follows : 



1. Migration to Breeding Area; 5. Incubation and Care of Eggs; 



2. Courtship and Mating; 6. Care of Young in Nest; 



3. Nest Building; 7. Care of Young out of Nest; 



4. Laying Eggs in Nest; 8. Migration to Feeding Area. 



Beginning at 2, 3 or 4, according to circumstances, the cycle may 

 be repeated one or more times within the breeding season, or a new 

 cycle may be begun, and stayed at any step from nest-building to laying 

 of the eggs. Again, an entire cycle may be brought near the close, and 

 then scamped, the young being left to die. 



The reproductive cycle may be graphically represented by a number 

 of tangent circles, each of which stands for a distinct sphere of influ- 

 ence or for a subordinate series of related impulses. It is evident that 

 these serial instincts must be in relatively perfect harmony, or if 

 regular perturbations occur, new and permanent adjustments must be 

 forthcoming to meet them, if the species is to continue to exist. One 

 act or series of related acts must be performed in preparation for that 

 which follows. The nest must " anticipate " the eggs, and not the egg 

 the nest. Upon the whole the serial instincts of birds are well attuned, 

 yet disturbances more frequently occur than is commonly supposed, 

 and by conditions of this kind much that is anomalous or eccentric in 

 the behavior of birds can be explained, as we shall later see. 



The cyclical instincts are profoundly affected by fear at terms 3 

 and 4, and the whole fabric of instinctive life is subject at nearly every 

 step to the modifying influence of intelligence. In the reproductive 

 cycle, as elsewhere, the same struggle is seen between competing or 

 conflicting instincts, especially where attunement is imperfect, leading 

 now to a fuller expression, and now to a total neglect of the usual activi- 

 ties. The number of terms, of which eight are given above, is unim- 

 portant, so long as it is recognized that they occur in serial form, and 

 that many activities such as brooding, and feeding the young, are 

 recurrent. 



Certain subordinate instincts rise and wane during the reproductive 

 cycle, thus adding to the complex of behavior. Song, which is pri- 

 marily instinctive, often begins in the male during the time of mating, 

 but that it is not wholly dependent upon the reproductive function is 

 proved by the fact that it is not always coincident with the breeding 

 period. Thus, it was shown as early as 1834 by Blackwall, that singing 

 may cease before the nest is built, or last long after the young have 

 flown. Again the fighting instinct usually emerges early (at 2 or 3) 

 and is long continued. In that inbred pugnacity, which characterizes 

 the breeding season of birds and higher animals, we possess the key, as 

 I believe, to the origin of the instinct of incubation. According to this 



