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 influence 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 activities. The number of terms, of which eight are given above, is unimportant, 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 primarily 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
