REPRODUCTIVE CYCLE. 95 



earnestness. It is sometimes accompanied with nods of the head, the beak being 

 held vertically downward. These nods are the more pronounced as the calls 

 increase in loudness and earnestness. The call is also accompanied by wing- 

 movements similar to those of the young in appealing for food. These vary in 

 amplitude with the earnestness of the call. The female's call is, in these forms, a 

 single husky, jarring, vibratory note with a duration of three-fourths of a second. 

 Its strength is less than that of the male, and it is given at a variable rate. It 

 is accompanied by slight head nods and the beak is slightly opened. 



(3) In egg-laying the bird exhibits diagnostic symptoms for several days pre- 

 vious to ovulation, and apparently the length of this period can be curtailed as a 

 result of experience. The symptoms mentioned are, sitting in the nest, drooping 

 wings, sickly appearance, and heavy movements. In laying, the bird moves well 

 forward in the nest, assumes an erect position with the head and fore-parts raised 

 and the rump lowered at an angle of 45, raises the tail, exhibits an attitude of 

 strain and sometimes of pain and agony, exhibits intermittent moments of labor, 

 gives in some cases the nest-call occasionally throughout the operation, raises the 

 wings to the horizontal with the head thrown back and half around and with 

 beak raised as the egg drops, and then stands motionless for a variable time with closed 

 eyes, cuddles the egg with the bill, and then usually settles down on the nest. The 

 egg invariably comes small end first. The number of eggs depends upon the 

 species. The first egg is invariably dropped in the afternoon, the exact time vary- 

 ing with the species and the time of the year. The time of the second egg and the 

 interval between the two eggs is a function of the species and the time of the year. 



(4) The onset of incubation is not necessarily synchronous with the appearance 

 of the first egg; the time varies with the species. With Ectopistes both birds start 

 incubation previous to egg-laying. Others begin after this event, the time being 

 somewhat variable. The impulse gradually rises in strength; at first, the birds 

 may merely stand over the eggs and sitting is somewhat intermittent, but shortly 

 after the second egg is dropped incubation becomes continuous. Both birds par- 

 ticipate in the nesting duties. The onset of the impulse seems to develop rather 

 independently in the two birds. Generally the male begins incubation later than 

 the female. The female takes the nest during the night, while the male roosts on 

 the perch as far away from the nest as is possible. During the daytime the two 

 alternate on the nest, with the male performing the major portion of the work 

 during this time. The length of this period in which alternation occurs is some- 

 what variable, probably being somewhat dependent upon conditions of light. 



With any defection on the part of either bird, the mate usually attempts con- 

 tinuous incubation until endurance is no longer possible. The exchange is fre- 

 quently stimulated by the mate's act of taking or leaving the nest. The incubation 

 impulse seems to reach its maximum at the time of hatching and persists for about 

 a week, when incubation becomes intermittent and finally ceases. Apparently 

 the male desists from his duties sooner than does the female. The length of the 

 incubation period is highly variable. Its duration, as represented in figure 1, is 



