CHAPTER IV. 

 THE INCUBATION PERIOD. 



By the incubation period we refer to that part of the reproductive cycle which 

 extends from the appearance of the first egg until hatching is completed. Some 

 species may begin to sit on the nest before the appearance of the eggs, while in 

 other cases incubation may be delayed until the advent of the second egg. Incu- 

 bation, in a sense, exists for a week or so after hatching, as the young are at first 

 covered almost as carefully as were the eggs. Our use of the term, however, may 

 be justified as a means of classifying and describing the various activities of the 

 reproductive cycle. The present chapter is concerned primarily with various 

 statistical data rather than with behavior proper. 



LENGTH OF THE PERIOD. 



The time of incubation varies considerably in the different species. The shortest 

 incubation period that I have known anything about is that of the wild passenger-pigeon, 

 which is only 12^- days; while that of the common pigeon is 17 to 19 days, and that of the 

 ring-doves or turtle-doves is 14 to 16 days. (SS 10.) 



T. H. Newman (A vie. Mag., May, 1908, 217-218) gives the incubation period of T. 

 pictaturus (Madagascar turtle-dove) as 12 days (reckoned from the date of the second 

 egg). I find these birds have a longer period, and furthermore, it is incorrect to start the 

 period with the second egg, as development begins in the passage through the uterus 

 before the egg is laid. (R18.) 



Fulton (op. cit., p. 39) says: 



"The period of sitting among (fancy) pigeons is 18 days, reckoned from the day on which the 

 second egg is laid, for one day intervenes between the first and second egg being produced. Some- 

 times, if the parent birds sit very close, the young are hatched on the seventeenth day in summer 

 weather; not infrequently, however, the period of hatching is delayed to the nineteenth or twen- 

 tieth day." 



Table 3 has been compiled from the records. Besides differences of species it 

 is to be noted that the period for the second egg is generally shorter than that 

 for the first egg. This fact is probably a result, in part, of the irregularity of early 

 incubation. Until the arrival of the second egg, incubation is intermittent and 

 sometimes lacking (see subsequent topic "Early Incubation," Chapter V). 

 This shortening of the time for the second egg secures a greater equality of the 

 two birds in feeding. This fact is important in that the stronger and better- 

 developed young is likely to monopolize the attention of the parents in feeding 

 (see topic "Feeding of Young," Chapter VI). There is some slight indication in 

 the table that the period tends to decrease with the advance of summer. 



In this table we have given the data for the pure species 1 only, not hybrids. 

 When two species with different incubation periods are crossed, the resultant 

 period approaches a mean, with the female exercising a preponderant influence, 

 Given two species A and B, with incubation periods of 12 and 19 days respectively, 



1 Domestic pigeons and the ring-doves are, however, included. 



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