tion while the set of eggs is being laid. That this is true 

 cannot be questioned, since it has been found (92) by 

 "candling" pigeons' eggs that the first egg shows indubitable 

 evidences of beginning embryonic development by the time 

 the second egg is laid. It is also wise here to recall that all 

 eggs respond developmentally to lower degrees of tempera- 

 ture in the early parts of incubation than they do in the 

 later or last portions. 



Hence, from the foregoing, it is evident that the diffi- 

 culty of fixing the real beginning of incubation must con- 

 tribute not a little to conflicts in the data on incubation 

 length, and has led to honest differences in the records made 

 on a given species by different observers. The correct and 

 exact method of measuring the length of incubation is to 

 mark each egg as laid and watch it daily until hatched. 

 This is often impossible because of psychic reasons, since a 

 bird may abandon a nest if too closely watched, or because 

 of physical reasons, as with birds nesting in holes. Some 

 errors have also been caused by lack of agreement as to 

 when the incubation terminates; thus some reports seem to 

 indicate that the observers date the end of incubation when 

 the egg is merely "pipped," while others fix the end when 

 the "chick" is completely hatched. 



In cases where several eggs make a set for a single 

 hatching, all the eggs may hatch at the same (relatively) 

 time, in which event I propose to name it a "simultaneous 

 hatching," as is seen with the domestic hen, and when the 

 eggs hatch one after another at intervals of "a day or two, 

 a "successive hatching." The effects of these two types of 

 hatching on the estimation of incubation length will be 

 considered later on. 



Conditions Necessary for Successful Incubation 



The growth of a new bird individual really begins 

 directly after fertilization, which may occur a considerable 

 time before the egg is completed and extruded ; consequently, 

 the incubation period embraces only part of a bird's em- 

 brj T onic development, which part is, however, by far the 

 largest portion of the process of development. 



Successful incubation depends on "keeping a fertile 

 egg * * for a sufficient period of time under certain con- 

 ditions of heat, moisture and position" (33)*. 



This combination of heat, moisture and position is 

 achieved through the brooding of the parent (real or foster), 

 or by a mass of decaying vegetable matter, or by hot springs, 

 or through the care of the parents plus the sun's heat. It 

 may begin at once, after the first egg is laid, or after part 



Numbers in parentheses correspond to the number of the authority 

 quoted, and as listed in the bibliography. See Table No. 2. 



