308 TJ. S. NATIONAL MUSEUM BULLETIN 217 



The progression of physiological changes toward reproduction 

 seems to be punctuated by actions or stimulation which must come 

 from another bird (Tinbergen, 1953). Thus copulation, which may 

 occur at any time during a certain reproductive period, is necessary 

 to start the formation of fertile eggs. Thereafter, in the Arctic the 

 eggs of most birds are laid on successive days, and once the phase of 

 egg laying is initiated subsequent progress is invariable in all climates. 

 Such internal processes occur in homoiothermous birds under condi- 

 tions separate from external environmental influence and must proceed 

 at rates prescribed by ancient phylogeny. Certain types of behavior 

 which are the outward expressions of these internal processes must 

 be equally inevitable and unmodifiable by local and transitory 

 climates. But during the intervals between phases in the reproductive 

 cycle stimulation from another bird or even from the environment 

 may be quicker to set in motion the operations of the next phase. 



Elaborate patterns of behavior concerned with possession of terri- 

 tory, courting, and mating are precisely followed by birds of each 

 species in sequences related to reproductive condition. Among indi- 

 viduals in the same phase of their reproductive cycle no scrutiny 

 is needed to determine that any bird of the same sex is physiologically 

 ready to compete for territory or mate. All birds of opposite sex are 

 disposed to the one phase of reproduction current in the local popula- 

 tion. In such a society no bird is likely to untimely action and one 

 of the common causes of social disorder is removed by the uniformity 

 of impulses in a population progressing synchronously through the 

 sequences of reproductive behavior. The duration of competition and 

 conflict in reproduction is shortened and actions decisive to the se- 

 quence of events in reproduction are prompt. 



I do not imply, in this respect, that a natural society ordered by such 

 well synchronized impulses would be favored for existence in all 

 environments. But it appears to be by the ordering of society rather 

 than by acceleration of the individual's physiological processes that 

 adaptation of the breeding populations of birds conforms with the 

 shortness of the arctic season. 



Incubation 



Mrs. Nice (1954) has pointed out the frequency of erroneous reports 

 of the time during which eggs of wild birds are incubated. For a 

 clutch of eggs laid on successive days, it is difficult to set as a single 

 figure the duration of incubation. If incubation is considered to 

 begin only when the last egg has been laid its termination may differ 

 by as much as several days in the successive hatching of individual 

 eggs. The stage of embryonic development which is attained when 



