A CASE OF APPARENTLY ADAPTIVE ACCELERATION 

 OF EMBRYONIC GROWTH-RATE IN BIRDS. 



HERBERT FRIEDMANN, 

 BIOLOGICAL LABORATORY, BROWN UNIVERSITY. 



Temperature experiments upon the egg of the domestic fowl 

 have shown that the period of incubation varies inversely with 

 the temperature at which the eggs are incubated. It may be as- 

 sumed that this is due to an increase in the increment of em- 

 bryonic development. If we consider any temperature well within 

 the margins of safety for the egg (high 105 F. ca., low 100 F. 

 ca.) such as 102.5 F., we may say that an increase of one degree 

 leads to an accelerated development while a decrease of one degree 

 leads to a depressed or inhibited development. However, as far 

 as I have been able to discover, no case is known in nature where 

 an adaptive acceleration of the rate of embryonic development 

 seems to have accompanied the evolution of new species within a 

 group. The purpose of this paper is to report on what seems to 

 be such an instance. 



The cowbirds are a group of icterine birds comprising half a 

 dozen species placed in three closely related genera : Agelaioldes, 

 Molothrns, and Tangavius. Of these three, the first named is in 

 every way the most primitive and doubtlessly represents the primi- 

 tive stock from which the other two have evolved. The last 

 mentioned is a side branch of the cowbird group and need not 

 be considered in this paper. The species of Molothrns (and 

 Tangavius) are parasitic in their breeding habits, i.e., they do not 

 make any nests of their own but deposit their eggs in nests of other 

 birds and leave them to be incubated and hatched by these strange 

 species. Agelaioldes is not parasitic but is not entirely normal in 

 its breeding habits. It breeds in old nests of other birds but cares 

 for its own eggs and young. It will build for itself only if it can- 

 not possibly get possession of a nest already built, showing that 

 it still possesses some of its original nest-building instincts, but 

 makes use of them only in case of emergency. In this genus the 



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