38 U.S. NATIONAL MUSEUM BULLETIN 223 



not have been attuned directly to any external factors, either of 

 obvious selective value or not. Aside from the loss of broodiness, 

 the fact that the vidumes appear not to breed until their second year, 

 whereas many small bh-ds, including their estrildine relatives, do so 

 in their first year, further suggests an endocrine "lag." 



Endocrine changes seem in light of current knowledge to be the 

 most promising approach to the discovery of the underlying mecha- 

 nism involved in the establishment of conditions that led to the origin 

 of brood parasitism. This approach does not reveal the other factors 

 that may have had their effect in making possible, in an evoluntionary 

 sense, the loss of selfbreeding to survive and to develop as brood para- 

 sitism, but the approach seems to be a necessary first step for further 

 investigation, even if it may not be as direct and as simple as one might 

 wish. Beach (1948, p. 77) recently summarized his conclusions by 

 sajang that apparently "seasonal correlation of nest building, egg 

 laying, and parental responses with glandular hypertrophy, in many 

 wild birds is in accordance \vith the thesis that such behavior is af- 

 fected by endocrine products; but it must be admitted that observations 

 of this type are of little assistance in any attempt to define the re- 

 sponsible hormones." Even in cases where gonadal hormones appear 

 to stimulate parental responses, it is not yet possible to indicate the 

 accuracy of the conclusions, especially since in other cases it was 

 equally apparent that broodiness can be delayed or even inhibited 

 by substances such as androgen. 



With current information as a guide, the factors underlying the 

 development of brood parasitism will probably be found to fall in 

 line wdth Zuckerman's general statement (1955, p. 249) that evolution 

 or change in the endocrine picture often consists of changes of hormonal 

 emphasis rather than of kind, or of the development of a new mode of 

 tissue response to hormones already in existence. Studies of the 

 endocrine situation in brood parasites are being planned, but what 

 results will come of them cannot be previsioned here. 



