5 6z 



THE QUARTERLY REVIEW OF BIOLOGY 



pseudo-scientific sentimental nature writers.], but is 

 due to the fact that he would have nothing to gain by 

 wandering. 



So then, it seems that the Shiny Cowbird is 

 monogamous, under normal conditions, but where 

 artificial conditions have caused a great, unnatural 

 increase of the species, the inherent instinct is not 

 strong enough to stand unmodified against the in- 

 creased competition, and frequently is modified so 

 extensively as to belie its original status. 



The sexual and territorial relations in 

 the North American. Cowbird are prac- 

 tically the same as in the preceding 

 species, except that they are usually less 

 violently modified, as the birds are not so 

 crowded. In a general way this is also 

 true of the Red-eyed Cowbird, but in the 

 latter species the numbers are usually low 

 enough so that the birds have sufficiently 

 large territories to avoid competition, and 

 monogamy is more easily observed. It is 

 only fair to say that less is known of the 

 habits of this species than of any of the 

 others, but for our purposes it is relatively 

 unimportant, as it is off the main line of 

 Cowbird descent. 



HOW DID THE PARASITIC HABIT COME ABOUT? 



Before utilizing the above data in the 

 formulation of an explanatory theory it 

 may be well to present and comment on the 

 leading current hypotheses. The first to 

 be considered is that the source of the 

 parasitic habit is to be sought in the 

 polyandrous condition which all parasitic 

 birds were supposed to exhibit. Pycraft 

 (40) and Fulton (2.4) are among the best 

 known exponents of this view. While it 

 is possible (though not probable) that 

 some parasitic cuckoos may be polyan- 

 drous, the cowbirds are certainly more or 

 less monogamous, and such promiscuity 

 as occurs is more likely to be a result 

 than a cause of the parasitic habit. 

 Vidua macroura, a parasitic African weaver- 

 bird, is also monogamous, and Chance 

 (13) writes of the European cuckoo that 



" . . . . whether cuckoos are poly- 

 gamous, polyandrous, or promiscuous is a 

 very open question. I am inclined to the 

 belief that they are, at least often, promis- 

 cuous. I should not, however, lightly 

 dismiss the theory that some pair as 



normal birds " Fulton admits 



that the question of polyandry and parasi- 

 tism is all in a circle and that it is hard to 

 say which came first. He inclines to the 

 view that polyandry causes parasitism. 

 In the cowbirds the circle is still open and 

 there can be no question that parasitism 

 is not caused by polyandry. 



The best theory advanced as yet, and 

 one which my studies tend to support in 

 part, at least, is that of Professor F. H. 

 Herrick. This writer studied the cyclical 

 instincts of birds and found that not 

 infrequently the cycle is interrupted by 

 various causes which result in a general 

 lack of harmony between its successive 

 parts. He suggested that the parasitic 

 habit may have originated from a lack of 

 attunement of the egg-laying and the nest- 

 building instincts which resulted in the 

 eggs being ready for disposition before a 

 nest was ready for them. His theory was 

 based largely on a study of the Black- 

 billed Cuckoo, Coccyxus erythro-phthalmus , 

 and a comparison of its life history with 

 that of the European Cuckoo, Cumins 

 canorus, which, of course, is parasitic, 

 while the former is not. 



He found that of all the perturbations 

 which are apt to arise at almost any step in 

 the cyclical sequence of instincts the 

 commonest was a failure in the "adjust- 

 ment of nest-building to the time of egg- 

 laying," and it was at this point that he 

 suggested the parasitic habit took its rise. 



"The door is thus opened wide to 

 parasitism in its initial stage, whenever 

 the acceleration of egg-laying or the 

 retardation of the building instinct be- 

 comes common, with or without irregu- 





