To illustrate the absence of a thought process we may recall 

 the incident related on page 28, and note the apparent uncon- 

 cern which the female Red-eyed Vireo showed when its youngest 

 nestling was jerked out of the nest and fell to the ground. Here 

 so serious an event as the loss of a young one did not seem to set 

 in motion aay reactions, because, as I interpret it, the stimulus 

 was unique, and could not excite any stereotyped or instinctive 

 response. The mechanism for instituting any new response was 

 lacking. 



Under another set of circumstances the Cowbird's eggs may 

 have been laid in the nest. There are, at least, four possible types 

 of response to the presence of the Cowbird's egg, viz., (1) the ac- 

 ceptance and continuation of incubation; (2) the abandonment 

 of the nest and egg; (3) the removal of foreign egg from the 

 nest by the hosts (and I do not know^ of any specific instance of 

 such behavior) ; (4) the building of a superstructure over the 

 intruded egg. 



The relative frequency of these responses has probably not 

 been determined. Nor will I attempt to discuss the relative com- 

 plexity of the several responses. The first one named is prob- 

 ably the most frequent one, and possibly the simplest, since it 

 involves only the supersedure by the highly developed chain- 

 refiexes of the breeding period of any other reflexes which might 

 be affected by the foreign egg. But in any of the possible cases, 

 to put the reaction in the class of intelligent behavior, we must 

 concede a knowledge on the part of the bird of the potential con- 

 sequences of the foreign egg. It seems doubtful to the writer if 

 the evidence will justify this conclusion, and that we are com- 

 pelled to seek a simpler explanation if it is possible. 



Before offering an explanation of the behavior in question in 

 terms of reflex action, it will be necessary to describe a simple 

 experiment performed at the Vireo 's nest. 



On Tuesday, July 11th, shortly after noon, the writer was a1, 

 the nest, and conceived the notion of placing a small piece of 

 green paper on the edge of the nest, for the purpose of ascer- 

 taining what effect it might have on the fairly well-established 

 routine of behavior of the parent birds. 



The male Vireo came first (visit No. 226), and fed a green 

 worm to Blue. He then picked up the green paper quickly, but 

 dropped it to catch the excreta sac voided by Blue. It had been 



35 



