i86 Bird - Lore 



stated definitely whether they were of two different species or merely variations 

 of the same species. 



The truth of the matter was revealed by the subsequent history of the nest. 

 By June 23 three of the eggs, including the one of slightly different color from 

 the other three, had hatched. (The other egg never hatched.) All three young 

 birds looked very much alike at first, but, by the 26th, differences began to be 

 noticeable. Two of them were covered with a down of grayish color and had 

 white-rimmed mouths, while the third one was covered with a darker down 

 and had a yellow-rimmed mouth. The first two were developing more rapidly 

 than the latter one. By June 30, when the young were about ready to leave 

 the nest, the differences were very evident. The two were much more 

 completely feathered than the one. They were of a brownish gray color 

 above and light underneath, with darker stripes becoming visible. The 

 one remained dark. The result clearly consisted of two Cowbirds and one 

 Towhee. 



While we do not class the Cowbird as a praecocial bird, the above example, 

 as well as others observed, indicates that it develops more rapidly than other 

 altricial birds. This is one characteristic which greatly aids it in being extra- 

 ordinarily successful in its parasitic life. 



While I observed only the one Towhee's nest, I repeatedly saw Towhees 

 feeding young Cowbirds during the latter half of June and the first half of July. 

 The Cowbirds in every instance received the entire attention of their foster 

 parents, there being no young Towhees present. They readily accepted all food 

 offered them and continued to call for more. On several occasions between 

 June 16 and June 25 I saw the same pair of Towhees feeding two young Cow- 

 birds — the male attending to one and the female the other. Although my 

 observations covered only a short period of time late in the nesting season, they 

 indicated a high percentage of Cowbird parasitism among Towhees. 



Some writers contend that Cowbirds are careful to lay their eggs in nests 

 where there are already eggs of another species. Others attempt to refute this 

 by citing instances of nests containing Cowbird eggs only. It seems to me, 

 however, that all the circumstances which produce such a condition have not 

 been considered. For example, it is not always possible to tell whether a nest 

 has been used in raising a brood of birds or whether it has been deserted. I 

 doubt very much whether one who did not know could have told on July i, 

 after the young birds had left the nest, whether the Towhee's nest referred to 

 above was one which had been used or whether it was a deserted one. Yet 

 there was one unhatched Cowbird egg, at least the hollow shell of one, remain- 

 ing in the nest. Who could say that it had been laid in an empty nest? Again, 

 on July 6, I found a nest, apparently of a Song Sparrow, containing one Cow- 

 bird egg. It did not seem to be in use. It was impossible to tell certainly 

 whether a brood of birds had been raised in it and this egg had failed to hatch, 

 or whether it had been deserted because the Cowbird egg had been found in it. 



