THE OOLOOIST 



Cowbird Study In Iowa 



By Emerson A. Stoner 



The writer's study of the breeding 

 habits of birds was begun without the 

 aid of books, and the first two nests 

 I found containing cowbirds' eggs 

 sorely puzzled me. 



It is well known to oologists that 

 Molothrus ater, the cowbird's scientific 

 name (meaning in Greek "black vaga- 

 bond"), does not build a nest of its 

 own, but parasitically deposits its eggs 

 in the nests of other birds who hatch 

 the eggs and rear the young of the in- 

 truder. 



My first eggs of this bird I found on 

 June 14th, 1906. The rest was a Scar- 

 let Tanager's, and contained three 

 Cowbird and one Tanager eggs. The 

 female Tanager was on the nest and 

 the male close at hand. Not knowing 

 anything about Cowbirds, I immedi- 

 ately recorded in my notebook a set 

 of four eggs of the Tanager. The dif- 

 ference in the ground color of the 

 eggs the Tanager's a light greenish 

 blue with brown blotches and the 

 Cowbird's, white with brown blotches, 

 — I pondered over for a while, and in 

 my ignorance finally decided that the 

 blue ones must hatch out female Tan- 

 agers and the white ones male Tana- 

 gers, or vice versa. Such are the dan- 

 gers of accepting notes and data writ- 

 ten by amateur oologists. 



On the same date as above, about 

 fifty feet away from the Tanager's 

 nest above referred to, I found an 

 empty Wood Thrush's nest. Both of 

 these nests were on horizontal limbs 

 of white oaks by the side of a foot 

 path running through a small ravine. 

 The Wood Thrush's nest I again visit- 

 ed on June 24th, and found it to con- 

 tain one Wood Thrush's egg and three 

 of the same species of bird that had 

 laid its eggs in the Tanager's nest. 

 Naturally, I called this later set in my 



note book "one egg of the Thrush and 

 three of the Tanager. 



But I began to grow doubtful about 

 such shiftless nesting, and during the 

 summer I had access to some bird 

 books in the City Library, and through 

 the study of these 1 straightened out 

 my problems. Cowbirds had laid three 

 eggs in the Tanager's nest, and three 

 eggs in the Thrush's nest, and the 

 rightful owners of these two nests 

 were sitting on and hatching the eggs. 



Since the experience with these tirst 

 two nests containing Cowbird's eggs, 

 I have discovered quite a number of 

 other nests, some containing eggs and 

 others young of the Cowbird. I give 

 below some of the species in whose 

 nests the bird lays its eggs, together 

 with the locations and seasons of the 

 year during which the Cowbird breeds. 

 All of the nests were found in the 

 city limits of Des Moines, Iowa. 



June 1, 1907— Nest of Wood Thrush, 

 15 feet up in crotch of oak tree. Con- 

 tained 4 Cowbird eggs, 1 Wood Thrush 

 egg. 



June 4, 1907— Nest of Wood Thrush, 

 6 feet up in low bushes in orchard. Con- 

 tained 4 Cowbird eggs, 3 Wood Thrush 

 eggs. 



June 7, 1907— Nest of Wood Thrush, 

 6 feet up in crotch of white oak sap- 

 ling. Contained 5 Cowbird eggs, 1 

 Wood Thrush egg. 



June 11, 1907— Nest of Wood 

 Thrush. Contained 5 Cowbird eggs, 

 none of Wood Thrush. 



June 27, 1907— Nest of Wood 

 Thrush, 4 feet up in low bushes. Con- 

 tained 1 Cowbird egg, 2 Wood Thrush 

 eggs (badly incubated). 



May 20, 1908— Nest of Wood 

 Thrush, 3 feet up in low bushes. Con- 

 tained 4 Cowbird eggs, 1 Wood Thrush 

 egg. On the following day this nest 

 contained one additional egg of the 

 Cowbird, and the Wood Thrush's egg 

 was gone. 



