10 IOWA STUDIES IN NATURAL HISTORY 
Not far from this Yellow-throat’s nest there was a Yellow 
Warbler’s nest which retained an addled Cowbird’s egg after 
the nestling warblers had left the nest. During the summer of 
1914 a study of nest of the Western Meadow Lark was carried 
on at the Laboratory; and an addled egg of this species re- 
mained in the nest throughout the brooding period. The ques- 
tion arises, then, as to what is the common practice of birds 
toward foreign egg in the nest, or toward an addled egg. 
THE COWBIRD 
The manner in which a portion of the shell of the Cowbird’s 
ege became adhered to one of the Yellow-throat’s eggs, pre- 
venting the latter from hatching has just been described. Thus 
33 1/3 per cent mortality may be directly charged to the pres- 
ence of the Cowbird. It will also be recalled that the Cowbird’s 
egg hatched a day before any of the Yellow-throat’s thus giving 
him a considerable advantage. 
The Cowbird was larger to begin with, and was able to lift 
his head higher when the old birds came with food. In Table 
II the facts show that the Cowbird received fifty-five per cent of 
all the food brought to the nest. He seldom was satisfied, and 
only a very few times did he fail to thrust his head forward 
for food. As he got older he acquired a habit of spreading his 
wings, and bringing them down over the /Yellow-throats. 
Sometimes they retaliated by climbing on top of him, thus gain- 
ing some advantage in position. By the 14th the young were 
very much crowded, and the nest walls were being pushed out. 
From now on there was more or less piling up of the young 
birds. 
The 16th was the last entire day of feeding of the nestlings, 
and it was a very busy one for the old birds. During the day- 
light hours they made a total of three hundred and forty-eight 
visits to the nest with food. This was an average of one feeding 
visit every 2.6 minutes. The Cowbird received all, or part, of 
the food on one hundred ninety of these visits. 
The eyes of the Cowbird opened on the fourth day; on the 
sixth he began to stretch and preen his feathers; on the eighth 
