422 



THE BIRDS 



egg. They had to deposit it somewhere and those that were fortunate 

 enough to find a fresh nest of another species left survivors, whereas 

 those that left their egg on the ground did not. In this way, accord- 

 ing to this hypothesis, the instinct to lay their eggs in the nest of other 

 birds was developed in the cowbirds by natural selection. 



A few birds reject the strange egg. The yellow warbler buries it 

 in the bottom of her nest often with several eggs of her own. She then 

 builds a second nest over the first one and lays a new clutch of eggs. 

 The robin, which has a blue egg very different in color from the pale 

 spotted egg of the cowbird, throws the foreign egg from the nest. 



Photo by Winchester 



Fig. 27.13. The stork. Although not found in the United States the stork is one of 



the best known birds because of its traditional baby carrying activities. This habitat 



group in the Chicago Natural History Museum includes a typical nest and scene as 



found in the coastal regions of Eastern Europe. 



Most other birds, however, seem not to notice the difference. A com- 

 monly parasitized species is the song sparrow which lays a spotted egg 

 rather similar in appearance to that of a cowbird, although smaller. 

 The incubation time of the cowbird is only eleven days, one day shorter 

 than that of the song sparrow. This generally results in the young cow- 

 bird hatching one day before the sparrows. When food is brought to 

 the nestlings, the larger cowbird manages to get most of it and may even 

 crowd the young song sparrows from the nest. It is an amusing, yet 

 somewhat tragic, sight to see a song sparrow stuffing food down the 

 huge mouth of a baby cowbird twice her own size while her own off- 

 spring are begging for food. As soon as the cowbird is able to feed 

 itself, it deserts its foster parents and joins a flock of birds of its own 



