38 U.S. NATIONAL MUSEUM BULLETIN 233 



So far, we have been discussing merely the ability of the various 

 hosts to feed and take care of a young cowbird. Other factors also 

 play a role in determining the suitability of a host. One is the acces- 

 sibility of its nest to a cowbird about to lay. Birds nesting in cavities 

 with very small entrance holes are not ordinarily "available" but 

 occasionally are parasitized, probably when the entrance to the nest 

 is unusually large or has been tampered with. Such a case is that of 

 the brown creeper listed on page 39. 



Another factor is the intended victim's alertness or pugnacity that 

 tends to protect it from being imposed upon by the parasite. One 

 group of passerine birds, the nests of which are suitable but which have 

 not been known to be parasitized, is the slirike family. I have long 

 ago attributed the immunity of the shrikes to their pugnacious, ag- 

 gressive nature, which would cause them to attack and to drive off, if 

 not actually kill, any intruding, would-be parasite. Many years ago I 

 expressed this opinion to a correspondent, L. B. Potter of Eastend, 

 Saskatchewan, who decided to test the white-rumped shrike, Lanius 

 ludovicianus, as a potential foster-parent of a cowbird. Potter (1939, 

 pp. 219-220) published a brief account of what he found, which may 

 be supplemented by the following, taken from his letter of August 

 1934. 



He placed a partly incubated cowbird's egg in a slirike's nest 

 containing 6 eggs, one of which he removed. A week later, revisiting 

 the nest, he found that the cowbird had hatched but the shrike's 

 eggs had not. The shrikes obviously had been feeding the young 

 parasite as it was m good condition. Tliree days later, the cowbird 

 had grown appreciably and the shrike's eggs remained unhatched. 

 The adult shrikes, busy with caring for their parasitic young, had 

 stopped incubating. Mr. Potter concluded that the shrikes treated 

 the young cowbu'd just as they normally would one of theu^ own chicks. 

 This crude experiment makes it seem all the more probable that it is 

 the pugnacity of the shrikes toward an approaching adult cowbird and 

 not an inability to hatch and feed a young cowbhd that is the reason 

 the shrikes have remained free from the attentions of the parasite. 



The following list contains 101 species, or a little less than half of 

 all the species known to have been imposed upon by the brown- 

 headed cowbird. While other birds undoubtedly will be added in the 

 future, it seems that the percentage of the total host catalog that 

 may be considered "successful" choices for the parasite will remain 

 about as given here. 



One further thought might be expressed before listing the rearing 

 hosts. From the standpoint of the population economy of the para- 

 site, there is a great difference between a host species that occasionally 

 manages to bring up one young cowbhd and a host that does so 



