HOST RELATIONS OF PARASITIC COWBIRDS 35 



theii' nests when parasitized by the brown-headed cowbird. Their 

 presence in this hst should not be taken to imply that this is their 

 regular response or even a fau'ly frequent reaction. In those species 

 that tend to desert readily the word "frequently" is added. In 

 two species the observational evidence suggested but did not prove 

 that desertion was due to cowbird parasitism; to these the word 

 "uncertain" has been added. 



verdin Maryland yellowthroat 



Bewick's wren (uncertain) yellow-breast chat (frequently) 



eastern bluebird redwinged blackbird 



Bell's vireo scarlet tanager (uncertain) 



red-eyed vireo cardinal (frequently) 



yellow warbler painted bunting (frequently) 



myrtle warbler field sparrow (frequently) 



Burying cowbird eggs under new nest-lining. — Several kinds 

 of cowbird hosts have been found which disposed of the parasitic eggs 

 by bur}dng them under new nest linings and then proceeded with 

 further egg lajdng on the new floor. Because this has the appearance 

 of clever resourcefulness, it has been described many times and has 

 often been overinterpreted in some anthropomorphic accounts. 



The explanation of this "flooring over," fu'st advanced by F. H. 

 Herrick (1910), still seems very plausible, and is, in fact, the only 

 one which presents the picture on its proper behavioristic level. 

 Herrick studied the cj^clical instincts of birds; i.e., migration, court- 

 ship, matmg, nest-building, egg-laying, incubation, etc. He found 

 that, if the cycle were disturbed at any point, the birds would go 

 back one stage in the cycle and start again from that point. Thus, 

 if the egg-laying stage was disturbed by some cause, the birds would 

 go back to nest-building: they would build a new nest and then 

 resume egg-la;^Tag. In the case of a cowbird victim, the egg-laying 

 is disturbed by the introduction into the nest of a cowbird's egg. 

 According to Herrick's analysis, the bird should desert — as many 

 bu'ds do — and build another nest. However, the victimized bu'd 

 may be so attached to its nestmg site that it cannot easily break 

 away. At the same time, the interference it has suffered prompts 

 it to build another nest. There is then a conflict between the two 

 instinctive drives — the one tending to hold the bird to the nesting-site, 

 the other tending to pull it away to build a new nest. The forces 

 appear to be fairly equal, and the outcome is not predictable in any 

 one case. The presence of eggs of its own in the nest strengthens 

 the attachment of the bird to the site and nest, and it is this force 

 probably that wins out in many cases. The new floor, covering 

 the parasitic eggs, is thus to be interpreted as comparable to a new 

 nest, such as the sequence of the cyclical instincts would demand, 

 but placed directly in or on the old one, due to the strong site attach- 



