HOST RELATIONS OF PARASITIC COWBIRDS 



37 



Birds that evict the parasitic eggs. — Throwing the parasitic 

 eggs out of the nest is the least frequent reaction recorded in cowbird 

 hosts. Most of the smaller species of victims probably would be 

 unable to do so if they wanted to, and some of the larger ones seem 

 to vary individually in this reaction. Egg eviction has been recorded 

 for the following hosts: 



eastern kingbird (evidence 



inconclusive) 

 purple martin 

 catbird (regularly) 

 robin (regularly) 



wood thrush (locally frequently) 

 Bell's vireo 

 Baltimore oriole 

 Bullock's oriole 



Hosts Known To Have Reared Young of Parasite 



The mere fact that a cowbird lays an egg m the nest of a given 

 species of bhd does not imply that the chosen victim is necessarily 

 able to act as a successful fosterer. The parasite frequently deposits 

 eggs in nests of unwilling or unsuitable bird victims. From the 

 standpoint of the cowbhd, these are lost eggs: the maintainance of its 

 species depends on its placing enough eggs in nests of the right Idnds 

 of bu'ds and in having enough of these eggs hatch young which reach 

 maturity. 



In compiling the list of successful fosterers, I have used as a mini- 

 mum criterion of success the situation of a nest containing a healthy 

 nestling cowbird which is more than half grown and largely feathered, 

 i.e., within a few days of fledging or leaving the nest. The degree of 

 development attained at this stage seems a fau'Iy safe indication that 

 the foster parents were able to feed properly and care adequately for 

 the growing cowbird chick. 



If we were to be more exactmg and require actual fledging as a 

 measure of success, our list would suffer unduly from the effects of 

 external causes, such as predators that frequently take young birds 

 out of nests regardless of whether they are parasites or rightful 

 young. Since our interest at this point is to determine the innate 

 suitability of the foster species, it seems overcritical to reject evidence 

 that is short of full fledging success. The ability to continue to the 

 fledguig stage may be safely deduced from a bird which has developed 

 rapidly and well, even if the observation does not continue to the 

 time of leaving the nest. There is no good reason for thinking that 

 any given host species which could hatch and rear a young cowbird for 

 the first week or more of its nestling stage could not be able to complete 

 the task. Any later factors of interference, such as predators, storms, 

 or accidents to the foster parents are outside the concept of host 

 suitability. 



