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The Wilson Journal of Ornithology 123(2):395-400, 2011 
Behavior of Warbling Vireos Ejecting Real and Artificial Cowbird Eggs 
Todd J. Underwood ‘' 2 ' 3 and Spencer G. Sealy 1 
ABSTRACT.—We videolaped nine ejections of real 
hi = 5) and artificial (ft = 4) Brown-headed Cowbird 
(Molothrus ater) eggs by Warbling Vireos (Vireo 
gilvus). All eggs were ejected within 6 min. There 
were no significant differences in time used for any 
ejection behavior by egg type, although artificial eggs 
were probed longer before ejection. Eight vireos ejected 
the cowbird egg using visual cues only because none sat 
on its nest before ejection. One male ejected the 
cowbird egg after sitting on the nest for a few minutes: 
consequently, both visual and tactile cues were 
available for its decision to eject the cowbird egg. 
'Department of Biological Sciences. University of 
Manitoba, Winnipeg. MB R3T 2N2. Canada. 
: Current address: Department of Biology, Kutztown 
University. Kutztown. PA 19530. USA. 
‘Corresponding author: e-mail:underwoo@kutztown.edu 
Most vireos identified the cowbird egg by sight and, in 
most cases, rapid ejection of the cowbird egg precluded 
the possibility of using tactile cues. Grasp-ejection was 
the only ejection method confirmed for real and 
artificial eggs. Two male vireos ejected cowbird eggs 
at two nests, which is the first documentation of 
successful ejection by male Warbling Vireos, and the 
third cowbird host for which males are known to eject 
cowbird eggs. The ability of males to eject cowbird eggs 
has important implications for the evolution of ejection 
behavior. Received 10 August 2010. Accepted 7 January 
2011 . 
High reproductive costs incurred by hosts of 
obligate brood parasites (Lorenzana and Sealy 
1999. Davies 2000) favor evolution of defenses 
against the parasitic egg. such as egg ejection, egg 
