398 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
eggs are rejected faster than mimetic eggs by 
many hosts of brood parasites (Underwood and 
Sealy 2002, Antonov et al. 2008). Covvbird eggs 
are non-mimctic to the eggs of most ejectcr 
species and many of these species eject most 
cowbird eggs within 24 hrs (Rothstein 1976, Sealy 
and Bazin 1995. Sealy and Neudorf 1995). 
However, few studies have measured the actual 
time until ejection of the cowbird egg to permit 
comparison of the speed of ejection: most 
measure the ejection frequency at 24-hr intervals. 
Baltimore Orioles ( Icterus galbula) also ejected 
cowbird eggs rapidly with 88% ejected within 1 hr 
(Sealy and Neudorf 1995), but Eastern Kingbirds 
(Tyrannus tyrannus) only ejected 14% within I hr 
(Sealy and Bazin 1995). Sealy (1996) recorded 
only 25% ejection of real, unpainted cowbird eggs 
( n — 16) within I hr in the same population of 
Warbling Vireos compared to 100% ejection (/; = 
9) in this study. Differences in nest stage at time 
of experimental parasitism cannot explain this 
discrepancy in time until ejection because nest 
stage does not influence this interval in Warbling 
Vireos (Sealy 1996, Underwood and Sealy 
2006b). The rapid ejection of real and artificial 
cowbird eggs in this study is likely because the 
paints applied to these eggs made them appear 
more strongly non-mimetic and, for artificial 
eggs, their response to repeated parasitism may 
have been faster, as it is by Eurasian Blackcaps 
(Sylvia atricapilla) (Honza et al. 2007). 
We found no significant differences in any 
measure of ejection behavior by egg-type. How¬ 
ever, vireos probed artificial eggs about six times 
longer than real eggs before ejecting them. The 
lack of a significant difference in probing time 
was likely due to the relatively small sample sizes 
of the two egg-types for comparison. The 
difference in probing time seems large, but a 
probing time of ~2 min to remove an artificial 
cowbird egg compared to 17 sec to remove a real 
cowbird egg does not likely represent true 
difficulty in egg ejection. For example. Baltimore 
Orioles took an average of 11 min to puncture 
eject real cowbird eggs (Sealy and Neudorf 1995). 
The difference in ejection experience of Warbling 
Vireos exposed to real versus artificial eggs had 
the potential to influence most of the time of 
ejection behaviors (e.g.. Honza et al. 2007). 
Experienced vireos took longer to eject artificial 
cowbird eggs than non-experienced vireos to eject 
real cowbird eggs. This non-significant difference 
suggests that egg material may have a stronger 
influence on ejection time than experience does 
for Warbling Vireos. 
Warbling Vireos damaged none of their own 
eggs when ejecting artificial cowbird eggs iUn¬ 
derwood and Sealy 2006a). A few other hosts, in 
comparison, were more likely to damage their 
own eggs while ejecting artificial eggs (Manin- 
Vivaldi et al. 2002, Prather el al. 2007), or were 
unable to eject artificial eggs made with certain 
materials (Rothstein 1976, 1977; Prather et al. 
2007: Antonov et al. 2009). The well developed 
grasp-ejection ability of Warbling Vireos, con¬ 
firmed in this study, likely explains why this small 
host had little apparent difficulty ejecting artificial 
eggs (Underwood and Sealy 2006a). Most species 
that have difficulty ejecting artificial eggs are 
small and are likely puncture-cjecters (Martin- 
Vivalidi et al. 2002, Prather et al. 2007, Antonov 
et al. 2009; but see Honza and Moskat 2008). 
Larger cowbird hosts, such as American Robin 
{Tardus migratorius) and Gray Catbird (Durm- 
tellu carolinensis). apparently have no difficulty 
ejecting artificial eggs (Rothstein 1976, Loren- 
zana and Sealy 2001). 
Warbling Vireos apparently eject cowbird eggs 
based on visual cues. Sealy (1996) observed four 
instances of ejection of experimental cowbird 
eggs by Warbling Vireos. Females always looked 
into the nest and attempted to eject the cowbird 
egg without settling on the nest to incubate. A 
host may use tactile stimuli detected during 
incubation to identify a foreign egg by size 
(Rothstein 1982), as suggested for Rufous Hor- 
neros (Fumarius rufits) and American Robins 
(Rothstein 1982, Mason and Rothstein 1986). 
Scaly (1996), out of seven observed ejection 
attempts, noted only one (male) Warbling Vireo 
settling on the nest before attempting to eject a 
cowbird egg. These results were confirmed in this 
study. Most vireos ejected the cowbird egg before 
settling on the nest. Only at one nest did the male 
vireo eject the egg after settling on the nest. Thus, 
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. 
Rothstein (1982), in contrast, suggested the 
quickest ejections by robins were due to use ot 
both visual and tactile stimuli. The different 
behavior of male Warbling Vireos may lie due 
to a lack of experience in ejecting cowbird eggs, 
as has been suggested for male Baltimore Oriolo 
(Sealy and Neudorf 1995), and not use of different 
stimuli. 
