SHORT COMMUNICATIONS 
397 
TABLE 1. Time (sec) for Warbling Vireos to respond to real and artificial Brown-headed Cowbird eggs experimentally 
added to their nests. “Time until" variables represent the time from egg addition until the vireos have responded. All 
measurements represent mean ± SE (sec) for each variable. 
Variable 
Real cowbird egg (n = 5) 
Plaster cowbird egg (n - 4) 
/ 
p 
Time until eject 
232.0 ± 64.5 
328.8 ± 126.5 
-0.73 
0.49 
Time until return 
171.0 ± 50.0 
164.3 ± 55.5 
0.09 
0.93 
Time for ejection 
60.8 ± 38.5 
164.5 ± 74.4 
-1.32 
0.23 
Probing period 
51.0 ± 37.0 
161.8 ± 74.4 
-1.43 
0.20 
Time probing 
16.8 ± 6.5 
115.5 ± 40.6 
-2.40' 1 
0.09 
Equal variances nol assumed. 
and. presumably, toward the eggs. We compared 
ihe time used for ejection of real cowbird eggs to 
that for artificial cowbird eggs because the latter 
may be more difficult to eject and may not reflect 
ejection times of real eggs (Martm-Vivaldi et al. 
2002. Prather et al. 2007). We also recorded 
whether vireos sat on their nest prior to egg 
ejection to identify the potential use of tactile and 
visual cues in egg discrimination. The method of 
egg ejection was identified, where possible, for 
each egg type. We also ascertained, where 
possible, whether males were responsible Tor 
egg ejection. Males were identified based on 
whether they sang before ejection because there is 
no evidence female Warbling Vireos of the 
eastern subspecies sing (Howes-Jones 1985a, b). 
RESULTS 
Warbling Vireos ejected both real and artificial 
cowbird eggs within 6 min on average after 
experimental parasitism (Tabic I). Much of the 
delay until ejection was due to the vireos’ absence 
from the nest, but once a vireo returned to the 
nest, egg ejection was rapid (Tabic 1). There were 
no significant differences in length of time for any 
ejection behaviors when vireos ejected real eggs 
compared to artificial eggs (Table 1). However, 
vireos on average spent six times as long probing 
artificial cowbird eggs compared to real cowbird 
eggs. No host eggs were damaged (e.g.. missing 
or punctured) in any of these ejections. 
All vireos immediately looked into their nest 
when they first arrived after experimental para¬ 
sitism and within 30 sec began probing inside it 
(mean ± SE = 6.7 ± 3.4 sec). Four of five vireos 
ejected real cowbird eggs without sitting on the 
nest. The remaining vireo sat on the nest a few 
minutes before ejecting the cowbird egg, although 
it probed within the nest before sitting. All four 
vireos ejected artificial cowbird eggs without 
sitting on the nest. Thus, vireos used only visual 
cues for egg discrimination in eight ejections, 
whereas in one ejection both visual and tactile 
cues were available for egg discrimination. 
The method of egg ejection was confirmed at 
four nests. Two real cowbird eggs were ejected 
by grasp-ejection and, as previously reported 
(Underwood and Scaly 2006a). two artificial 
cowbird eggs were ejected by grasp-ejection. The 
exact method of ejection (grasp- or puncture- 
ejection) could not be confirmed at three nests 
parasitized with real eggs and al two nests 
parasitized with artificial eggs due to the position 
of the vireos during ejection. The video evidence, 
however, indicated these eggs were ejected with 
the bill. 
Male Warbling Vireos ejected cowbird eggs at 
two nests (one each in 1998 and 1999). Four 
minutes after experimental parasitism at nest 
1998-38, a vireo landed above the nest, looked 
into it and then settled on the nest. Several 
seconds later, Ihe vireo began singing, which 
identified it as a male. The male sat on the nest for 
~3 min where it mixed bouts of singing with 
probing inside the nest. After a final bout of 
probing for 6 sec, the vireo ejected the cowbird 
egg 3 min and 33 sec after arrival. Five minutes 
after experimental parasitism at nest 1999-75, a 
vireo landed on the nest branch and, after probing 
inside the nest twice, it sang. This male sang 
intermittently as it continued to probe or peck 
inside the nest until finally ejecting the cowbird 
egg 6 min and 23 sec after arrival. 
DISCUSSION 
Warbling Vireos rapidly ejected cowbird eggs 
from their nests. Cowbird eggs differ in two 
parameters (size and spot pattern) from Warbling 
Vireo eggs (Underwood and Sealy 2006b), 
making them strongly non-mimetic. Non-mimetic 
