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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
burial, and nest desertion (Rothstein 1990). Egg 
ejection appears to be the most effective and 
efficient defense against parasitism (Rothstein 
1975a. Peer et al. 2005, Underwood and Sealy 
2006a), although it is sometimes associated with 
costs, especially for small hosts (Lorenzana and 
Sealy 2001. Martm-Vivaldi et al. 2002). Experi¬ 
ments documenting egg ejection have been 
conducted for many years (e.g M Rothstein 
1975a); however, previous knowledge about 
ejection behavior relied on rare anecdotal obser¬ 
vations of ejection by hosts and a few intense 
observational studies involving experimental par¬ 
asitism (Sealy and Neudorf 1995). More recently, 
use of video cameras has revealed additional 
details regarding ejection behavior. The method 
by which hosts eject eggs has been ascertained for 
several species (Moksnes et al. 1994, Soler et al. 
2002, Underwood and Sealy 2006a, Rasmussen et 
al. 2009). However, for most species, less is 
known about whether the male or female ejects 
the parasitic egg (Sealy and Neudorf 1995, Soler 
et al. 2002, Honza et al. 2007). the behavior 
leading to egg ejection (Antonov el al. 2008. 
2009), and time required to discriminate and eject 
foreign eggs (Sealy and Bazin 1995. Antonov et 
al. 2008). We report details of videotaped 
observations of Warbling Vireos (Vireo gilvus) 
ejecting real and artificial Brown-headed Cowbird 
C Molothrus uter) eggs that offer insight into the 
experimental use of different types of eggs, egg 
discrimination behavior, and the evolution of 
ejection behavior. 
methods 
Our study was conducted at Delta Marsh 
Manitoba (50 11' N. 98'23’ W) where we locat¬ 
ed Warbling Vireo nests from May to July 1998 
and 1999. Warbling Vireos, at 15 g with a'tomial 
bill length of 17.2 mm. are the smallest known 
ejecters of cowbird eggs and are capable of both 
puncture- and grasp-ejection (Sealy 1996, Under¬ 
wood and Sealy 2006a). Incubation in this 
sexually monomorphic species is by both males 
and females (Gardali and Ballard 2000); thus, 
males have the opportunity to be directly involved 
in egg discrimination. 
Warbling Vireo nests were experimentally 
parasitized as soon as they were available (i.e., 
during laying or incubation) because nest stage 
does not influence the response of Warbling 
Vireos to parasitism (Sealy 1996. Underwood 
and Sealy 2006a). We videotaped Warbling 
Vireos ejecting real (n = 5) and artificial (n = 
4) cowbird eggs in conjunction with two other 
experiments (Underwood and Sealy 2006a. hi We 
calculated the length of lime vireos were involved 
in ejection behaviors, whether only visual cues w 
a combination of visual and tactile cues were used 
by vireos lor cowbird egg discrimination, and 
whether males ejected eggs. 
Single real cowbird eggs were experimental I\ 
added to vireo nests in 1998. One nest was 
parasitized during the laying stage, whereas four 
nests were parasitized during the incubation 
stage. Real cowbird eggs were painted over with 
non-toxic, acrylic paints to match the appearance 
of a cowbird egg as a control for various 
treatments of painted eggs as part of a study of 
the parameters of egg discrimination (Underwood 
and Sealy 2006b). Vireos ejected these painted 
eggs at about the same frequency as real, 
unpainted cowbird eggs (Sealy 1996, Underwood 
and Sealy 2006b). Vireo nests in 1999 were 
experimentally parasitized with a single artificial 
cowbird egg made of plaster and painted with 
non-toxic, acrylic paints to match the appearance 
ol real cowbird eggs (Underwood and Sealy 
2006a). All of these nests were parasitized during 
the incubation stage. Individuals at these nesls 
previously ejected real eggs in several treatment'' 
as part of another study (Underwood and Sealy 
2006b). 
Nests in both years were videotaped for I hr to 
record ejection behavior after a cowbird egg had 
been added. We recorded for each ejection the 
time from egg addition until ejection (time until 
ejection), the time from a vireo’s arrival at the 
nest until ejection (time for ejection), the time 
Irorn the start of probing the cowbird egg unid 
ejection (probing period), and the actual time the 
vireo probed (time probing; Table I ). The probing 
behavior of Warbling Vireos involved a combi¬ 
nation of pecks and tremble-thrusting movement 
prior to ejecting cowbird eggs. Tremble-thrusting 
(i.e.. pushing of the bill into the nest with a 
vibrating motion) is used by some songbirds to 
turn or rotate eggs during normal incubation 
(Deeming 2002). We were unable, however, to 
consistently differentiate these behaviors or the 
strength of pecks (sen.su Soler et al. 2002. 
Antonov et al. 2008) because the deep nest cups 
and height of the vireo nests precluded a clear 
view ol the bird’s head some of the time. Thus, we 
used probing behavior to describe any pecking ot 
bill-thrusting movement directed into the nest cup 
