SHORT COMMUNICATIONS 
399 
One of five real cowbird eggs was ejected by a 
male Warbling Vireo in 1998 and, in 1999, one of 
four model cowbird eggs was ejected by a male. 
Thus, at least 22% of ejections were by males, 
which is the first confirmed documentation of 
male Warbling Vireos ejecting cowbird eggs. This 
represents a lower estimate of the proportion of 
ejections by males, however, because we relied on 
song to identity them. The two ejections by males 
had the longest times until ejection, which 
confinns Sealy and Neudorfs (1995) suggestion 
that males are less experienced ejecters and Honza 
et al.'s (2007) observations that female Eurasian 
Blackcaps eject more quickly than males. 
Sealy (1996) observed two male Warbling 
Vireos attempt unsuccessfully to eject cowbird 
eggs, but his other observations implicated 
ejection only by females. Ejection by males has 
also been observed in two other Brown-headed 
Cowbird hosts. Gray Catbirds and Baltimore 
Orioles (Sealy and Neudorf 1995). Alexander 
Wilson recorded ejection of Brown Thrasher 
(Toxastoma rufum) eggs by a male catbird 
(summarized in Sealy and Neudorf 1995), but 
male ejection in this species has not been 
confinned because catbirds are sexually mono- 
roorphie. Sealy and Neudorf (1995) predicted that 
species where males incubate or feed females at 
the nest are the most likely to have evolved egg 
ejection by males. Ejection by male Warbling 
Vireos supports this prediction because males also 
incubate (Gardali and Ballard 2000). Experiments 
with a few host species of Common Cuckoo 
(Cuculus canorus) also support this prediction. 
Soler et al. (2002) found that in species where 
males also incubated, both males and females 
ejected artificial cuckoo eggs, but in those where 
males did not incubate, only females ejected them. 
This trend has been supported in studies of three 
additional cuckoo hosts where both males and 
females eject eggs (Lee et al. 2005, Honza et al. 
2007) and where only females eject (Pozgayova et 
al. 2009). Previously, females have generally been 
assumed to be solely capable of ejection because 
in many species they conduct most activities at 
the nest (e.g., Rolhstein 1975b). However, in 
Brown-headed Cowbird hosts, identifying which 
gender ejects has been difficult because most 
species that eject are not sexually dimorphic 
(Sealy and Neudorf 1995). Only two species of 
—20 known ejecter species (Underwood 2003, 
Peer and Sealy 2004), Baltimore Orioles and 
Bullock’s Orioles (Icterus bullockii), are strongly 
sexually dimorphic; most observations of ejection 
have not been made in populations that are 
uniquely color marked. Thus, involvement of 
males in cowbird egg ejection may have been 
underestimated. Ejection by males has theoretical 
implications because ejection by both males and 
females should increase the spread of the ejecter 
trait in a population (Rothslein 1975b, Kelly 1987, 
Sealy and Neudorf 1995). 
ACKNOWLEDGMENTS 
We thank C\ M. McLaren and R. M. Underwood for field 
assistance. We also thank the staff of the Delta Marsh Field 
Station (University of Manitoba) for providing logistical 
support and the Portage Country Club. Delta Waterfowl and 
Wetlands Research Station, and private landowners who 
permitted us to work on their property. This study was 
funded by grants from the Natural Sciences and Engineer¬ 
ing Research Council of Canada to SGS and a post-graduate 
fellowship from the University of Manitoba to TJU. 
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