SHORT COMMUNICATIONS 
153 
(2010) confirm that females are breeding, but it is 
not known which species they parasitize. Several 
hypotheses may explain why no information is 
available. (1) Competition with Brown-headed 
Cowbirds. This hypothesis is difficult to test, 
because of the scarcity of both species of 
cowbirds in regions where they co-occur (Post 
and Gauthreaux 1989, Stevenson and Anderson 
1994, Beaton et al. 2003). Parasitism rates are 
low: Whitehead et al. (2002) on the central coast 
of South Carolina found only 13% of 346 nests 
parasitized, all by Brown-headed Cowbirds. 
Prather and Cruz (2002) in southwestern Florida 
found only 2% of 108 nests parasitized. Other 
studies in the southeastern U.S. have found 
cowbird-parasitized nests (Sargent et al. 1997, 
Kilgo and Moorman 2003) but, on the upper 
coastal plain and piedmont, outside the range of 
the Shiny Cowbird. (2) The similarity of eggs and 
young of the two cowbird species. This hypothesis 
has not been tested, because studies conducted in 
areas where Shiny Cowbirds occur have docu¬ 
mented few cases of parasitism by any cowbirds 
(Prather and Cruz 2002); another study found 
cowbird eggs, but all were believed to have been 
laid by two color-banded Brown-headed Cow¬ 
birds (Whitehead et al. 2000). (3) Lack of research 
in areas occupied by Shiny Cowbirds. This 
cowbird occurs on the coast from Alabama to 
South Carolina during the passerine breeding 
period but, other than Prather and Cruz (2002) 
and Whitehead et al. (2002), no recent studies of 
breeding songbird communities on the coast 
appear to have been published. 
Several species widespread in the southeastern 
U.S. are parasitized by Brown-headed Cowbirds 
and presumably would be used by Shiny Cow¬ 
birds. Whitehead et al. (2002) found 37% of 30 
Yellow-breasted Chat ( Icteria virens ), 36% of 14 
Painted Bunting ( Passerina ciris ), and 24% of 17 
Blue Grosbeak (P. caerulea) nests parasitized in 
coastal South Carolina. The clutch sizes of three 
species were reduced by cowbird parasitism, and 
the seasonal fecundity of Blue Grosbeaks was 
lowered (Whitehead et al. 2000). These authors 
found two Red-winged Blackbird nests parasit¬ 
ized. This species is potentially a highly suitable 
host, considering its similarity to the Yellow¬ 
shouldered Blackbird ( Agelaius xanthomus), 
which is heavily parasitized in Puerto Rico (Post 
1981). Prather and Cruz (2002) also found Red¬ 
winged Blackbirds parasitized in southern Florida, 
where they nests in mangroves, the habitat in 
which Shiny Cowbirds most often breed in Puerto 
Rico (Post and Wiley 1977), and in cordgrass 
(Spartina alterniflora ), which is similar to grami- 
noid vegetation used by Yellow-hooded Black¬ 
birds ( Chrysomus icterocephalus) parasitized by 
Shiny Cowbirds in Trinidad (Cruz et al. 1990). 
The Shiny Cowbird’s population growth is 
correlated with a decrease of Yellow-shouldered 
Blackbirds in Puerto Rico (Post 1981), but 
cowbird control appears to have helped in slowing 
the blackbird’s decline (Wiley et al. 1991, Cruz et 
al. 2005). If Shiny Cowbirds continue to increase 
in North America, it is important to examine their 
effect on potential hosts such as Painted Buntings, 
which, because of other factors, are already at risk 
in portions of their range (Sykes and Holzman 
2005, Sykes et al. 2006). 
ACKNOWLEDGMENTS 
The paper benefited from the reviews and useful 
comments of Peter Lowther, an anonymous reviewer, and 
the editor of this journal. Steve Calver, U.S. Army Corps of 
Engineers, arranged access to the study site. 
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