SHORT COMMUNICATIONS 
151 
The Wilson Journal of Ornithology 123(1): 151-154, 2011 
Reproductive Status of the Shiny Cowbird in North America 
William Post 1,3 and Paul W. Sykes Jr . 2 
ABSTRACT.—We collected 17 (13 females, 4 
males) Shiny Cowbirds ( Molothrus bonariensis) during 
the passerine nesting season in July 1999 and 2003 in 
Jasper County, southwestern South Carolina. Five 
females (38%) were laying eggs, as ascertained from 
the condition of their reproductive organs. Two females 
collected on 1 July 1999 and 19 July 2003 had eggs in 
their oviducts, and would have deposited eggs within 
1 day. Shiny Cowbirds have been in North America for 
at least 24 years, but only males had been collected 
before this study. Most of those collected had enlarged 
testes, as did the four collected in the present study, but 
these data are not proof that breeding actually occurred. 
The reproductive condition of the females we collected 
provides material evidence that the species breeds in 
North America. It is not known which species are being 
parasitized by Shiny Cowbirds, but several species 
widespread in the southeastern United States are highly 
suitable hosts. Received 23 August 2010. Accepted 3 
November 2010. 
The Shiny Cowbird ( Molothrus bonariensis ) in 
its expansion from South America was first 
recorded in Cuba in 1982, the northern Florida 
Keys in 1985, and the Florida mainland in 1987. 
Shiny Cowbirds were reported at nine coastal 
localities north of Tampa, Florida, north to 
northeastern North Carolina and west to south¬ 
western Louisiana from 1988 to 1990. They also 
appeared at interior localities, including Ft. Hood 
Texas and Winbom Springs, Oklahoma (Post et al. 
1993, Cruz et al. 2000). Despite the Shiny 
Cowbird’s expanding distribution, and relatively 
extended residency in North America, their 
breeding status has not been documented. This 
ma y be due, in part, to the similarity of Shiny 
Cowbird females and fledglings to Brown-headed 
Cowbirds (M. ater ), making it difficult to distin¬ 
guish them in the field. Shiny Cowbirds lay 
immaculate and spotted eggs (Lowther and Post 
1999); the latter are similar to those of Brown- 
' Charleston Museum, 360 Meeting Street, Charleston, 
S C 29403, USA. 
"USGS, Patuxent Wildlife Research Center Athens, 
darnel] School of Forest Resources, The University of 
Ge ^gia, Athens, GA 30602, USA. 
Corresponding author; e-mail: grackler@aol.com 
headed Cowbirds (Lowther 1993). Shiny Cowbirds 
have been on the North American mainland for at 
least 24 years, but only a few reports provide 
evidence, all indirect, that breeding has actually 
occurred. Other than direct observations of females 
laying eggs, breeding can be verified by finding 
Shiny Cowbird eggs or young associated with an 
identified host species, and by genetic analysis of 
eggs, nestlings, and fledglings to distinguish them 
from Brown-headed Cowbirds. 
METHODS 
The study area bordered a dredge spoil-site next 
to the Savannah River in southwestern Jasper 
County, South Carolina (32 04.52' N, 80° 57.83' 
W). We captured Shiny Cowbirds in mist nets 
placed in coastal scrub at the edge of the spoil- 
site. About 30% of the net site consisted of open 
ground, either bare or covered with patches of 
grasses and forbs <30 cm in height. The 
remainder consisted of stands of woody vegeta¬ 
tion which, in order of importance, were com¬ 
posed of hackberry (Celtis laevigata ), cherry 
(Prunus spp.), sweet gum ( Liquidambar styraci- 
flua ), and blackberry (Rubus spp.). 
We captured birds at one site, using three mist 
nets (6 m length, 30 mm mesh) placed in a 
triangular array around an elevated feeder provi¬ 
sioned with millet ( Panicum milleaceum ) seeds 
(Sykes 2006). The cowbirds were euthanized and 
then frozen. We recorded diameters of the three 
largest follicles and of any oviducal egg, the area 
(length X width) of the ovaries (females), and the 
dimensions of the testes (males) during examina¬ 
tion of the thawed specimens. We assigned age of 
the birds from plumage characteristics (Pyle 
1997). Female Shiny Cowbirds and Brown¬ 
headed Cowbirds are about the same size, based 
on birds collected in or near our study site (mean 
mass of 13 M. ater = 34.2 g; 13 M. bonariensis = 
33.6 g). We assumed the reproductive physiology 
of the two species is similar, and used the criteria 
of Scott and Ankey (1983) to ascertain laying 
rates. We estimated that laying would occur (1) 
within 1 day if an egg was in the oviduct, or (2) 
within 2 days, if the ovaries contained at least one 
