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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 1, March 2011 
TABLE 1. Measurements of reproductive tracts of 
female Shiny Cowbirds collected in Jasper County, South 
Carolina in 1999 and 2003. SY (second-year): hatched the 
previous year. ASY (after-second-year): hatched at least 
2 years earlier. 
Charleston 
Museum 
specimen # 
Date 
Age 
Area of 
ovaries 
(mm 2 ) 
Diameter (mm) of: 
Largest 
follicle 
Oviducal 
egg 
99.22.075 
1 Jul 1999 
SY 
2.3 
02.46.051 
1 Jul 1999 
ASY 
71.4 
5.0 
05.2.036 
1 Jul 1999 
ASY 
117.5 
7.2 
13.0 
05.2.043 
1 Jul 1999 
ASY 
110.0 
7.8 
05.2.044 
1 Jul 1999 
ASY 
41.9 
1.3 
99.22.037 
2Jul 1999 
ASY 
160.2 
8.8 
99.22.048 
2 Jul 1999 
SY 
8.1 
99.22.069 
2 Jul 1999 
SY 
116.5 
9.7 
99.22.072 
2 Jul 1999 
ASY 
13.3 
05.2.041 
18 Jul 2003 
SY 
32.0 
2.5 
05.2.033 
19 Jul 2003 
ASY 
21.8 
8.3 
10.3 
05.2.040 
19 Jul 2003 
ASY 
33.0 
2.5 
05.2.042 
19 Jul 2003 
ASY 
30.0 
2.5 
pre-ovulatory (round, cream-colored or yellowish) 
follicle >7.9 mm diameter. Specimens were 
identified by bill shape and wing formula (Pyle 
1997) and were saved as study skins. Identifica¬ 
tions were verified by comparisons with a series 
of M. b. minimus collected in the West Indies. 
rviiCiUJLtj) 
We collected 17 (13 female and 4 male) Shiny 
Cowbirds in 1999 and 2003. We collected nine 
females on 1-2 July 1999; six were after-second- 
year (AS y ) and three were second-year (SY) birds 
# ns 9 t° ne / SY female (Charleston Museum 
UP.-.036), had an oviposited (oviducal) egg 
3 mm in diameter, indicating she would have 
# 99 a 22 e 03 8 7 r tl ! i , n ' day ‘ An ° ther ASY female 
( V—037) With an unovulated follicle 8.8 mm 
lameter had an extremely dilated cloaca, and 
presumably had laid an egg the day she was 
C # P 99^n^l f* f6males (# 99 - 22 -048 and 
: i ? 9 ' 22 ' 1 ° J 69) had follicles >7.9 mm diameter 
nd would have laid eggs within 2 days. 
1 A ^‘r^'° Ur females (3 ASY and 1 SY) on 
u y 2003; one ASY female (# 05 2 033) 
had an oviposited egg 10.3 mm in diameter’ and 
mheTth 1316 She W ° Uld haVC Iaid within 1 da r the 
(3 h :r,r r Y n0t Iaying (Table D- Thus five 
we examined 
Four males collected on 1-2 July 1999 had 
enlarged Mea. The lengths of ,he larges, lesres of 
two ASY males were 8.4 and 7.3 mm; those of 
two SY males were 6.2 and 6.3 mm. The testes of 
non-breeding Shiny Cowbirds usually diminish to 
<2 mm in diameter (WP, unpubl. data). Neither 
males nor females were molting. 
DISCUSSION 
The first suggestion that Shiny Cowbirds breed 
in North America was based on a 1991 observa¬ 
tion near Homestead, Florida. A Red-winged 
Blackbird (Agelaius phoeniceus) was seen feeding 
a “very young” Shiny Cowbird which was “just 
starting song” (Pranty 2000: 516). The report is 
questionable, because fledglings of the Brown¬ 
headed Cowbird and Shiny Cowbird are not 
known to sing (Lowther 1993, Lowther and Post 
1999) and, if they do, it is not known if their 
vocalizations can be used to differentiate them. 
Larry Manfredi (pers. comm, in Pranty 2000) saw 
a female cowbird, which he identified as a Shiny 
Cowbird, in April 1998, near Kendall, Florida, fly 
to an unattended Red-winged Blackbird nest. The 
cowbird sat on the nest 3^4 min, but it is not 
known if she laid an egg (Pranty 2000). 
Enlarged testes suggest the possibility of 
breeding, but verification depends on the repro¬ 
ductive status of females. The four males 
collected in this study had enlarged testes, as did 
one collected 10 August 2000 at the same site. 
Seven males collected in South Carolina and 
Florida during 30 April-25 July 1989-1991 had 
enlarged testes (Post et al. 1993). A male obtained 
at Ft. Hood, Texas on 23 May 1990 had enlarged 
testes (Greg Lasley, pers. comm.) as did one 
collected 21 May 2009 on Sullivan’s Island, South 
Carolina (WP, unpubl. data). 
We provide evidence that Shiny Cowbirds were 
laying eggs in South Carolina in 1999 and, based 
on the sizes of preovulatory follicles and the 
presence of oviducal eggs, estimate that five 
(38%) of 13 females collected in South Carolina 
in 1999-2003 were laying (Table 1). Additional 
evidence of breeding in North America has been 
provided by collection of two females, each with 
an e gg in her oviduct: in Georgia in 2000 (Sykes 
and Post 2001), and in northcentral Florida in 
2009 (Reetz et al. 2010). 
Shiny Cowbirds occur during the passerine 
breeding period on the Atlantic and Gulf coasts 
from South Carolina to Alabama (Lowther an 
Post 1999, Pranty 2000). They have been in the 
southeastern U.S. at least 24 years; this study and 
those of Sykes and Post (2001) and Reetz et al. 
