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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 2012 
We did not observe the fledgling from the 
parasitized brood >50 m front the nest until 
12 days after fledging and it did not move >100 m 
from the nest until 25 days after fledging. 
Fledglings front non-parasitized broods tracked 
for at least 25 days (n — 16) were observed >50 lit 
front nests 2-10 days (.v = 4.1 r 0.71) after 
fledging, and >100 m from nests 3-16 days (.v = 
9.5 ± 1.14) after fledging We observed host 
adults with the fledgling front the parasitized 
brood during three (33%) of nine observations 
during the cowbird fledgling superaclive phase 
(days 13-23). We observed adults present with 
fledglings from non-parasitized broods during 105 
(68%) of 155 observations during that period 
(difference = 0.35; 95% confidence interval = 
0.034-0.666). 
DISCUSSION 
Cowbird parasitism reduced Ovenbird clutch 
size, brood size, and number of young fledged in 
northcenlral Minnesota. These findings are similar 
to results of studies of Ovenbird populations in 
other regions (Hann 1937. Hersek et al. 2002). Our 
observations of apparent parental neglect and 
starvation of a fledgling from a parasitized brood 
is consistent with claims by Rasmussen and Scaly 
(2006) that effects of cowbird parasitism likely 
extend beyond the nesting period. Rasmussen and 
Sealy (2006) suggested that host adults spend most 
of their time provisioning cowbird fledglings, 
thereby neglecting their own offspring. The host 
fledgling we monitored was accompanied by adults 
less often than any fledgling from a non-parasitized 
brood during the cowbird superaclive phase (days 
13 to 23), and was the only fledgling to apparently 
starve (i.e., empty stomach and no sign of 
predation). This fledgling also remained closer to 
its nest than any other fledgling we monitored, 
suggesting the cowbird fledgling may have reduced 
brood movement. We acknowledge that inference 
from our post-fledging observations is speculative 
because of the small sample siz.e. It is possible the 
fledgling’s mortality may have been caused by 
adult mortality or by disease. However, adult 
Ovenbird survival is high during the post-fledging 
period (Bayne and Hobson 2001). and we observed 
no signs of additional stressors (e.g., blowfly 
infection or heavy tick load). Brood" parasitism 
could significantly impact reproductive success in 
populations with high incidence of nest parasitism 
it our observations arc representative of fledglings 
in broods with cowbirds. 
Ovenbirds renest up to five times (more 
commonly once or twice) after nest failure t Harm 
1937, Podolsky et al. 2007). However, pairs rarely 
double brood (Hann 1937, Zach and Falls 1976, 
Podolsky et al. 2007). The presence of cowbirds in 
successfully-fledged broods may reduce seasonal 
reproductive success more than cow bird-induced nest 
failure. On average, an adult Ovenbird has two 
nesting seasons (Hann 1937). Thus, if cowbirds 
reduce the survival ot host fledglings, a single 
cowbird fledgling could substantially reduce the 
lifetime reproductive success of an average Ovenbird. 
Models of songbird reproductive success typically 
rely on estimates of nest success and assumptions 
about fledgling survival. Our observations suggest 
brood parasitism may decrease reproductive success 
by decreasing nest productivity and by reducing 
survival of young fledged from parasitized nests 
Each of these influences could result in overestimates 
of reproductive success if all successful nests are 
treated equally. Wc recommend further investigation 
of the relationship between cowbird brood parasitism 
and host fledgling survival, especially in areas where 
brood parasitism is relatively common. 
ACKNOWLEDGMENTS 
These data were collected during a project funded by the 
U.S. Geological Survey and the US. Fish and Wildlife 
Service through Research Work Order 73 at the Minnesoi;. 
Cooperative Fish and Wildlife Research Unit with in-kind 
suppon from the U.S. Forest Sen ice. We handled, banded, 
and attached radio transmitters to birds following Priutvol 
#0806A35761, approved by the University of Minnesota 
Institutional Animal Care and Use Committee We dunk 
D. D. Robinson. A. C. Edmond. E. S. Michel. A. P- 
Monroe. T. L. Eisenhauer. J. L. Hammers. J. M. Rcfsnider. 
and K. J. Iknayan for assistance in field data collection 
Brian Scholtens for assistance with dissection and stomach 
content identification, and L. I. Berkeley and S. R Loss lot 
helpful comments on the manuscript. 
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