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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 2012 
nests and explaining reduced predation when Sora 
nests were present in the marshes with breeding 
Marsh Wrens. Previous experimental work by 
Pieman (1977) showed that Marsh Wrens are not 
able to destroy quail eggs; thus, the stimulus 
presented by these eggs remained for the duration 
of our 48-hr trial period. 
We had expected corvids and mammals to be 
significant predators of blackbird and Sora nests 
in addition to Marsh Wrens based on many years 
of work at Ihis site. We may have overestimated 
the importance of crows and ravens (Corvus spp.) 
because ihey arc observable. Mammalian preda¬ 
tion tends to be intermittent but widespread. 
Common raccoons (Procxon lotor) in particular 
have decimated entire Red-winged Blackbird 
colonies in one or two nights. 
Our experimental design contained one unavoid¬ 
able element that may have contributed to lower 
predation of blackbird nests in the presence of Sora 
nests. As artificial nests were used, there were no 
attending adult blackbirds or Sora. A female Sora or 
Red-winged Blackbird would be in nearly constant 
attendance at the nest during incubation under natural 
conditions (Kaufman 1989, Beletsky 1996) and, for 
blackbirds, the territorial male would defend against 
Marsh Wren intrusions (Beletsky 1996). Marsh 
Wrens are known to attack Sora nests (Allen 1934) 
and it is possible that in the absence of adult Sora, 
Marsh Wrens had greater than normal access to the 
experimental nests. However, nests do remain 
uncovered during egg-laying before the onset of 
incubation by both blackbirds (Beletsky 1996) and 
Sora (Kaufman 1989) and a 2-day exposure of the 
eggs as in our experiment would not be unusual. 
ACKNOWLEDGMENTS 
Funding was provided for this project by the Natural 
Sciences and Engineering Research Council of Canada 
(NSERC) and the University of Winnipeg. We thank two 
anonymous reviewers and C. E. Braun for helpful 
comments on the manuscript. 
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