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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 1. March 2012 
The Wilson Journal of Ornithology 124(1): 188-190. 2012 
Do Sora Nests Protect Red-winged Blackbirds from Marsh Wren Predation? 
Leanne A. Grieves 1 - 2 and Scott Forbes 1,3 
ABSTRACT.—We report an apparent protective 
effect of neighboring Sora {Porzana Carolina) nests on 
Red-winged Blackbird {Agelaius p/weniceus) nests in 
an experimental study. We suggest that quail eggs used 
in Sora nests acted as a supernormal stimulus drawing 
Marsh Wrens (C is tot horns palustris), the main predator 
in the system, from Red-winged Blackbird nests. 
Received 16 August 2011. Accepted 6 January 2012. 
Nest predation is the most significant source of 
mortality in many populations of nesting Red- 
winged Blackbirds (Agelaius phoeniceus) (Cacca- 
mise 1976, Martin 1988, Pieman et al. 1988. 
Westneat 1992; reviewed in Beletsky 1996, 
Weatherhead and Sommerer 2001). These black¬ 
birds often nest with a variety of other species in 
their wetland habitats and an obvious question is 
whether the presence of interspecific neighbors 
affects rates of predation by providing additional 
cues for predators. Our logic was that once 
predators are in the area, they are more likely to 
locate a blackbird nest. We tested this hypothesis 
by performing an artificial nest experiment that 
placed Sora ( Porzana Carolina) nests near black¬ 
bird nests at randomly assigned locations within 
study marshes. We chose Sora as the interspecific 
neighbor as their nests are common on our study 
site (near Winnipeg in southern Manitoba), and 
they do not cooperate with blackbirds in any 
obvious way to deter predators (e.g., mobbing). 
We knew from field work at this site since 1993 
(Forbes 2010) that nests of both blackbirds and 
Sora are often robbed by a variety of avian and 
mammalian predators. Our expectation was that if 
Sora nests provided a conspicuous cue for 
predators, nests of blackbirds with nearby nests 
of Sora would experience higher rates of preda¬ 
tion; we did not expect to find the opposite effect. 
Department of Biology, University of Winnipeg, 51' 
Portage Avenue. Winnipeg. MR R3B 2E9, Canada. 
.. De ( f a " me nl of Biology, McMaster University, 1280 
Main Street West, Hamilton. ON L8S 4KI. Canada 
Corresponding author; e-mail; s.forbes@uwinnipeg.Ci 
METHODS 
The study area consisted of three artificial 
wetlands in Rosser, Manitoba, Canada (49 57' N, 
97 19' W) which we refer to as the Northwest 
(NW), Northeast (NE). and Southeast (SE! 
marshes with nesting populations of Red-winged 
Blackbirds and Sora. The sites were excavated 
during construction of a highway overpass, 
creating relatively shallow and uniform Typlta 
spp. marshes used by a variety of nesting birds. 
We used old. abandoned or flooded Red-winged 
Blackbird nests as experimental nests. These nests 
were removed from their original locations and 
placed on bamboo tripods 0.5 m above the ground 
or water. We obtained experimental eggs for ihe 
blackbird nests from two sources: flooded nests 
and unhalehcd eggs collected from nests during 
routine surveys. We had limited availability ol 
blackbird eggs, and used Brown-headed Cowbird 
(Molothrus ater) eggs to complete clutches of 
three eggs (2 blackbird, I cowbird) in nine of 66 
experimental clutches. This mimics the natural 
situation as Brown-headed Cowbirds are Irequent 
brood parasites in this system (Glassey and Forbes 
2003, Royie et al. 2011). The modal clutch size in 
this population is four, but clutches of three are 
common in the study population (Forbes 2019). 
Thus, with a limited supply of Red-winged 
Blackbird eggs available, a dummy clutch o: 
three allowed us to increase our .sample size. Sora 
were used as an interspecific model and Japanese 
Quail (Cotumix japoniea) eggs purchased com 
mercially were used to mimic their eggs following 
Pieman et al. (1988). We used both nests that had 
been abandoned during flooding and artificially- 
constructed nests from mounds of Typha spp. tor 
the experimental Sora nests. Experimental Sora 
nests were placed at ground level and tilled with 
three quail eggs to remain consistent with the 
number of eggs used in the blackbird nests, 
although the average clutch size for Sora i> 
between eight and II (Bent 1926, Lowther 
1977, Kaufman 1989). 
The three marshes were subdivided into 20 X 
20-m quadrats (69 quadrats in NW. 59 in NE, and 
