Do Sora nests protect Red-winged Blackbirds from Marsh Wren predation?
Abstract: We report an apparent protective effect of neighboring Sora (Porzana carolina) nests on Red-winged Blackbird (Agelaius phoeniceus) nests in an experimental study. We suggest that quail eggs used in Sora nests acted as a supemormal stimulus drawing Marsh Wrens (Cistothorus palustris), the main predator in the system, from Red-winged Blackbird nests.
Article Type: Report
Subject: Red-winged blackbird (Research)
Nest building (Research)
Commensalism (Research)
Authors: Grieves, Leanne A.
Forbes, Scott
Pub Date: 03/01/2012
Publication: Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491
Issue: Date: March, 2012 Source Volume: 124 Source Issue: 1
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Canada Geographic Code: 1CANA Canada
Accession Number: 285207265
Full Text: Nest predation is the most significant source of mortality in many populations of nesting Red-winged Blackbirds (Agelaius phoeniceus) (Caccamise 1976, Martin 1988, Picman et al. 1988, Westneat 1992; reviewed in Beletsky 1996, Weatherhead and Sommerer 2001). These blackbirds 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 blackbird 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 predation; we did not expect to find the opposite effect.


The study area consisted of three artificial wetlands in Rosser, Manitoba, Canada (49[degrees] 57' N, 97[degrees] 19' W) which we refer to as the Northwest (NW), Northeast (NE), and Southeast (SE) marshes with nesting populations of Red-winged Blackbirds and Sofa. The sites were excavated during construction of a highway overpass, creating relatively shallow and uniform Typha 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 the blackbird nests from two sources: flooded nests and unhatched eggs collected from nests during routine surveys. We had limited availability of blackbird eggs, and used Brown-headed Cowbird (Molothrus ater) eggs to complete clutches of three eggs (2 blackbird, 1 cowbird) in nine of 66 experimental clutches. This mimics the natural situation as Brown-headed Cowbirds are frequent brood parasites in this system (Glassey and Forbes 2003, Royle et al. 2011). The modal clutch size in this population is four, but clutches of three are common in the study population (Forbes 2010). Thus, with a limited supply of Red-winged Blackbird eggs available, a dummy clutch of three allowed us to increase our sample size. Sofa were used as an interspecific model and Japanese Quail (Coturnix japonica) eggs purchased commercially were used to mimic their eggs following Picman et al. (1988). We used both nests that had been abandoned during flooding and artificially-constructed nests from mounds of Typha spp. for the experimental Sora nests. Experimental Sora nests were placed at ground level and filled with three quail eggs to remain consistent with the number of eggs used in the blackbird nests, although the average clutch size for Sora is between eight and 11 (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 44 in SE). The experimental nests were placed in the marshes at the center of randomly chosen quadrats between 7 and 30 June 2010, spanning the period of peak nesting activity and extending to the end of the nesting period for most blackbirds in the study population (Forbes 2010). Three eggs were placed in each nest and were removed 48 hrs later. Nests were checked daily for the occurrence of predation. Red-winged Blackbirds and Sora nested in all three marshes. Marsh Wrens (Cistothorus palustris) and their nests were observed in the NW and NE marshes but not in the SE marsh. Evidence of egg predation consistent with Marsh Wrens was found in NW and NE marshes but not in the SE marsh.


Between 28 and 67% of experimental Red-winged Blackbird nests were depredated, consistent with estimates of nest predation in active blackbird nests (reviewed by Beletsky 1996). Avian predators were responsible for most predation of experimental nests in all three study sites. Twenty-nine (44%) of the 66 experimental blackbird nests placed into the marshes were depredated: 28 of 29 depredated nests were due to avian predators, and at least 16 of the 28 nests (57%) were depredated by Marsh Wrens based on the presence of punctured eggs in the nest (Picman 1977). There was no obvious relationship between nest fate of experimental blackbird and Sora nests (Fisher exact test, two-tailed: P = 1.00).

We used two-tailed Fisher exact tests to examine the effects of interspecific neighbors (Sora) on nest predation of blackbirds. The results for individual marshes were not statistically significant (Table 1). However, when data for the two marshes (Northeast and Northwest) where nesting Marsh Wrens were present were combined, a significant effect was observed (Table 1; Fisher exact test, P = 0.031). Conversely, in the one marsh where nesting Marsh Wrens were absent, no effect of a Sora nest on the likelihood of predation on a blackbird nest was observed (Table 1; Fisher exact test, P = 1.00).

We found no obvious effect of the presence of an active Red-winged Blackbird nest in the same quadrat as a dummy nest on predation. There were active blackbird nests in most quadrats at the beginning of the season, but the attrition of active nests both due to predators and fledging meant that few of our dummy nests overlapped with active blackbird nests. Further, some dummy nests were placed in quadrats with active Marsh Wren nests with no obvious effect. We suspect this was not because the proximity to Marsh Wrens was unimportant, but rather because the spatial scale of quadrats and territories was dissimilar. We observed Marsh Wrens ranging widely in the NW and NE marshes, but they were not present in the SE marsh.

We examined the likelihood of predation on active Red-winged Blackbird nests in relation to distance from a Marsh Wren nest in the NE marsh where the location of all Marsh Wren nests was known. We found a positive but non-significant effect: 10 of 11 nests within 20 m of a Marsh Wren nest were depredated, compared to 23 of 34 nests >20 m from a Marsh Wren nest (Fisher exact test, P = 0.24).


Red-winged Blackbird nests in marshes with Marsh Wrens, placed near Sora nests, were less likely to be depredated than solitary blackbird nests. This result was unexpected. Rather than attracting additional predation as we initially expected, the presence of a Sora nest appeared to confer some level of protection to a blackbird nest. The effect was not due to group defense by blackbirds as only artificial nests were used and there were no adults associated with the experimental nests.

The behavior of Marsh Wrens seems most likely to account for the observed results. Marsh Wrens were the main predator in the NW and NE marshes but were absent from the SE marsh. We suggest that quail eggs in the experimental Sora nests acted as a supernormal stimulus, distracting the Marsh Wrens from the neighboring blackbird nests and explaining reduced predation when Sora nests were present in the marshes with breeding Marsh Wrens. Previous experimental work by Picman (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 this site. We may have overestimated the importance of crows and ravens (Corvus spp.) because they are observable. Mammalian predation tends to be intermittent but widespread. Common raccoons (Procyon lotor) in particular have decimated entire Red-winged Blackbird colonies in one or two nights.

Our experimental design contained one unavoidable 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.


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.

Received 16 August 2011. Accepted 6 January 2012.


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Leanne A. Grieves (1, 2) and Scott Forbes (1, 3)

(1) Department of Biology, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada.

(2) Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada.

(3) Corresponding author; e-mail:
TABLE 1. Number of experimental Red-winged
Blackbird nests that survived (no predation) or failed
(depredation) when an experimental Sora nest was present
or absent.

                          Fate of Red-winged
                          Blackbird nest

Location      Sora nest   Survived   Failed

Northwest      Present       8          1
  marsh (a)    Absent        9          9
Northeast      Present       4          3
  marsh (b)    Absent        3         11
Southeast      Present       4          1
  marsh (c)    Absent        9          4

(a) Fisher exact test P = 0.0912.

(b) Fisher exact test P = 0.1564.

(c) Fisher exact test P = 1.000.
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