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Article Excerpt
The Sedge Wren (Cistothorus platensis) is a rare breeding bird in eastern Canada and adjacent United States, occurring in small, scattered populations and inhabiting wet meadows and grass-dominated wetlands (Herkert et al. 2001). It is currently listed as Endangered by the States of Maine, New Hampshire, Vermont, and New York, and is considered Imperiled and Critically Imperiled in the Provinces of Quebec and New Brunswick, respectively (NatureServe 2007). Sedge Wrens typically exhibit low site tenacity (Burns 1982), probably because they nest in habitats that are susceptible to drying or flooding, vegetative succession, and disturbance caused by agriculture (Herkert et al. 2001). At least 91 sites have been occupied by Sedge Wrens since 1959 in Quebec, where they reach their northeast limit in terms of breeding distribution, of which only 12% have been occupied >4 years (Regroupement QurbecOiseaux, unpubl. data). Quebec has only one area, Lake Saint-Francois National Wildlife Area (NWA) and vicinity, where Sedge Wrens have been reported consistently in large sedge (Carex spp.)-dominated fens (low-lying marshes or minerotrophic peatlands) since 1970 (Fragnier and Robert 1996, Robert 2002).
The Lake Saint-Francois NWA was established in 1978 by the Canadian Wildlife Service to protect exceptional wetland habitats for waterfowl and several rare species. Concerns have been expressed regarding the invasion of fens in the NWA by speckled alder (Alnus incana ssp. rugosa) (Jean and Bouchard 1991, Gratton 1996). The reason for this invasion remains unclear, but may be related to a decrease in fire frequency (Jean and Bouchard 1991) and to regulation of water levels of Lake Saint-Francois (Brisson et al. 2006). Alder encroachment into fens of Lake Saint-Francois NWA could compromise the integrity and biodiversity of these exceptional wetlands (Jean and Bouchard 1991, Brisson et al. 2006), and may constitute a threat to Yellow Rails (Coturnicops noveboracensis), a species at risk that breeds in the area (Robert et al. 2000).
Methods for controlling alder invasion in Lake Saint-Francois NWA were tested by the Canadian Wildlife Service, some of which may be readily applicable (Brisson et al. 2006). Information on habitat characteristics associated with Sedge Wrens breeding in the NWA are needed to fully consider the impacts of alder control on this species at risk. This is especially true for shrub cover, which is known to influence habitat selection by Sedge Wrens at the landscape level (Bakker et al. 2002, Grant et al. 2004) but which has not been described in detail at the breeding territory level. Sedge Wrens are known to breed in dense, tall graminoid habitats with scattered shrubs (Niemi and Hanowski 1984, Niemi 1985, Grant et al. 2004, NatureServe 2007), but few details are available on local shrub attributes (e.g., height, density, patchiness) and thresholds. Shrub removal in Minnesota wetland brush ecosystems resulted in more Sedge Wrens (Hanowski et al. 1999), whereas Baldwin et al. (2007) reported the occurrence of Sedge Wrens wintering in Texas to be independent of shrub densities.
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Our objectives were to: (1) describe habitat variables associated with Sedge Wren breeding territories in Lake Saint-Francois NWA, (2) compare local habitat variables of territories occupied and unoccupied by Sedge Wrens, and (3) estimate Sedge Wren breeding density.
METHODS
Study Area.--The Lake Saint-Francois NWA is a protected area extending over 1,445 ha on the south shore of Lake Saint-Francois, an open-water broadening of the St. Lawrence River in the southwest corner of Quebec Province in close proximity to New York State (Fig. 1). The well decomposed organic material averages 1.5 m thick (max = 4 m) and rests on top of a clay deposit from the last postglacial era. Numerous rare plant species are found in this fen (Auclair et al. 1973, Brisson et al. 2006).
Sedge Wren Surveys.--Territory mapping surveys (Bibby et al. 2000) were conducted by two experienced observers from 25 May to 28 June 2005 along three fixed-width transects (total length = 4,925 m) in fens where Sedge Wrens had been located regularly (Regroupement QuebecOiseaux, unpubl, data). Transect width was 300 m (150 m each side) (Fig. 1). Each transect was surveyed on nine or 10 occasions, between sunrise and 0900 hrs (EDST) on days when there was no rain and winds were [less than or equal to] 20 km/hr. Each survey was completed by an observer walking slowly ([less than or equal to] 2 km/hr) along the transect and stopping every 200 m for a 1-min listening period, and approaching each Sedge Wren heard or seen within the transect. Once located, a wren was observed passively for 5-10 min to note its movements and to register (afterwards) as many locations as possible using a Global Positioning System (GPS) (Garmin GPS 76[TM], Garmin Ltd., Olathe, KS, USA), before proceeding along the transect. Observers alternated transects from day to day, as well as walking direction while surveying transects.
We mapped all wren locations (n = 707) using MapInfo 7.0 GIS software and assumed that clusters of locations from separate visits originated from individual wrens (Bibby et al. 2000). Sedge Wrens occupy small territories (Burns 1982) and we identified 48 "occupied" stations by placing a 20-m radius circle (0.13 ha) on the centroid of each cluster of locations where wrens had been detected in [greater than or equal to] 2 surveys (85.4% of stations had wrens detected in [greater than or equal to] 3 surveys). We identified 101 "unoccupied" stations by outlining the maximum number of non-overlapping 45-m radius plots (i.e., 20-m radius territories...
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