Date of Award

5-2012

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Brian J. Olsen

Second Committee Member

William E. Glanz

Third Committee Member

Cynthia S. Loftin

Abstract

Virginia rail (Rallus limicola) and sora (Porzana Carolina) (hereafter "rails") populations are thought to be declining and we examined habitat variables potentially affecting productivity of 75 Virginia rail and 22 sora nests in Maine during 2010-2011. We identified the mechanisms for nest failure (nest scale) and characteristics of wetlands (wetland scale) that predict reproductive success with logistic-exposure models and an information-theoretic approach. Our results suggest that water-level variation is a positive predictor of rail nest success and rail density in Maine wetlands. Hydrologic variation creates low-sloped wetland edges where emergent plants thrive. Wetlands with large areas of shallow depths and abundant emergent vegetation improve habitat condition for nesting rails. The presence of a waterfowl impoundment did not affect water-level variability, nest survival, or clutch size in our models.

The National Marsh Bird Monitoring Program is being initiated to document marsh bird population trends with call-broadcast surveys. We examined effects of site-estimated rail density, breeding stage, call type, and sex differences on marsh-bird response probability to improve marsh bird surveys. We conducted 335 surveys on 113 rail nests in ten wetlands during 2010 and 2011. We determined important variables for predicting response probability of Virginia rails and soras to broadcast surveys 10 m from known nests with generalized linear models. The odds of both rail species responding to broadcast increased as rail density increased. Nest age and recent nest failure owing to predation significantly decreased the odds of sora response. For Virginia rails, the post-predation stage decreased the odds of response to broadcast calls and rail response was marginally less, although not significantly so during the post-hatch stage. Rails responded similarly to broadcast during egg laying, incubation, and hatching. Virginia rails and soras both used "peep" call late in late nesting/post-hatching stages, and this call could be used during surveys later in the breeding season as an index of nest success. The "kadic-kadic" (Virginia rail) and the "per-weep" (sora) calls are used primarily during the pre-nesting phase (and not prior to replacement clutches) and may be indicators of unpaired birds. These vocalization differences could be used to strengthen population estimates by differentiating between presence and active breeding throughout the season. Spectrogram analysis of recorded vocalizations showed that male Virginia rails responded to broadcast calls with louder (possibly related to approach distance), longer, and faster calls (hence higher detectability) than females. We recommend that large-scale marsh-bird population trend estimates take density and sex detectability issues into account and recognize that wetlands with low response rates may underestimate population estimates more than those with high response rates owing to lower bird densities and differences in sex ratios and breeding stage.

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