Density, sex, and nest stage affect rail broadcast survey results

Abstract

The National Marsh Bird Monitoring Program is being initiated to document population trends with call-broadcast surveys. We examined effects of site-estimated vocalization density (birds calling per ha), breeding stage, call type, and sex differences on marsh bird response probability to improve marsh bird surveys. We conducted 335 broadcast surveys 10 m from 113 known Virginia rail (Rallus limicola) and sora (Porzana carolina) nests in 10 wetlands over 2 breeding seasons (2010–2011) to determine important variables for predicting response probability. The odds of both species responding to broadcast increased as vocalization density increased. Recent nest predation decreased the odds of both species responding and increasing nest age additionally decreased sora response. Both species responded similarly to broadcast during egg laying, incubation, and hatching. In addition, we present methods to confirm nesting stage during broadcast surveys using vocalization type. Both species possess 1) calls that we detected exclusively during the pre-nesting phase but not prior to replacement clutches, which may indicate unpaired birds; and 2) calls detected exclusively during the late incubation through brooding stages, which may be useful for inferring nest success from broadcast surveys. 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 marsh bird population estimates account for detectability biases in density, stage, and sex and use call type to infer nonbreeding status and nest fate. © 2014 The Wildlife Society.