Monitoring Least Bitterns (Ixobrychis exilis) in Vermont: Detection probability and occupancy modeling

Northeastern Naturalist
By: , and 



Ixobrychus exillis (Least Bittern) is listed as a species of high concern in the North American Waterbird Conservation Plan and is a US Fish and Wildlife Service migratory bird species of conservation concern in the Northeast. Little is known about the population of Least Bitterns in the Northeast because of their low population density, tendency to nest in dense wetland vegetation, and secretive behavior. Urban and agricultural development is expected to encroach on and degrade suitable wetland habitat; however, we cannot predict the effects on Least Bittern populations without more accurate information on their abundance and distribution. We conducted surveys of wetlands in Vermont to assess the efficacy of a monitoring protocol and to establish baseline Least Bittern abundance and distribution data at a sample of 29 wetland sites. Surveys yielded detections of 31 individuals at 15 of 29 sites across 3 biophysical regions and at 5 sites where occupancy had not been previously reported. Probability of occupancy was positively related to wetland size and number of patches, though the relationships were not strong enough to conclude if these were true determinants of occupancy. Call—response broadcast surveys yielded 30 detections, while passive surveys yielded 13. Call—response broadcasts (P = 0.897) increased the rate of detection by 55% compared to passive surveys (P = 0.577). Our results suggest that call—response broadcast surveys are an effective means of assessing Least Bittern occupancy and may reduce bias in long-term monitoring programs.

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Publication type Article
Publication Subtype Journal Article
Title Monitoring Least Bitterns (Ixobrychis exilis) in Vermont: Detection probability and occupancy modeling
Series title Northeastern Naturalist
DOI 10.1656/045.025.0104
Volume 25
Issue 1
Year Published 2018
Language English
Publisher Eagle Hill Institute
Contributing office(s) Coop Res Unit Leetown
Description 16 p.
First page 56
Last page 71
Country United States
State Vermont
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