Secretive Marshbird Response to Invasive Wetland Plant Management in the Prairie Pothole Region of Minnesota

Abstract

Marshbirds are difficult to survey due their secretive nature and association with dense wetland vegetation. Recently developed standardized survey protocols are used to monitor patterns of abundance, primarily at large spatial scales, but also can be used to assess marshbird response to management. We estimated abundances of 5 species of marshbirds (American bittern [Botaurus lentiginosus], least bittern [Ixobrychus exilis], pied-billed grebe [Podilymbus podiceps], sora [Porzana carolina], and Virginia rail [Rallus limicola]) in relation to vegetation management techniques of Prairie Pothole wetlands. In northwestern Minnesota, management in autumn 2105 included herbicide application to wide-spread cattail (Typha spp.) mats with the goal to break up dense vegetation patches and restore wetlands to hemi-marsh conditions. In a before-after, control-impact study design we conducted standardized call-broadcast surveys for marshbirds during breeding seasons 2015 – 2018. We observed that American bittern, pied-billed grebe, sora, and Virginia rail abundances initially decreased, and then increased at 2nd and 3rd seasons post-treatment at sites where herbicides had been applied. In west-central Minnesota, long-term vegetation management included varying frequencies of multiple control methods. Using a habitat-informed detection probability we transformed bird counts to densities to compare abundances of marshbirds across survey locations surrounded by variable amounts of suitable habitat. We compared abundances of marshbirds among categories of wetlands with management histories of low frequency of prescribed fire, high frequency of prescribed fire, and high frequency of prescribed fire and grazing. Fire and grazing as applied in the system we studied did not appear to influence Prairie Pothole Region wetland characteristics enough to result in changes in marshbird abundance, but abundance of marshbirds was related to characteristics of individual wetlands that did not appear to respond to fire and grazing. Pied-billed grebe abundance was positively associated with higher areas of open water, soras were more abundant in wetlands with high ratios of open water to emergent vegetation, and Virginia rails were more abundant in wetlands with scrub-shrub wetland cover types.