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

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. 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.沼鸟类（Marshbirds）生性隐秘且常依附于茂密的湿地植被，因此调查难度极高。近年开发的标准化调查规程（standardized survey protocols）主要用于大空间尺度下的种群丰度格局监测，同时也可用于评估沼鸟类对生境管理措施的响应。本研究针对草原坑洼湿地（Prairie Pothole wetlands）的植被管理措施，对5种沼鸟类的种群丰度进行了估算，分别为美洲麻鳽（American bittern *Botaurus lentiginosus*）、小苇鳽（least bittern *Ixobrychus exilis*）、斑嘴巨䴙䴘（pied-billed grebe *Podilymbus podiceps*）、索拉秧鸡（sora *Porzana carolina*）以及弗吉尼亚秧鸡（Virginia rail *Rallus limicola*）。在明尼苏达州西北部，2015年秋季的管理措施包括对宽叶香蒲（Typha spp.）群落施用除草剂，旨在破碎化密集植被斑块，将湿地恢复为半沼泽（hemi-marsh）生境。本研究采用前后对照-影响控制试验设计，于2015至2018年的繁殖季开展了沼鸟类的标准化鸣声回放调查（call-broadcast surveys）。调查结果显示，在施用除草剂的样地中，美洲麻鳽、斑嘴巨䴙䴘、索拉秧鸡以及弗吉尼亚秧鸡的种群丰度在处理初期有所下降，但在处理后的第二、第三个繁殖季出现回升。在明尼苏达州中西部地区，长期植被管理采用了多种调控手段，并设置了不同的实施频率。本研究针对三类具有不同管理历史的湿地开展沼鸟类丰度对比：分别为低频率计划性火烧（prescribed fire）、高频率计划性火烧，以及高频率计划性火烧结合放牧的湿地类型。本研究区域内所采用的火烧与放牧措施，似乎并未对草原坑洼湿地的生境特征造成足够显著的影响，进而未导致沼鸟类种群丰度发生变化；但沼鸟类的种群丰度仍与单个湿地的生境特征密切相关，而这些生境特征并未因火烧与放牧措施发生明显改变。斑嘴巨䴙䴘的种群丰度与开阔水域面积呈正相关；索拉秧鸡在开阔水域与挺水植被比值较高的湿地中更为常见；而弗吉尼亚秧鸡则更多分布于具有灌丛湿地覆被类型的湿地中。