Waterbird Response to Conservation Grazing in Western Minnesota Tallgrass Prairies

Abstract

Conservation grazing is becoming a popular management tool in prairie systems that evolved with grazing ungulates. Whereas it has been found to be an effective tool for maintaining upland prairie cover, its impacts on wetlands and the species that rely upon wetlands is not well-studied. Our objective was to investigate the impacts of conservation grazing on 2 groups of waterbirds: ground-nesting waterfowl and secretive marsh birds. As an economically important group of birds, much of the public land in Minnesota is managed for the production of waterfowl. Meanwhile, secretive marsh birds are believed to be an indicator species of wetland health, and their response to grazing could tell us how the whole wetland system is responding to management. In 2017 and 2018, we conducted waterfowl pair and brood counts and call-response surveys for secretive marsh birds in the tallgrass prairie of western Minnesota. We estimated abundance of blue-winged teal (Spatula discors) and mallard (Anas platyrhynchos) pairs and broods in response to grazing frequency and years since a site was last grazed. We used generalized linear models to estimated pair response and generalized linear mixed effects models to estimate brood response. Grazing had no effect on blue-winged teal pair abundance. Years since grazed had a weak, positive effect on mallard pair abundance, though increasing grazing frequency had no significant effect. Insufficient observations of mallard broods prohibited analysis of brood abundance, but we combined all dabbler broods together to estimate overall brood abundance in relation to grazing. Neither grazing variables influenced blue-winged teal or combined dabbler brood abundance. Using robust design occupancy models in Program MARK, we estimated marsh bird response to the same two grazing variables. American bittern (Botaurus lentiginosus), sora (Porzana carolina), Virginia rail (Rallus limicola) all responded positively to at least one grazing variable, whereas pied-billed grebe showed no response. Our study suggests that the use of frequent, short-duration grazing to manage upland cover neutrally or positively impacts occupancy of secretive marsh birds and abundance of ground-nesting waterfowl pairs and broods.