Grassland bird community response to vegetative diversity in restorations in northwestern Iowa
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Grassland bird populations have experienced significant population declines due to the loss of grassland habitat. In an attempt to bolster grassland bird populations, grassland restorations have been implemented throughout the Midwest to increase the total amount of habitat available on the landscape. Understanding the potential of these restorations to be used by birds is imperative for the future conservation of grassland birds. We compared grassland bird use among restored grasslands that were established with seed mixes of varying plant species diversity. We also evaluated shifts in bird community structure over time among a diversity of restorations. We selected 5 grassland planting types commonly used by managers in the Spring Run Wetland Complex in northwestern Iowa: 1) cool-season, 2) warm-season established between 2005-2007, 3) warm-season established before 2005, 4) medium-diversity, and 5) high-diversity. We performed line-transect surveys in 2015 and 2016 to quantify grassland bird use among the planting types. We also compared bird densities calculated from our study to those calculated from an identical study conducted in 2007-2009 to look for differences in bird densities over time. The most common bird species we observed were common yellowthroat (Geothlypis trichas), bobolink (Dolichonyx oryzivorus), red-winged blackbird (Agelaius phoeniceus), and sedge wren (Cistothorus platensis). Grassland bird community structure differed among the planting types. Cool-season and diverse plantings had dissimilar bird communities while warm-season bird communities were similar to all other planting types. The environmental variables most correlated with differences in bird community structure among planting types were the percent cover of exotic cool-season grasses and Shannon’s Diversity Index of vegetation cover classes, suggesting that vegetation composition and structure play a role in differentiating grassland bird communities. Grassland bird community structure shifted over time within all planting types and the depth of the litter layer was the environmental variable most correlated with these shifts. The buildup of litter may be partially responsible for deviations in bird community structure through time in grassland restorations. This study demonstrates that grassland restorations need to be established in varying years using a diversity of planting mixes to conserve grassland bird populations and reiterates the complexity of grassland ecosystems.