Effects of habitat heterogeneity on grassland songbirds in an experimental landscape
Landscape heterogeneity promotes higher biodiversity in natural ecosystems. In North America, tallgrass prairie was historically maintained as a mosaic of heterogeneous habitats by an interaction between fire and selective grazing by large herbivores. In recent decades, intensification of agricultural practices in managed grasslands has led to more homogeneous landscapes, which has been linked to widespread declines in songbird populations both in North America and Europe. Patch-burn grazing management increases heterogeneity in vegetative structure, because only a third of the pasture is burned each year in a three-year rotational scheme. Movements of grazing cattle are not restricted by cross-fences in the pasture, increasing heterogeneity in vegetative structure due to the cattle’s preference for recently burned areas. Patch-burn grazing management has been found to increase bird diversity, and has species-specific effects on bird abundance. However, the consequences on bird reproductive output are still unknown. In this study, we assessed how patch-burn grazing affected the reproductive output of three species of declining grassland songbirds in tallgrass prairie.
Our study was conducted at Konza Prairie Biological Station, a tallgrass prairie reserve in northeast Kansas. Between 2011 and 2014, we monitored nests of Dickcissels (Spiza americana), Eastern Meadowlarks (Sturnella magna) and Grasshopper Sparrows (Ammodramus savannarum). We compared nest survival and brood parasitism rates by Brown-headed Cowbirds (Molothrus ater) among three pastures managed with a patch-burn grazing regime. A fourth pasture with annual burns and season-long stocking of cattle served as a negative control, and a fifth pasture with annual burns, but no grazing, served as a positive control.
We found that nest survival of Dickcissels was highest on the patch-burn grazing treatment, but found no treatment effect for the other two species. Rates of cowbird parasitism were high on all treatments, but highest on pastures that were grazed by cattle. Negative effects on the reproductive output of all species was substantial, as parasitized nests faced a 50% reduction in the number of host eggs, and gained on average two to three cowbird eggs in return. Bird abundance of all three species was similar among treatments and did not reflect the effects of different management regimes on the reproductive output, and therefore the viability of songbird populations. Using bird abundance alone might therefore not be a sufficient tool to determine the effects of grassland management regimes on wildlife.