Date of Award

January 2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Brett J. Goodwin

Abstract

Grassland birds have been under threat since tallgrass prairie was first used for agricultural cultivation and grassland acreage continues to be converted for row-crop agriculture. The loss of suitable native habitat for grassland birds makes effective management of the little grassland that remains that much more important. While it is widely recognized that grassland birds use vegetation cues for habitat settlement during the breeding season, little study has been done on how vegetation structure influences grassland bird abundances in the northern tallgrass prairie, particularly vegetation structure as it is shaped by different forms of management. Therefore this study aims to determine the most important forms of vegetation structure to breeding grassland birds, and how management actions influence that structure. The effects of grazing, haying, and idle management were examined across 17 sites in 2014, and 16 sites in 2015 in Grand Forks County, ND. I surveyed the bird community during the peak of the grassland bird breeding season. Vegetation structure was measured in early July each year. Measures of vegetation highly overlapped between hayed, grazed, and idle sites, though there was significantly more bare ground and vegetation was less dense in grazed versus hayed and idle sites. I analyzed the six most abundant grassland bird species using N-mixture modeling in package ‘unmarked’ in program R. For the majority of species, bird abundance was most related to vegetation density and showed no significant effect of percentage of grass, bare ground, and number of woody plants. However, Clay-colored Sparrow was positively associated with number of woody plants. During the same two seasons the bird community and vegetation structure were monitored at Oakville Prairie Field Station in Grand Forks County, ND. The site had prescribed burning applied during fall 2014, after lacking management for more than 30 years. There was little change in most measures of vegetation after burning, though litter depth and live vegetation height both declined in burned units relative to unburned. Five grassland bird species were examined for vegetation associations using N-mixture modeling and a multi-step modeling procedure. Three of the five species had a significant change in abundance after burning. Bobolink and Sedge Wren abundance declined in burned units, while Western Meadowlark abundance increased. The vegetation measure that best explained Bobolink and Sedge Wren abundance changed from vegetation density in the pre-burn year to litter depth in the post-burn year, suggesting that litter depth may be an important indicator of disturbance for these two species. Overall, these results suggest that a variety of management (e.g. haying, grazing, burning, and idling) could be more beneficial to multiple bird species, by providing a range of vegetation structure from sparse to dense vegetation. Additionally prescribed fire may not have a strong effect on vegetation structure in the first year of burning after a long idle period, though birds may still respond to these small changes in vegetation.

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