Songbird Response To Mixed Ungulate Herd Reduction Through Changes In Vegetation At Sullys Hill National Game Preserve
Date of Award
Master of Science (MS)
Susan N. Ellis-Felege
Big game management traditionally focused on single species management with less emphasis, until recently, on community interactions. Sully's Hill National Game Preserve (SHNGP) is a 678 hectare preserve where extirpated Plains bison (Bison bison) were reintroduced along with Rocky Mountain elk (Cervus elaphus) and white-tailed deer (Odocoileus virginianus). These large ecosystem engineers have been managed at varying densities within a fenced area of deciduous woods, wetlands, and native prairie since 1918. Grazing, browsing, and rubbing by ungulates, along with additional refuge management prescriptions (e.g. prescribed fire), shape the understory and midstory and in turn determine which birds will nest and forage in the area. The 2008 SHNGP Comprehensive Conservation Plan recommended a 50% reduction in herd sizes based on vegetation surveys which indicated little forest regeneration. The suspected cause was overbrowsing by ungulates. An ungulate density of 37 ungulates/km2 was reduced to 19 animals/km2. This density has been annually maintained since 2008. The primary objective of this study was to determine if large ungulate reduction has improved forest regeneration with an ensuing increase in bird abundance and diversity. Vegetation and avian surveys were conducted pre-treatment (before three herd reduction) and repeated post-treatment in 2012-2013.
Repeat forest surveys revealed a return of regeneration when analyzed using negative binomial regression, but regeneration is lower in grazed areas as compared to ungrazed. Species accumulation curves revealed that species richness is not returning with regeneration in grazed habitat for both upland and bottomland habitat types.
Repeat bird surveys of four target species, ovenbirds (Seiurus aurocapilla), yellow warblers (Dendroica petechia), American redstart (Setophaga ruticilla), and red-eyed vireos (Vireo olivaceus) were analyzed for changes in abundance using Royle repeat measures and changes in species diversity using species accumulation curves. Changes in target bird density before and after the herd reduction treatment revealed lower abundance and downward trends in ovenbird. No significant changes in abundance were observed in yellow warblers, American redstarts or red-eyed vireos, but yellow warblers and redstarts had higher abundance in exclosure (100% ungulate reduction) than grazed (50% ungulate reduction) and were similar to ungrazed abundance.
Bird abundance relative to post-herd reduction vegetation was analyzed to determine if there were specific associations with species or guilds which may be impacted by herd reduction. We found that ovenbird abundance is higher with midstory basswood saplings and native sedges and lower with native forbs. Yellow warblers had higher abundance with native sedges and lower with more midstory cover. American redstarts had higher abundance with deeper litter and lower abundance with midstory ash saplings. We found no specific vegetation associations for red-eyed vireos. Some of the associations were weakly significant and all surveys conducted at one site so there was no replication for increased inference. We concluded that more time, more browser reduction, and additional disturbance techniques may be needed to improve forest vegetation recovery and associated increases in bird abundance.
Walters, Bethany J., "Songbird Response To Mixed Ungulate Herd Reduction Through Changes In Vegetation At Sullys Hill National Game Preserve" (2014). Theses and Dissertations. 1727.