The influence of cattle grazing intensity on grassland bird occupancy in southeast North Dakota

Background/Question/Methods

Grassland songbird populations are in decline across the tallgrass prairie. Loss of habitat is considered the primary cause, and much of the remaining grassland is in private ownership, where cattle grazing is common. The objective of this study was to evaluate the influence of grazing intensity on songbird occupancy of grasslands in a cattle-dominated landscape. The study took place in southeastern North Dakota on The Nature Conservancy’s (TNC) Brown Ranch preserve and the U. S. Forest Service’s Sheyenne National Grasslands. TNC recently reduced the grazing intensity on Brown Ranch, allowing for a comparison with the higher grazing intensity experienced on the National Grasslands. We used point counts to survey songbirds on six TNC pastures and five Forest Service pastures, and we used modified belt transects to measure vegetation structure. We used the program PRESENCE to run occupancy models for four species of grassland songbird, grasshopper sparrow (Ammodramus savannarum), bobolink (Dolichonyx oryzivorus), marbled godwit (Limosa fedoa) and upland sandpiper (Bartramia longicauda). We evaluated the influence of grazing intensity, measured in animal months per acre (AM), and structural vegetation characteristics on occupancy and detection probabilities.

Results/Conclusions

Preliminary results from the first two years of the study varied by species. For all species, detection probability was influenced by vegetation height and varied by visit throughout the season. For all species but the bobolink, detection probability was lower at sites with taller vegetation. Models for the upland sandpiper, marbled godwit and bobolink showed some support for the influence of grazing intensity on site occupancy. Upland sandpiper and marbled godwit occupancy increased with grazing intensity, whereas bobolink occupancy decreased as grazing intensity increased. The best-supported model for the grasshopper sparrow included a positive relationship with grazing intensity as well as vegetation density. With the exception of the grasshopper sparrow, model results were generally consistent with habitat preferences for these species, upland sandpipers and marbled godwits prefer the shorter stature vegetation created by higher grazing intensities while bobolinks prefer the denser vegetation structure created by lower intensity grazing.