Grazing Impact on Brood Parasitism

Abstract

Anthropogenic land use changes can have tremendous direct and indirect effects on biota. A prevalent land use change in Texas is conversion of land to grazing. Grazing facilitates foraging opportunities for brown-headed cowbirds (Molothrus ater), a brood parasite. Cowbirds can reduce productivity of their hosts, causing some host species to decline in abundance. Thus, grazing indirectly influences productivity of some songbirds. The black-capped vireo (Vireo atricapilla) is an endangered songbird with most of its breeding range occurring in areas of livestock and other ungulate grazing. A contributing factor to its endangered status is brown-headed cowbird parasitism. I monitored 382 black-capped vireo nests from 2012-2013 in Real, Kerr, Bandera and Edwards counties, Texas and described parasitism. I investigated how grazing system related to parasitism; I defined grazed systems by the presence of enclosed ungulates and wild ungulate only systems by the absence of enclosed ungulates. I also examined how grazing intensity (represented by distance from nest to nearest ungulate water source), nest concealment, and grazing in the landscape (represented by proportion of grassland within 3 km of a nest), related to parasitism. Overall parasitism was 30% (n = 166) in 2012 and 31% (n = 216) in 2013, moderate compared to other research, but above a proposed threshold of concern. Grazing system and grazing in the landscape interacted in predicting probability of parasitism. Grazing in the landscape was not important in predicting brood parasitism in wild ungulate only grazing systems, but was important in predicting brood parasitism in grazed systems. In grazed systems, there was low probability of brood parasitism with low grazing in the landscape and high probability of parasitism with high grazing in the landscape. Nest concealment and grazing intensity were not good predictors of brood parasitism. Land managers could use this information to prioritize cowbird management or preservation efforts.