Effects of cattle grazing and food availability on avian reproduction in Conservation Reserve Program grasslands

Abstract

Grassland birds have benefitted from the Conservation Reserve Program (CRP), which creates grassland habitat through restoration of marginal cropland. Grazing by domestic cattle (Bos taurus), which is currently restricted in CRP, might improve habitat structure for some bird species. However, changes in habitat structure, and the presence of cattle, might hinder nest concealment from predators, attract brood-parasitic brown headed cowbirds (Molothrus ater), and alter invertebrate food availability for birds. Higher abundance of arthropods, which constitute the diet for most songbird nestlings in grasslands, might allow greater parental provisioning and, consequently, improved nestling condition and survival. During the summers of 2017–2019, I investigated the effects of experimental grazing on nest survival and brood parasitism (where appropriate) of five bird species that utilize grassland habitat in Kansas. Additionally, I examined nestling condition of dickcissels (Spiza americana) in relation to abundance of arthropod prey across sites. Experimental grazing by cattle, which occurred during the first two years of study, had inconsistent effects on nest success and parasitism by cowbirds among the bird species analyzed. Negative consequences of grazing included reduced nest success and increased brood parasitism in dickcissels, as well as reduced nest success in meadowlarks (Sturnella spp.), but some effects varied over years or were

conditional upon conservation practice. Management had no effect on arthropod biomass, and nestling condition showed no clear relationships with field-level variation in food availability. Instead, nestlings in larger broods were generally in poorer condition than those in smaller broods. Thus, parents might be more limited in their capacity to feed nestlings in large broods than limited by the availability of food within CRP fields. Negative consequences of grazing tended to be weak, so short-term grazing, as might be implemented for mid-contract management of CRP fields, might not have long-lasting effects on grassland bird reproduction