Cowbirds follow groups of cows, or other grazing animals, eating the insects kicked up by the walking animals. The herbivores are not bothered by the cowbirds. This relationship is an example of:

Commensalism

Explanation:

Commensalism can be defined as a symbiotic relationship between two different species which are living in close association with each other. In commensalism one of the species receives the benefit other receives no harm nor benefit.

According to the given situation, the cowbirds feed on the insects on the body of walking grazing animals and the grazing herbivore animals are not affected by this process so this is an example of commensalism.