The sound of a mower/conditioner is a call to the feast to ravens, seagulls, and even foxes. As the mower moves from the outside toward the center the voles scamper for cover. The birds follow and pounce on anything they see moving. They are mouth sized morsels that go down in one gulp. Cutting this field I saw a large bald eagle swoop down and grab a victim without even touching the ground.
In case you are worried about the survival of the voles- see what Wikipedia says, "Voles are small rodents that grow to 3–9 in (7.6–22.9 cm), depending on the species. They can have five to 10 litters per year. Gestation lasts for three weeks and the young voles reach sexual maturity in a month. As a result of this biological exponential growth, vole populations can grow very large within a very short time. Since litters average five to 10 young, a mating pair can birth a hundred more voles in a year".