Yep, voles for sure. You can get rid of them by pouring stale human urine mized with glycerine and cut hair from any barbershop in a few of them mounds. Open up a few of them mounds and pour in the mixture then recover. The vole (moles too) will get that stuff on them and they will vacate the area. Or, invite a cat into your back yard for a snack.
Voles are migrating North this time of the year, by this time next week they should have worked there way at least 2 zip codes north of your place.
No real need for concern yet as they are not seeking Summer residency yet.
In about 2 weeks you should start monitering your yard from 1am to 3:45am....this is the peak time for the male vole to perform the mateing dance ritual. If he finds a suitable partner, they may choose residency in your yard.