My guess would be that they are the field ants. They are probably eating the 'honeydew' produced by aphids. Check to see if there are aphids nearby and use the garden hose to knock the aphids off any plants in your garden. The ants don't pose an
immediate threat to the Forsythia but as
@Moonhowl said, over time the cavities/holes in the soil will have a negative impact of the shrub. I would send the ants an invitation to live elsewhere.
