Sparrows have the reputation for that Posy. Might well be them as mine primroses are not attacked and I never see a sparrow
Mine are all nibbled as well, even the polyanthus coloured ones, but we never see a sparrow here so it must be a favourite of another bird as well.
Sparrows definately do the damge in our garden, They begin with yellow flowers and then move on to pink as a second choice but they also like blue
We have a bird feeder and enjoy seeing the birds so a downside to that.