On my mountain bike along the new trails in the arboretum. I was following the loop on the eastern side of River Road at the southern end of the arboretum. All the trees are still very small there so I am not sure where this bird came from. As I was coming close to River Road, having been travelling south along the loop, I heard a whoosh and one beak snap, but not too close. The magpie only did it once and did not appear to pursue me.