Birds are funny creatures. I was at Kingdom Trails yesterday and was on one of the newer trails - Good 'n You - and as I was near the top I came across a rather aggressive bird (a grouse, maybe?) defending its nest. It never actually attacked me, but it made it abundantly clear I wasn't welcome as I rode past.
One of the parking lots at the office park at my company's NJ office in Somerset has a hawk nesting in it. Being a protected bird in NJ, it basically would take an act of god to get the nest moved, so the management has part of the lot taped off so casual walkers and what not don't get attacked by defending hawks.