It can sometimes end up in a weird kind of stand off, where they can see your path is through their nest or perch but they don't really know what your intent is so you just end up staring at each other. The most aggressive I came across was a big seagull, which I'm pretty sure was protecting it's eggs since it clearly wasn't going to move, thankfully that line was easy enough that I could just figure out an alternate way around. Climbing you also sometimes get to see some of the horrific remains from the predators - One time towards the end of covid lockdowns some peregrines had reclaimed a pinnacle due to the unusual absence of climbers. When I got to the top it looked like a sacrificial temple, filled with the carcases of all the other bird species, not to mention the bees nest en route, that one was a mini adventure.