No general answer to your question. Some bird species are very territorial. They will attack any other bird in their "space". Including yours if it's there. Raptor's are not only territorial, but they are predatory. They will attack both defensively AND offensively. During breeding season, males of many species become extremely aggressive. During brooding season, certain species also can become very protective in an aggressive way. So, there you have it. It all depends on which bird you might come across. I've been approached aggressively by many species of hawks and falcons. Never hit yet, but close. My action now is simple. If I see any raptor in the air near me, I land. They are just too unpredictable to mess with.