I WAS out jogging the other morning and saw a mockingbird chase a hawk out of his territory. Wow. You see mockingbirds chase crows twice their size all the time, but watching one chase a bird of prey that must have three or four times larger that he was, was shocking. Maybe hawks that size don't eat mockingbirds; I've seen them with mice occasionally, but mice are much smaller. Still, it's the equivalent of seeing a human chase a bear away. The human may not be the bear's preferred prey, but the bear could do a lot of damage if he wanted to.
'Wanting to' is the key, isn't it? All the mockingbird had on his side was raw courage and determination. Why did the hawk flee? Maybe fighting the mockingbird was more trouble than it was worth, the hawk knew there was easier prey elsewhere. Maybe the hawk knew he was in the 'wrong', he had strayed into the mockingbird's nesting territory.
Psychological elements play a large role in animals' defense of their territory. Stickleback fish will mark out territories underwater. One invades the another's territory, the other pushes it back out, and keeps on going until it is within the first fish's territory. Then the aggressor starts to lose its courage, and the first fish turns around, and chases its pursuer out of its own territory. Eventually these forays get shorter and shorter, until both fish are nose to nose at the invisible line that marks the division between their two realms. That invisible line is pure psychology. Each fish is indomitably courageous inside its own territory, but becomes increasingly fearful and weak-willed once it is in the other fish's.
Other animals are the same way. Bucks mark out invisible territories in the woods, and lock horns at the borders. "One tiger to a hill" is an old Chinese saying. Inside his territory, the little mockingbird is valiant, and the mighty hawk is timid, but let the mockingbird chase the hawk to the edge, and the mockingbird loses his nerve, turns back, and chases the hawk no further.
The territorial urge is strange, and I don't think we acknowledge it enough, or understand it very well, although you can see marks of it throughout human life. The urge to create a territory and protect it is probably coded in our subconscious, left over from all the species we are descended from.
Now, if you'll excuse me, I have to go. There's a hawk at the front door I need to attend to.