Why did a bird slap my dog's ?

Ok. My shih-tzu dog was just minding her own business when she heard a noise by the fence. She went to go check it out. She sat by the fence for a moment and then started to walk away then this black bird comes and sits on top of the fence and sees her walking away. Then the bird starts flying towards her and slaps her with its wing. My dog stops and looks at the bird like "what da hell?" and then she starts walking again. The bird does it two more times and then my dog stops and barks at the bird and then it flew away. Why did the bird do that? Does it want my dog ually or something???