Grackle Problem

I can't seem to keep my car clean, the grackles keep dumping on my dark green Ford Taurus. They do not dump on any other car in the area, just mine. The more I wash it the more they mess it up. Is it possible that a grackle is smart enough to realize that I harass them, so they keep messing on my car? I put food out for all birds and squirrels but the grackles steal everything. When they come around I make loud noises, and they fly away. I even tried taping a picture of Susan Boyle on the roof but that did no good either, they just dumped on her picture!

Common Grackles are blackbirds that look like they've been slightly stretched. They're taller and longer tailed than a typical blackbird, with a longer, more tapered bill and glossy-iridescent bodies. Grackles walk around lawns and fields on their long legs or gather in noisy groups high in trees, typically evergreens. They eat many crops (notably corn) and nearly anything else as well, including garbage. In flight their long tails trail behind them, sometimes folded down the middle into a shallow V shape.