That's true, so closely spaced markings are recommended. I had the same problem and this dotted tape solved it completely. Personally, I didn't want clip art graphics on my windows, and I like the fact that the dotted tape minimizes interference with my view outside. You can find more information on other strategies here though (like drawing deterrent patterns with bar soap).

*Two inch spacing worked for me, but you might need to make it denser depending on the species you're accidentally luring