Well, I was thinking for the most part illusions of predatory animals, both sight and sound, along with other loud noises or intense stimuli.

[She taps a finger against her lip, thinking.]

There are also genjutsu that cause the subject to fall asleep or experience disorientation, but I'm not sure that would be as effective at driving the birds away. It would have the advantage of not being something they could get used to or fight off, but...