Birds nest

How do I discourage birds from building a nest in my eaves? I have been chasing them away with a broom all day. Everytime I knock the nest down, they start rebuilding it.
I wouldn't mind so much, but it is right above a garden (bay) window and they crap all over it.
I tried WD40 but no luck.
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Update.
By spraying the shit out of the area with WD40 it seems to have done the trick. The little buggers came back a few times, but packed their bags and headed out.
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Tanglefoot.
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