What keep birds from seeing you when you use a porthole spectacle bird house?

I would like to buy a bird house that you attach to your window next to suction cups, & theres a piece of clear acrylic on the back of it so you can watch them manufacture a nest, etc., from inside your home. But my question is (as stupid as it may be) what keeps the birds from seeing YOU contained by the house watching them? Seems to me they would fly out of their little house when they see you walk up to the window, thus defeat the purpose. Does anyone have one of these & could give me some insight beforehand I buy one?

Answers:    We have two, one in the kitchen windowpane and the other in my 3 yr olds window. The birds slowly take used to seeing you and then stop being worried and stay to feed.

Go for it, they are wonderful.
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