Pigeon shooting
PETER THEOBALD says:
Pigeons recognise the human shape, whether it is standing, kneeling or even lying flat out and very quickly take avoiding action if they feel threatened.

Movement on your part will obviously draw attention to yourself needlessly, which is why we employ hides and camouflaged clothing.

But our biggest ally is the decoy pattern itself, and provided you learn to recognise when a pigeon is fully committed to it, and is looking for a landing area, it is surprising what you can get away with.

I have often been caught outside the hide as pigeons have approached, but by remaining perfectly still, still had them pitch right in.

So, to answer your question, I would say that movement is the first giveaway, closely followed by the sight of you, if the bird is not focused on your layout.