1Are these stickers reusable? If I peel them off accidentally, can I stick them back on the window?

For sure you can. All you need to do to put them on your window is clean your window before use. If they fall accidentally, just spray some water and stick them back on!
2How can I make sure the birds are able to see these window clings inside the window?
If you put these window stickers on windows from outside, they will create a vivid visual rainbow effect that birds will know it is better not to be there.

3How many pieces does it contain in your product?

42 in total. The package comes with 42 stickers and one plastic scraper.

4Why is there no rainbow in my window when I apply them?
Stickers do not produce a rainbow effect. They only REFLECT sunlight when light shines through. Please make sure your room/window has enough sunlight coming in.
5Why it won't stay on my window?
Our stickers are made of prismatic vinyl which relies on ELECTROSTATIC ADSORPTION between water molecules and glass surfaces to stay on your windows. Spaying some water before use can help stick well on your window.