Build a bird house nestbox and attach it to a window with sturdy suction cups so you can watch the birds nest.
This is a link to a Google 3D SketchUp drawing for nest box where birds enter through open front gable and drop down into box. Upper floor can be lifted out for cleaning.
This is a link to a Google 3D SketchUp drawing for a tree swallow nest box with hinged side allows access and is weatherstripped to keep out rain. You will need the SketchUp software to download this drawing and its freely available online.
This is a link to a Google 3D SketchUp drawing for an unusual Northern Flicker nest box design. Top pulls out for cleaning.