For birds? Redring sight.

With the slug barrel, I'd look for a good 1-4x, 1-5x, or 1-6x. Or a 1.5-6x, if you don't want a true 1x. I don't think any more magnification is necessary on a slug gun. There's tons of options in that range. If you're concerned about dawn/dusk performance, go with something low magnification, large objective. Good example would be the Vixen 1.5-6x42, low mangification with wide field of view, and enough objective to collect usable ambient light before dawn and after dusk. Pricey, but just an example of the type of scope I'd look for.