Product review (5 stars): LOVE THIS!!!
Okay people, this is the one you need to get to keep the birds OUT!  I've had the handyman replace these louvers in the past twice.  And the birds can still get through it because it is just a piece of plastic with screws holding it in place and the birds just lift it up and enter.  In contrast, this one is great.  There are no openings that the birds can lift with their wings or beaks.  Also, there is a plastic circle device INSIDE the vent hood that allows one way in and now way out.  Essentially, you have to get your finger, lift the circle plastic piece up and enter through a very slim slit, that's if you are a skinny bird.  And trust me, no bird can get through the plastic cage blocking the circle plastic disc.  And even if the bird went it, there is no way to get out, the bird is TRAPPED.  So no more hearing birds in my roof.