Screen on ER side Vents - How To?

[Alegria]

Question I was wondering if anyone has found a good way to mount screening to the ER vents. Mine are covered, but they are open to the ducts and the sparrows love to build nests in there in the spring.

I just want to cover the opening to discourage them from nesting in there.

I have some simple screening just wondering how to mount it? Screw it on? Glue it on? Don't want to do more harm then good..

Thanks for any ideas!!!!