It's more than the birds it's the bees as well.
This is what I do year round. I do it year round not because the birds and bees try to get in year round but as soon as I don't the birds will decide it's time to nest. Once they decide your airplane is a great nesting place you'll have a VERY difficult time keeping them out.
I take ordinary aluminum window screen cut to just over the grill opening size and weave it into the grill so birds don't go in. This will not work once they've found the compartment. They will just push the screen out of their way. I use a screen in case I forget to remove it before flight there will be no damage (it's happened).
I then hang a empty plastic soda bottle about 6 inches under the gasolator with some safety wire. I don't know whether it's the movement in the wind or maybe a sound from the bottle but I haven't had a bird enter the underside of the cowl since starting this practice. Empty plastic soda bottles are generally free in the trash so if it gets lost, no big deal. They also double for a great fuel sump tester.
I have some black plastic window screen glued inside the fresh air vent intakes on the wings using dabs of silicone sealer. This keeps the bees and flies out nicely. I have a piece of the same material folded over the fuel vent on top of the cabin and held in place with a wire tie to keep the mud dabbers out.
You'll also learn to put your prop at either the eleven or one o-clock position so the birds don't have a big perch to sit on an crap on your prop and cowl will they eye up your grill area for a nest.
All of these things work best when your tied down next to a derelict unprotected airplane which is an easier target for the birds.
