Roof Ridge Vents and Rodents

What construction features do ridge vents use to keep out rodents; e.g. squirrels? Wire mesh?
I ask because I have one of those crappy powered vents that uses mesh, but the mesh was anchored so poorly that the little devils soon learned to push on it hard, and voila, they were inside.
TIA, HaroldWho
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We've have several houses with ridgevents all made different ways. Rodents were never a problem from them.

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