One easy way is to make sure the obvious openings that we create like the range hood exhaust (shown in the picture) have bars on the windows so-to-speak.
Be sure to purchase all such vents with maximum quarter-inch grates. That will be sufficient to keep the small critters like birds, bats, mice and squirrels from having an easy access.
Also this one has a metal flap in it that open when the fan is on and closes shut when it is off.
;-)