I am sure many have seen the white streaks that form from lead flashing. Its not real attractive, especially on dark roofs. This is the lead oxidizing over time and effectively rusting onto the shingles during rain storms. Some get it more than others, this is because of the quality of the lead and the environment they are in. This also may explain why squirrels will eat some vents and not others, the lead quality is extremely variable so maybe some taste better than others. My squirrel transation abilities are limited.
Anyway, Critter quitters will stop this streaking because they fit snugly around the vent and keep both sunlight and rain away from the outside where the oxidized lead will leach onto the roof. No rain, no streaks. Plus they look better than those wrinkled up lead pipes. We will put out some more pictures of recent installations this week.