These are the plastic, big vents builders install on new construction, here in southeast Texas. UV protection is necessary but like most, not applied. 7 of these on a 13yr old home. The 4 on one side of the home were decayed. The other 3 that didnt get the direct sun were brittle, but not like this.
Now. You’re probably thinking, squirrel, coon, rat?
That was my first thought. No scratch marks. When we touched them, they just crumbled. I dont think it was heat, because the 3 on the opposite side of the gable showed no damage. We concluded the sun.
Spend the extra money and buy metal.
Side note: This call was for a waterheater type B storm collar install for a plumber. The owner said they didnt have leaks. When i showed them the photo they went in the attic and found boxes of wet papers.