This is the time of year commensal rodent (mice) exclusion becomes a priority. Here is a view of a crawl space vent, taken at a residential home around Boston.
When viewed from a normal standing position it looks like there are no rodent entry points.
When viewed from below, best done using a mirror, such as a mechanics mirror with a handle, you can see the huge gap along the top of the vent. Notice the brown rodent rub mark on the cement edge just to the left of the metal frame of the crawl vent. Rub marks are left on surfaces when rodents run them repeatedly and the grease and dirt from the fur of the rodent leaves a stain on the surface they run over.
Here is another visual example of a rub mark. When controlling mice it is essential to locate entry points. That is why rodent rub marks are important in pest control.
Standard exlusion materials are a supply of copper or stainless steel wool and snips to cut it. Don’t use standard steel wool, since moisture will cause it to rust and stain. Also black polyurethane is useful to go over the copper and seal out airflow points that rodents will sniff out.
This is the mirror view with the black polyurethane over the copper. The polyurethane is not essential, but helpful to hold copper in place.