So as to not fall victim to hearsay:
1. Iíve personally watched a mouse zigzagging in all directions on a smooth concrete wall.
2. I was shown a video of a mouse triggering a WiFi doorbell when its tail swung through the view of the motion sensor as it sat on top of the doorbell. The doorbell was mounted on a wall with brick siding.
Neither of these will be surprising to most here, but it is confirmation of their abilities on two surface types. In both of the above cases, the material was porous enough for a mouse to easily grab and walk vertically. The smooth concrete wall really surprised me because it felt smooth to the touch.
Iím highly confident that a mouse could not climb the outside face of a downspout. Iíve looked at the tapering gap between vinyl siding and a downspout and have wondered if a mouse could navigate it. My conclusion has always been Ďno,í and Iíve never had to go back and exclude a downspout because mice keep getting in after all other exclusion work was completed. In other words, I have plenty of customers who are mouse-free and I did not exclude their downspouts. This isnít proof, but it is a large sample size that would seem to deny that mice scale the siding/downspout gaps.