dirt streaks on siding

[rfd578]

I have streaks of what looks like dirt on the siding under the trim board which is under the angled part of the roof. There is a gap between the siding (hardiplank) and the board at the top of each run of siding due to the angle of the siding, and it looks like this stuff is coming out of those gaps. It almost looks like bird poop. It is in ten or twelve runs of siding, only on one side of the building. Could it be bats or bugs? Can I fill/seal these gaps with caulk or great stuff or something else, or would that cause problems?

[pushkins]

Sounds more like dust and dirt migrating out of the attic area, that is not so uncommon, though there should be house wrap on the home if it's newer and that would stop it. Yes you could seal the gaps with a good quality exterior rated caulk (NOT silicone). Do not use spray foam of any nature.
I can't say it wouldn't be bats, they like to get into every nook and cranny if they can, though usually they are much more obvious.