I have a simple mechanism like this in the comment fields of a few of my sites. And it shoots down 99.9% of mechanical spams. Yet, although my sites are not t

I have a simple mechanism like this in the comment fields of a few of my sites. And it shoots down 99.9% of mechanical spams. Yet, although my sites are not that popular, I have a few tens of spam attempts per day, and once in a while they succeed to come through. What's interesting is that they do seem to monitor the result. If I leave the succeeded spam comments for a while, the attacker adapts their strategy to follow the succeeded pattern. (Usually the initial attempts are just a neutral comment, irrelevant to my contents but doesn't contain any URLs or product names. Once it penetrates, they start spamming with URLs.) I'm not sure if there're human behind this, monitoring bots activities, or bots are actually sophisticated enough to adapt.