Kobolds are stupid, yet it cannot be denied that they are bold. Magmawiki states that they attack fortress when the defences seem weak, but I found otherwise. I have five rings of defences, with guard animals, and kobolds (and other enemies) never breached all of them. The fortress is 15 years old, and the lizardkin were trying to rob it many times. Sometimes they were sending waves of kobold thieves, four or five at once, during sieges performed by more intelligent creatures, like zombie dwarves for instance. They were able to kill a grizzly bear or a two, not counting dogs, but generally they were rather harmless. And hardly successful.
So when one time my dogs reported that a kobold commando attacked I was quite shocked. Six kobold ninjas hacked to pieces a team of guard animals at one gate, and were content to occupy it. I found it insulting, so sent a team to get rid of kobolds. I also was curious how would they fight, they seemed quite timid and did not press the attack for some reason. It turned out it was a ruse. When the quickest of my dwarves was in the middle ground between the occupied gate and the internal wall, he was suddenly attacked by six other kobold-ninjas, who jumped out from behind the trees. Despite all this the kobolds couldn't stand a chance, they are too small for dwarves and were quickly dispatched. Then I ordered one team to search area beyond the wall, and another to return to the castle. And what happened? The last attack was another ruse, because returning dwarves found a kobold thief, in exactly the same spot the second team of ninjas was lying in ambush.
Now, it would seem that kobold are not that stupid, if they are able to execute such a plan. Probably they were really desperate to steal anything, with their angry wives waiting at home hole and only eggs to eat. But what if they were successful, and the thief would sneak into my castle, assuming he evaded the guards somehow? There's a way in, and a way out, and he couldn't expect than his sword-wielding comrades would help him escape from inside, I think. So, are there more uintelligent sentient critters than kobolds?