An update. The date is Obsidian 14th. Three days ago liaison finally made it to the courtyard, so my estimate was 100% correct. Anyway, the hamsters are still standing at the edge of the map, trying to understand what the hell did they forget here. Meanwhile my miners have made the tunnel that goes all the way from the courtyard to the hamsters location, and turned the tunnel into a deathroad. Fourteen cage traps, followed by 12+ stonefall traps. And if they somehow pass them, there's still a long way to go so I can always seal the exit from the tunnel, leave them inside, and devise another way to deal with furry grain-eating menace.
One strange thing I noticed though is that hamsters do not own anything at all, neither in the way of armor nor in the way of weapons. I wonder what's the big difference between a pikeman and a lasher is.
Another small problem. Due to neglect and lots of other, more important things, all the food that I bought finally succumbed to rot and rodents. So the farmer had to get back to work. Just an inconvenience really as the food situation is improving even as I type these very words.
Obsidian 17th. Ta-da-da-dam! The siege is lifted! After losing 9 pikemen to cage traps, the remaining hamster forces turned tail and fled in disarray. A special Dwarven Medal of Courage goes to the Stray Kitten. Despite the great risk of death and injury, it bravely charged forward through the tunnel, luring hamsters inside and onto the traps. If not for the brave kitten, the enemy would probably lose no more than 4 soldiers instead of the resulting 9.
I'm not sure what I'm going to do with prisoner hamsters though. Probably I'll just play until the end of Winter and then pass this problem to the next player. Even with currentl number of critters, the sheer size of the fort puts a heavy strain on my PC, so I'd rather not experience trying to play when immigrants show up in numbers.