something weird just happened... I wanted to test if I could put two goblins (captured siegers) in the same cage, so I set up a safe area to do it patrolled by several champion wrestlers, and built the two cages containing said goblins next to each other. They were a highwood cage containing Song Gosmabosa and a pine cage containing Ngebzo Ngokanguxum respectively, this is important later... I assigned ngebzo to Song's cage and an idle job came and did the job and then left...or so I thought. I then checked the highwood cage and found it still only contained Song, and that the Ngebzo had not escaped; there was no spam about anyone being interrupted by a goblin, and he was nowhere to be seen. I looked around, checked the units list and zoomed to him. I found him in my animal stockpile in a highwood cage... Confused, I assumed there had been some sort of mixup and built that cage next to Songs and intiated the transfer again. This time, he actually made it into Song's cage, but there was an extra pine cage pulled out afterwards. Is this a known bug, or did I just discover dwarven quantum cage mechanics?
Either way, I am now testing to see if it repeats and to see how many of my captured siegers I can get in one cage.
Edit; I've done a little science.
As near as I can tell, if you have a cage with a goblin already in it (may be true for other invaders/animals, haven't done animal testing yet) and add a goblin from an adjacent cage(haven't tested diagonals) unless there are multiple goblins in the adjacent cage an additional cage will be brought by a hauler. What appears to happen is that the adjacent goblin is put into the hauled cage, and then the cage is either emptied into the target cage, or another hauler gets to it first and drags it to a stock pile. The latter occasionally leads to the alarming sight of a hauler dragging an uncaged goblin from the stockpile to the target cage, but fortunately in all instances so far the goblin just kind of goes with it and doesn't try to escape/kill anyone. as near as I can tell cages aren't actually created and are instead hauled, but since I have a large number of cages it's hard to tell.