Only a limited number of mobs can be spawned at once, so after the first couple of minutes the limit has been reached, unfortunately these mobs have been spawned elsewhere.
You need to stop mobs spawning outside of the spawners the easiest way of doing this is by lighting up all the tunnels in the area.
It would be most efficient to light up the tunnels between you and the spawning pads as the mobs spawning there do not de-spawn like the ones further than 144
Update: According to Notch's blog post about changes coming in 1.6, the torches-on-fast-leaves bug is getting fixed:
- Fixed buttons and levers being placeable on leaves in “fast graphics” mode
This means that monsters can no longer spawn on leaves, either, as they are always considered transparent, even when using Fast graphics.
Monsters can spawn on any solid, non-transparent block which is at light level 7 or lower.
Signs, torches, rails, and the like are ignored when spawning monsters; they'll spawn on the block underneath just fine. This is why one-block-wide railways need to be lit.
Transparency means whether or not light passes through a block rather than whether you can see through it. Half-blocks, doors, stairs, and glass all count as transparent and prevent monster spawning. Leaves, oddly, count as transparent only when Fancy graphics are enabled. An easy way to test a block for transparency is to try placing a torch or sign on it — neither can be placed on transparent blocks. (Meaning that torches can be placed on leaves with Fast graphics, but not with Fancy.)
Torches put out light level 14, so monsters can't spawn within 5 blocks of a torch orthogonally or 3 blocks diagonally. If you put your torches in a diamond pattern, they can be up to 9 blocks apart, but it's best to move them closer than that on uneven terrain. (I use 5 myself.)
If you like, you can actually torch your treetops by changing your graphics to Fast, placing the torches, and switching back. Your torches will stay rooted, even though you wouldn't normally be able to place them that way. Personally, I think it makes them look rather cheerful. :-)
Best Answer
No sources seem to indicate that they can spawn (naturally) with a block in their hand. According to the Minecraft Wiki, they instead pickup blocks within a 4x4x3 area of where they are standing.
Commands can be used to spawn them in holding a block, but other than that, if there is no dirt/grass for them to pickup, then they will never be holding said blocks.