My memory is that in last year's discussion on this, it was decided that birthday megathreads were a clusterfuck. I can attest that merging them all now would be superconfusing - when you add posts to a thread, they are ordered by timestamp, which would cause a lot of weird overlaps. One big thread also makes the avatar changing confusing, as the thread progresses. I personally see no problem moving bdays to LJ, or just not doing them, or even giving them a 24 hour autolock. A stickied megathread, though, would be logistically messier.