LJ, how do the nurses move to the QR colony? Do they walk out the front, outside the hive and into the other nuc?
I honestly haven't a clue ...
When I began to suspect this was happening, I set up a pair of 6-frame nuc boxes (each divided into 2x 3-frames) side-by-side as a test. So we're talking four 3-frame nucs in very close proximity.
I then took a medium strength 12-frame colony and divided the frames and bees as equally as possible into the four cavities. Each nuc was given a mature queen cell, and the resident queen was left in one of them - I didn't know which one. (There was an ulterior motive to this experiment - I wanted that queen superceded by a good strain, but couldn't easily find her - so I thought I'd let a virgin do the job for me). I then left the nuc boxes alone for a week.
On the next 'quick peek' inspection what I had expected to see was all four queen cells opened (and maybe torn down) and just the one nuc still perhaps with open brood, if the mated queen had survived. But what I discovered was three almost empty nucs - maybe a dozen or so bees left in each - and 12 frames-worth of bees crammed solidly into a 3-frame queen-right nuc box !
Just how those bees had migrated is anyone's guess - I hadn't noticed anything odd going on, except excessive activity outside just one of the four entrances, which I'd put down to attempted robbing. But it wasn't robbing - it was just all the bees trying to get in, and were sniffing the cracks and so forth looking for an entrance.
Both nuc boxes were on a common platform, so they could have walked across, or some perhaps could have flown that short distance guided by Queen odour - I really don't know. But although this was an example of extreme behaviour, it mirrors what I've been seeing with the less extreme haemorrhaging of bees from Q-ve nucs placed over a Q+ve colony.
To be honest I was begining to lose faith in this means of raising nucs, until I 'did a Google' for "double screen boards" which returned many results of people successfully raising nucs in this way. Even the Dave Cushman site recommends doing this "to raise the odd nuc". So I'm hoping that double screening is the solution to this problem I've been looking for - but that will have to wait until next year now, as we're out of time this season.
And as for that 'dodgy' queen ? Well, I just had to buckle down and find her the hard way - she's now enjoying a vodka bath, and yet more good queen cells have been donated. It's a tad late in the year for getting queens mated, but I have a few surplus Q+ve nucs which can be used for combining if the virgins don't get lucky.