Thanks for this insight. My experience of co-brooding is confined to one case, and is the one last year where the first born died on the nest apparently of dehydration (perfectly formed, no physical injury visible, had been running around, yolk absorbed and consumed by the time it died, and had been cheeping very loudly in the nest box the night before it died). It had hatched under Idris 2 or 3 days earlier, run around the coop a bit on the next day, while both broodies sat tight waiting for the last to hatch (I assumed, but they were both young novices so maybe didn't recognize the urgency of its cheeps), and it ended up under Rhondda in the box next door, dead in the morning in exactly the same place it had been alive the day before. Still kicking myself over that one; it was grey, like Amadeo, and probably his grandson/daughter.
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Anyway, I can change the nest boxes by removing the central divider, so if I get co-broodies again, I'll do that, thanks to your shared experience 
@BDutch . Great thread you've started!