I came across this forum because I was intrigued by the close "companionship" of my resident dunnock and a pair of blackbirds. I feed the birds in winter and have noticed that the dunnock is the only bird the blackbirds will tolerate. Indeed the male blackbird chases off the female blackbird sometimes but never the dunnock. When the blackbird comes to feed, the dunnock arrives too and vice versa. The observation of a blackbird feeding dunnock chicks sounds odd, but I have found a reference to the same behaviour from 1968. see:
www.britishbirds.co.uk/search?model=pdf&id=4429
It's a pdf and worth a read. The author kept a record of the visits. The feeding was reciprocal between blackbird and dunnock families. Any ideas? Do the blackbirds mistake the dunnock for a blackbird chick?