It's not that uncommon DB ... and there are at least a couple of previous threads on this forum complete with photos (of blackbirds).
In my own experience, this appears to occur (or at least is observed) more frequently in male blackbirds than in any other species, although also occasionally seen in corvids and in some finches ... male chaffinches in particular.
Normally feathers are moulted in a sequence, so that dropped feathers are replaced by new ones ... apart from appearing untidy for a while, there usually isn't any apparent 'baldness'.
However, sometimes ... and for a variety of reasons ... the normal moulting pattern can become suspended or arrested, such that when it resumes there is a rapid loss and replacement of the plumage. To the casual observer, this is much more noticable when it involves the feathers on the head and neck of a bird.
Because we work 'up-close and personal' with those birds that are in our long term care, I can vouch that the condition isn't caused by the presence of mites or any other exoparasite ... nor is it due to any apparent dietary deficiency (although those are quite possibly factors in some birds).
Since it appears to occur more often in male birds than in females, I suspect that some hormonal imbalance might be implicated.
In some birds that have exhibited this condition, all moults in successive years have occurred normally, so the fact that it has happenned once does not predispose that it will occur again in the same bird.
At the moment, we have a magpie who is in the process of sprouting all new feathers on the head and neck, having spent last week resembling a mini-vulture ... in another week he'll have regained his overall glossy plumage and be well insulated again for the winter ... so thankfully we won't have to resort to knitting him a wee balaclava.
