Ruffled feathers, and worse

I first noticed this poor bird at the end of last year and took this photo on 2nd January.  As described in a previous post, it had abnormal body and tail feathers and discoloured wing feathers.  I sent pictures to the BTO but they didn't know what was wrong with it.

In a follow up post on 4th February the feathers looked worse but the bird was still behaving normally.

The last time I saw it was on 27th February, by which time it had lost its tail feathers altogether, plus a lot of body feathers.  I presume it died or was predated.

At the same time as the last photos above I noticed a similar but less severely affected bird in the garden but I didn't manage to photograph it until 8th March.  It still has fairly normal tail, head and wing feathers but deranged body feathers and a lot of missing feathers around its neck.  Like the other one it flies and feeds normally but it spends a lot of time preening, as if the feathers are irritating.  I have had a good look around on the internet and I wonder if these two birds might have a severe feather mite infestation.

This is the same second bird now.  It doesn't look significantly worse than two weeks ago and still behaves normally.

In recent days I have also seen this blue tit in the garden.  It has lost its tail, which happens, but its body feathers are abnormally fluffed up and it doesn't fly well.  It rarely gets on the peanut feeder but behaves like a dunnock, catching what the other birds drop from the feeder.  It also has an obvious bill deformity, so it not just a missing tail or just a feather problem.

And then there is this bird with a localised and rather different feather abnormality.  I don't know if all these problems are connected, or the cause of any of them.  If you know what they are please leave a comment.