Are there any infections currently affecting Siskins?
I have, recently, found three dead Siskins in the garden – on different days with no obvious signs of being attacked. A sick-looking bird was also around the garden this morning (see picture) until it was taken by one of the marauding cats that have infected our neighbourhood. Hopefully, if the bird was diseased, the cat will also be sick.
Glamorgan Bird Club 
Trichomonosis, which has done for Greenfinch in recent years, is perhaps most likely – BTO have some info http://www.bto.org/volunteer-surveys/gbw/gardens-wildlife/garden-birds/disease/trichomonosis but could also be salmonella
Thanks Peter. I’ll make sure to log them on my Garden Birdwatch submissions.