Hi Alan,
In response to your querie your Siskin have most probably got the same disease as the Greenfinches. It is a virus that affects the throats of some birds and prevents them from swallowing their food. When they spit this out it then gets passed on to other birds feeding in the same area, it is particularly virulent at breeding time when it can be passed on to young. Unfortunately keeping the feeding area and equipment clean has little effect on the virus, the recommended course of action is to stop feeding for several weeks until infected birds die off and hopefully the virus goes with them. The disease is unusual apparently as it has been around for some time and used to affect pigeons and doves but it has transferred to greenfinches and now siskin. One of the problems is feeding the birds as they are concentrated in a small area. I have had problems in Eglwys Fach with this, last winter (2009-10) I had a total of about 20 birds die. Most of these were Greenfinches and Siskin but one Chaffinch, one Goldfinch and a Collard Dove were seen suffering. I stopped feeding all of last summer and left it until November 2010 until I put the feeders out. I haven't seen any poorly birds this winter but have only had occasional greenfinch, one or two instead of the 15 to 20 I can usually expect, though chaffinch and siskin numbers have usually been above 15. I did submit a report to the BTO about this and they were keen to have feedback about all such cases.