One bird watcher researched online and reported that the cause was probably salmonella because the symptoms match. Pine siskins are especially susceptible because they are small, tend to feed a lot on the ground, and in large numbers.
The cause of the salmonella could be either suet cakes containing contaminated peanuts or from contaminated feeders or perhaps the ground below.
Recall of Suet Cakes
Scott is recalling some suet cakes that contain peanuts obtained from the Peanut Corporation, the source of the recent human salmonella outbreak. Most of these cakes are under the Morning Song or Royal Wing brand name. The pdf file link has images of the recalled items. I think I've seen them at Target.
Care of Feeders
Dr. Ellen Reynolds wrote about this problem and recommended increased attention to bird feeder maintenance:
In her post, you will find photos of sick pine siskins and much more detailed information on taking care of wild birds in the wintertime. She also has some good images of recommended bird feeders (Thanks to Clyde, local naturalist for the tip)."You can help prevent a build-up of potentially lethal bacteria at your feeding stations through weekly cleaning and routine disinfection of bird tables, feeders and drinkers. The areas beneath the feeders can also quickly become contaminated and should be kept as clean as possible, with any uneaten food removed on a routine basis."
At Our Feeders
I have only noticed one dead pine siskin and I suspect it flew into a window. My dog was carrying it around the yard (that pesky Kookie). I will observe the birds to see if they exhibit any of those symptoms but so far, so good.
This afternoon, I raked and disposed of the sunflower seed hulls on the ground as Dr. Reynolds recommends. I do this more often the rest of the year but the critical time for this type of maintenance is winter. I try to clean the feeders once a week.