The Pine Siskin is migratory and will only be seen here in Georgia in the winter months. Cornell says “Flocks of tiny Pine Siskins may monopolize your thistle feeder one winter and be absent the next.” . We haven’t seen them that often here, but maybe that’s because we are late filling our thistle feeders. (bet you can’t say ‘thistle feeder’ three times really fast)
These birds are fairly small, being around the same size as the widespread American Goldfinch.  In fact, it may be easy to confuse a Pine Siskin for a female goldfinch.
Adults are brown on the upper parts and pale on the underparts, with heavy streaking throughout. They have short forked tails. Their bills are conical like most finches but are more elongated and slender. 
Pine Siskins forage in trees, shrubs and weeds. They mainly eat seeds, plant parts and some insects. In winter, they often feed in mixed flocks including American Goldfinches. Small seeds, especially thistle, red alder, birch, and spruce seeds, make up the majority of the pine siskin’s diet.  In a part of their esophagus called the crop, the species can store up to 10% of their body weight in seeds overnight, providing extra food on cold days. 
In summer, they will eat many insects, especially aphids, as well as a few spiders and grubs, which they then feed to the young as a protein-rich food that contributes to their growth. By the time of winter, even first year siskins predominately eat seeds.  
References and Additional Information
 Wikipedia: Pine Siskin
 The Sibley Guide to Birds, second edition – David Allen Sibley
 Cornell – All About Birds: Pine Siskin
 Cornell – All About Birds: Pine Siskin – sound
 The Breeding Bird Atlas of Georgia – UGA Press: Schneider, Beaton, Keyes and Klaus, Eds.
 Cornell – All About Birds: Pine Siskin – life history