The winter of 2012-2013 has seen record number of Pine Siskins (Spinus pinus) in the Pacific Northwest. Numbers in the thousands have been reported on some Christmas Bird Counts. What's going on? It turns out that siskins are cyclic, on the average peaking in a given area in alternate years, and the peaks on occasion are very high. Thus they have a two-year cycle. The cause of this cycle is usually thought to be food availability, which varies in both space and time.
Siskins are among our smallest birds. They are members of the finch family Fringillidae, a family of interest because of its adaptive radiation into the seed-eating niche. The biggest species eat large seeds, the smallest very small seeds, and so forth. Pine Siskins eat a great variety of seeds of weedy herbaceous plants in the summer, but in winter in the Pacific Northwest they are strongly associated with red alders (Alnus rubra).
Alders bear their seeds in small conelike structures, and the slender bill of the siskin is well adapted to extract these seeds. Flocks of siskins visit alder trees and spend quite a bit of time working through the abundant cones. These flocks can be of surprising large size, up to and over 100 birds, but are usually smaller.
Pine Siskins also eat a variety of other seeds, from dandelions to hemlocks, and are frequent visitors to bird feeders, especially preferring thistle seeds. Unfortunately, this can be their undoing, as they seem very subject to infection from salmonella, which is spread in their feces. Birds may defecate while at feeders, and thus the salmonella bacteria are spread from bird to bird. Most winters, people who feed birds report sick and dead siskins at their feeders, sometimes dying right on a feeder.
Pine Siskins are very aggressive little birds. Perhaps they have to be because they go around in flocks, and a bird has to defend its feeding site against others. But they are also aggressive toward any other species, and at feeders, it is an impressive sight to see a siskin chase away a larger finch or sparrow. Siskins often have yellow in the wing and tail, and larger yellow patches may enhance a bird's success in driving away others.