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.
Dennis Paulson
