Pine Siskins – streaky relatives of goldfinches – are so distinctive as a species that we tend to overlook their considerable individual variation. Siskins are unusual among the small finches in that males and females look nearly identical. Subtle individual differences in the amount or intensity of streaking, or in overall color, have no bearing on identification and we simply blend them all together in our minds as “typical siskin”. They do show one infrequent variation that is quite noticeable, however, the so-called “green morph”.
A lot has been written about the “green morph”, and a good recent summary with photos and links is by Mandelbaum and Young. They end with a statement of uncertainty: “Whether green morph Pine Siskins represent a true “morph,” aberrant plumage, or simply the result of variations between individual birds is a question open to further study.”
Looking carefully at the flock of siskins that frequented our feeders in Deerfield, Massachusetts this winter, it is clear to me that the “green morph” is neither aberrant nor a distinct color morph separate from all other Pine Siskins. It is simply the bright extreme of variation in male Pine Siskins. There is an unbroken cline of variation from normal looking birds, to many that show some subtle hints of green morph features, to a few that stand out with obvious green morph features, and a very few extremely bright birds. The number of individuals showing these features drops off as the features get more extreme. That is, birds with less obvious “green morph” features are more frequent than birds with more extreme features, and the most extreme variants are the rarest of all.
Once you start looking carefully, you will find that the more subtle “green morph leaning” variants are not rare at all (like variation C, below, which could be 5% or more of all siskins). In fact, if all “green morph leaning” birds are adult males (like all “green morph”), and assuming that adult males make up roughly 25% of all siskins in winter, then about 20% of all adult males could be “green morph leaning”, and 4% of adult males are full “green morph”.
There is still a lot to learn about variation in Pine Siskins, and lots of interesting questions about these brighter males, but it is clear that the “green morph” is simply the bright extreme of a wide range of variation in male Pine Siskins, and not a distinct color morph.