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Another apparant green-morph Pine Siskin (Carduelis pinus); this one from Davis Mountains State Park, Jeff Davis county, western Texas on March 06, 2009:

I know of two articles describing the green- morph PISI: "Eurasian Siskins in North America - distinguishing females from green-morph Pine Siskins" by McLaren, Morlan, Smith, Gosselin, and Bailey; American Birds, Vol 43 number 5 ( Winter 1989); "Identifying Eurasian and Pine Siskins" by Lethaby; BIRDING vol 30 number 2 ( April 1998).

The first-referenced article above did the ground-breaking work on the hithero-undescribed green-morph PISI, and Lethaby largely re-stated the conclusions in his piece (that focused more on separating normal PISIs from EUSI). To summarize green-morph birds:
a) they are (thus far) all males, presumed to be examples of schizochroism, in that the brown pigments are reduced/absent, while the green and yellow pigments are enhanced.
b) they have larger-than-normal amounts of yellow in the tail and in the wing base - much more than on EUSI.
c) they have yellow on the flanks and undertail coverts, but not across the upper breast - EUSI has white undertail coverts and flanks, with a yellow wash on parts of the face and across the upper chest (note that Lethaby feels some EUSIs can also have some yellow on the flanks).
d) the underparts streaking is reduced such that the upper (chest) streaks resembe the thin streaks of EUSI, but the lower flank streaking is also thin - very different from EUSI's thick, well-defined lower flank streaks. The extent of streaking is typical for PISI in that it meets across the breast, while most EUSIs have little or no central streaking.
e) The wing bars are typical of PISI in that they are both rather narrow, with a white median covert bar and a slight tinge of yellow in the greater covert bar (some of this effect comes from the bright yellow transverse primary patch showing though the feather tips), - EUSIs have thicker median and greater covert bars and both are genuinely tinged with varying amounts of yellow.

As always, I would value informed comment on this page - please indicate whether your comments are to be considered private/anonymous - thank you.

Typical Pine Siskins from the same location and date:

- this individual has an unsual amount of yellow visible on the greater coverts: