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.