Distinguishing the subspecies of Purple Finch

Summary
The two subspecies of Purple Finch are fairly distinctive with well-defined ranges. They are best distinguished by overall impression of color and pattern, and more objectively by details of back and head pattern. Virtually all individuals should be safely identifiable in the field, but the frequency and extent of intergradation is unknown.

Eastern Purple Finch   C. p. purpureus

Pacific Purple Finch    C. p. californicus

Distinguishing features of female-plumaged birds

• upperside relatively uniform brown with faint dark streaks but no paler markings on Pacific (vs. more patterned with fairly obvious dark streaks on Eastern, and with paler edges on feathers adding to the tricolored varied pattern)

In addition Eastern often shows white blotches where feathers are parted on nape and upper back (Pacific is always relatively uniform olive-brown with slightly darker streaks there). See additional photo here

• usually with distinct green tinge on upperparts and secondary edges on Pacific (vs. tinged reddish brown on Eastern; Cassin’s can be greenish on scapulars but never (?) on secondary edges)
• secondaries usually edged greenish on Pacific (vs. usually edged reddish-brown)
• face pattern much less distinct, with pale eyebrow and malar filled with fine dark streaks on Pacific(vs. distinct with clean white supercilium behind eye, and contrasting whitish malar)
• rump olive, very faintly streaked and unicolor with the mantle on Pacific(vs. rump distinctly streaked in three colors like back, slightly paler than back)
• greater coverts tipped grayish-olive on Pacific(vs. whitish)
• all markings slightly paler and colder olive-brown, including cheeks, crown, streaks on underparts, etc. on Pacific (vs. all slightly darker and slightly reddish brown)
• Nasal tufts brownish like forehead on Pacific (vs whitish creating very narrow pale band across base of upper mandible)
• undertail coverts often streaked on Pacific (vs. very rarely streaked in Eastern)
• underside averages subtly more streaked, and streaks more blurry, blending together in places, on average more heavily marked overall below on Pacific (vs. slightly more spotted with triangular marks like Fox Sparrow, averages slightly less heavily marked with more white space between marks)

But both species are variable in the shape and density of dark marks on the underparts, Pacific averages more heavily marked and often has blurry streaks, reaching extremes in both of those features that are apparently outside of the range of variation of Eastern, but this will identify only a few individuals. The “sum of the parts” or gestalt of underparts pattern is probably a fairly powerful indicator of subspecies, but to confirm the identification you must look for head pattern or back pattern differences that are more objective.

• throat always spotted, sometimes heavily spotted on Pacific (vs. throat less spotted and occasionally unmarked clean whitish)

This is a fairly subtle difference with a lot of overlap, but combined with the rest of the underparts pattern it adds to the gestalt impression and might be worth loooking for as a quick indicator. Combined with the following feature this can create a subtly different appearance on many birds.

• dark lateral throat stripe continues to base of bill on Pacific (vs tapers and leaves small area of white at base of bill)
• breast sides overlapping bend of wing olive-gray with darker streaks on Pacific (vs white with contrasting dark streaks)

Distinguishing features of adult males

• back more uniform with blurry dark red-brown streaks on paler red-brown background on Pacific (vs. distinct dark streaks on pale pinkish)
• greater coverts tipped dark drab reddish on Pacific (vs. greater coverts tipped pale whitish with some pinkish-red)
• rump darker, maroon-red, not contrasting with back much on Pacific (vs. rump bright pink-red, more conspicuous)

Although some caution about variation in shade and intensity of red color applies here, rump color and contrast distinguishes the vast majority of adult males. see photo of specimens here

• often (usually?) shows distinct greenish edges on secondaries on Pacific(vs. pale brownish edges)
• underparts color very similar in both but Pacific tends to have buffy belly with brown-red smudges on lower flanks (vs. white with pink smudges, faint streaks)
• crown color averages darker and redder, less rosy-pink on Pacific. But the shade and intensity of red is quite variable, and both subspecies can rarely show orange or yellow instead of red
• no difference in nasal tuft color

Structure

Structure differs only slightly and on average, not likely to be useful in the field

• bill of Pacific averages slightly thicker and larger with more curved culmen; although some observers report the opposite impression, which suggests that this is too variable and subjective to be of any use.
• tail averages less deeply-notched, 3-6 mm (vs 6-10 mm on Eastern) This is simply impossible to ascertain in the field under most conditions, since the tips of the shortest (central) tail feathers will almost always be invisible from below, the tail notch in silhouette is created by the inner webs of some of the outer feathers and does not reflect the length of the central tail feathers.
• wing formula averages slightly less pointed, slightly longer primary projection and p7 longest (vs p8 longest) This is an extremely subtle difference of only a millimeter or two and is probably variable as well

Voice

Flight call a dry, abrupt pik or tink gives an impression of being slightly harder and more wooden-sounding in Pacific birds, but this is almost certainly not distinctive enough to allow identification of a bird out of range.

Song may be slightly different, as described in the Sibley Guide: Pacific birds sing a lower-pitched, less varied, duller-sounding warble, while Eastern birds sing a more varied, brighter-sounding warble with some higher notes. This is complicated by the fact that Purple Finches sing two distinct song types in different contexts, and the songs are quite variable. More study is needed to confirm any differences in song.

Distribution

Eastern purpureus nests across most of Canada and the northeastern US, wintering to the southeast. This subspecies wanders widely in irregular incursions south and west to Colorado, and has also been recorded west of there on a few occasions (e.g. San Miguel Island and Death Valley, CA; Tucson, AZ).

The Pacific subspecies californicus nests along the Pacific coast and is relatively sedentary. It moves short distances in winter to adjacent regions and has been recorded east only to Arizona.

The two subspecies meet in extreme southwestern Canada, but the situation there and any potential intergradation is essentially unknown. Based on specimens (MCZ) and band recoveries (see map), the Purple Finches nesting in much of British Columbia, west of the Rocky Mountains, are typical of Eastern purpureus and migrate southeast to winter in the eastern US. The dividing line between the subspecies is farther west, very close to the coast of British Columbia. More study in that region is the key to figuring out how much interbreeding is occurring.

References

Additional photos of specimens from MCZ can be seen here https://www.sibleyguides.com/bird-info/purple-finch/supplementary-info-on-purple-finch/

Song samples at http://www.xeno-canto.org/XCspeciesprofiles.php?species_nr2=3962.00