These two species (recently split) are extremely similar in appearance. No significant differences exist in size or shape, and plumage differences are subtle and mostly overlapping. Songs differ slightly but consistently, and call notes (year round) are the most different. Voice, especially call, will be the best way to distinguish them, and identifying individual birds from photographs may be impossible in some cases.
Arranged generally with more useful features at the top, less useful or unconfirmed at the bottom.
- Overall – Pacific darker, more rufous, and less patterned; vs. Winter Wren paler, grayer, and more patterned. The combination of reddish-brown eyebrow, cheeks, and throat, with smooth unmarked breast and neck-sides may give Pacific a distinctive overall appearance, even though no single feature is reliable.
- Neck sides – Pacific tends to have the pale sides of the neck more reddish and much less patterned than on Winter.
- Throat color – Pacific usually has throat almost as dark and richly-colored as breast; vs. pale throat contrasting with brighter breastband. Note that some (Eastern) Winter Wrens can have smudges of pale buff/orange on throat, and all Pacific have the throat paler than the breast (just not as pale as Winter).
- Pattern on underside – Pacific usually has breast unmarked or with a few faint bars; vs. speckled with faint, short dark and light bars.
- Eyebrow – Pacific tends to have the pale eyebrow stripe tinged rust/orange; vs. slightly paler and more clay-colored on Winter.
- Sides of breast – Pacific may have the darker and more reddish back color spreading down onto the sides of the breast; vs. sides of breast paler than the back on Winter (matching center of breast rather than back)
- Color of upperside – Pacific is dark, deep, chestnut-rufous; vs. paler, grayer brown on Winter. A subjective and subtle distinction that can be quite difficult in the usually brief and shaded glimpses these species afford. The difference might be most pronounced on the crown.
- Color of underside – Pacific darker rufous-brown; vs. paler gray-brown. Some near overlap in color makes this subjective.
- Pale spots on primaries (forming bars on the folded wing) – Pacific Wren has the pale spots on the primaries darker and more brownish on average; vs. Winter Wren with paler off-white to clay-colored spots. Note that even on Pacific these spots are paler than the back, and there is some overlap in color between the two species, so this is a fairly subjective feature and difficult to judge.
- Pattern on upperside – Pacific tends to have upperside smooth rufous-brown with faint dark bars or none; vs. upperside mottled and barred with dark and white. This helps accentuate the darker upperside of Pacific.
- White tips on greater coverts – Both species may or may not have tiny white spots at the tips of the greater coverts. On Pacific these tend to be smaller, but at the same time more conspicuous because they are surrounded by uniform dark brown; vs. Winter tends to have larger white dots, but these are less conspicuous alongside the whitish mottling of back and breast-sides.
All of this requires field testing, and now that birders have a new species to look for there will surely be a lot of new discoveries and refinements over the next few years.