Separation from European Siskin
Pine Siskin in its typical morph is a drab bird, whereas European Siskin, in many plumages, is much brighter. Adult male European Siskins are bright green and yellow with a black cap, and an unstreaked throat and breast; Pine Siskin does not have a corresponding bright plumage. Adult female European Siskins also usually have green and yellow plumage tones: for example, yellow in the supercilium and on the sides of the breast, green tones in the mantle and yellow in the rump. Adult Pine Siskins of the typical morph do not have green and yellow tones, although juveniles can have a yellowish-buff wash on their underparts and buff-toned wingbars, for a short period prior to their autumn migration. The ground colour of the underparts of European Siskin is normally pure white, whereas on Pine Siskin it is usually a dirtier colour.
In female and juvenile European Siskin, the centre of the belly and lower breast are often largely or entirely unstreaked, whereas in most Pine Siskins the streaking extends across the whole of the underparts. The wingbars of European Siskin are broad and yellow (with the tips white) whereas they are normally narrower and buffish-white in Pine Siskin, contrasting with the bright yellow flash at the base of the primaries. Pine Siskins have a longer bill, usually with a straight culmen, compare with a short bill in European Siskin, with a decurved culmen. There is a green morph of Pine Siskin, closer in appearance to European Siskin; these birds make up only 1% of the population. These are closer in appearance to female European Siskin, but differ in that they have a yellow-wash on the undertail-coverts (white on European Siskin), no yellow in the supercilium, reduced underparts streaking, and much yellow at the base of the tail and remiges; there may also be a difference in the extent of yellow in the underparts but this needs further study.