Separation from the Eurasian siskin

The pine siskin in its typical morph is a drab bird, whereas the Eurasian siskin (a bird the species does not naturally co-exist with), in many plumages, is much brighter. Adult male Eurasian siskins are bright green and yellow with a black cap, and an unstreaked throat and breast; the pine siskin does not have a corresponding bright plumage. Adult female Eurasian 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 the Eurasian siskin is normally pure white, whereas on the pine siskin it is usually a dirtier colour. In female and juvenile Eurasian siskins, 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 the Eurasian siskin are broad and yellow (with the tips white) whereas they are normally narrower and buffish white in the 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 Eurasian siskins, with a decurved culmen. There is a green morph of the pine siskin, closer in appearance to the Eurasian siskin; these birds make up only 1% of the population. These are closer in appearance to female Eurasian siskins, but differ in that they have a yellow wash on the undertail-coverts (white on the Eurasian 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.^[9]

Feeding

Pine siskins feeding on thistle seeds

These birds forage in trees, shrubs and weeds. They mainly eat seeds, plant parts and some insects. In winter, they often feed in mixed flocks including American goldfinches and redpolls. Small seeds, especially thistle, red alder, birch, and spruce seeds, make up the majority of the pine siskin's diet. In a part of their esophagus called the crop, the species can store up to 10% of their body weight in seeds overnight, providing extra food on cold days.^[7] They will alternately eat the young buds of willows, elms and maples, and the soft stems and leaves of weeds and even young garden vegetables. They glean the seeds of grass, dandelions, chickweed, sunflowers and ragweed. Bird feeders often attract pine siskins, where they may eat fragments of heavy-shelled seeds, such as black oil sunflowers, left behind by heavier-billed bird species. In summer, they will eat many insects, especially aphids, as well as a few spiders and grubs, which they then feed to the young as a protein-rich food that contributes to their growth. By the time of winter, even first-year siskins predominately eat seeds.^[7]

Pine siskins can survive in very cold temperatures. The metabolic rates of this species are typically 40% higher than a "normal" songbird of their size. When temperatures plunge as low as −70 °C (−94 °F), they can accelerate that rate up to five times normal for several hours. They also put on half again as much winter fat as their common redpoll and American goldfinch relatives. They also can protect their young from cold as well. Nests are often heavily insulated with thick plant materials and females may literally never cease incubating their eggs and hatchlings, while being fed by their male mate.^[7]