The movements of the pine siskin are highly erratic and are usually in response to food availability rather than weather conditions (2) (3) (5), with true migration not thought to occur in this species (3). During times of cold weather, the pine siskin is able to increase its metabolic rate to keep warm, raising it up to 40 percent higher than other similarly sized birds (2).
The diet of the pine siskin is mainly composed of young buds and the seeds of conifers, grass, weeds and deciduous trees. It also takes insects and grubs (2) (3) (5), although these only constitute a small percentage of its diet (3). This species is regularly seen around bird feeders throughout its range. It is able to store food weighing up to ten percent of its total body mass within a part of the throat known as the ‘crop’ (2).
Mating pairs of pine siskins are formed between March and May, when the male performs a courtship display involving flying, singing and feeding the female (3) (6). Mating pairs remain monogamous throughout the nesting period (3).
A gregarious species (2) (5), the pine siskin nests in small colonies (2) (3) (5) (6), although a territory around the nest is defended while eggs are being laid and incubated (3) (6). Breeding individuals forage in small groups (5) (6) and occasionally visit the nests of other pairs. The female pine siskin chooses the nest site and builds the nest over five or six days, although the male may contribute small amounts of nest material (2). The nest is a shallow saucer-shaped structure made with grass, twigs, moss, lichens, bark and roots and is lined with moss, hair, fur and feathers (2) (3) (6).
The female pine siskin commonly lays a clutch of three or four eggs between April and late May (3) (6). The eggs are pale green-blue and spotted with brown and black (2) (3) (6), and are incubated for around 13 days by the female alone, who is fed by the male during this period (2) (3) (6). The young pine siskins fledge the nest after around 15 days, and a second brood may be raised after the first (2).