The nest of the siskin is very like that of the goldfinch, but seldom so neatly built; the eggs, except in their smaller size, much resemble those of the greenfinch.
A larger and more brightly coloured species, C. spinoides, inhabits the Himalayas, but the siskin has many other relatives belonging to the New World, and in them serious modifications of structure, especially in the form of the bill, occur.
Thus the siskin perhaps may be regarded as one of the less modified descendants of a stock whence such forms as those just mentioned have sprung.
Among the smaller birds may be enumerated finches, the siskin, bullfinch, pipit, titmouse, wagtail, lark, fine-crested wren, hedge-sparrow, corn-wren, nut-hatch, starling, swallow, martin, swift, thrush, butcher bird, shrike, dipper, yellow-hammer, ortolan and a warbler (Accentor alpinus).
Aristotle seems to recognize eight principal groups: (1) Gampsonyches, approximately equivalent to the Accipitres of Linnaeus; (2) Scolecophaga, containing most of what would now be called Oscines, excepting indeed the (3) Acanthophaga, composed of the goldfinch, siskin and a few others; (4) Scnipophaga, the woodpeckers; (5) Peristeroide, or pigeons; (6) Schizopoda, (7) Steganopoda, and (8) Barea, nearly the same respectively as the Linnaean Grallae, Anseres and Gallinae.
Hybrids are also common, the canary breeding freely with the siskin, goldfinch, citril, greenfinch and linnet.
On the upper verge of the pine forests, or in the scrubby vegetation just beyond, the following are not uncommon - black woodpecker (Picus martius), ring-ousel (Turdus torquatus), Bonelli's warbler (Phylloscopus Bonellii), crested tit (Parus cristatus), citril finch (Citrinella alpina), siskin (Chrysomitris spinus), crossbill (Loxia curvirostra), nutcracker (Nucifraga caryocatactes), blackcock (Tetrao tetrix), and the alpine varieties of the marsh-tit (Parus palustris, borealis) and tree-creeper (Certhia familiaris, costae).