The Red Crossbill may move out of their home range when food is scarce. When this occurs, they may breed in areas far south or west of their normal range.
The extent of the irruptions depends on a combination of population levels and the availability of food. Like the white-winged crossbill, red crossbill breed when food is abundant, and they can breed in almost any month of the year when they find a mature crop of the appropriate species of conifer.
Multiple broods are often produced when a particularly abundant food source is found. Breeding typically ceases at the autumn equinox and may resume after the annual moult, which occurs in December or January. In summer, incubation typically begins after the entire clutch is laid, in cold weather though, the female will begin incubating immediately after laying the first egg.
The Red Crossbill is a stocky, medium-sized finch with a thick conical bill and a large head. The mandibles curve toward each other and cross at the tip. The brown wings lack the broad, white wing bars of the similar white-winged crossbills. The forked tails are also brown. Most males are red or orange-red, but some may have orange or yellowish-orange colours.
Males are brightest on the forehead, crown, and rump. The mantle and back are coloured like the crown but may be mottled with darker feathers. The underparts are almost entirely red in typical individuals or are yellowish-orange in some variants. The under tail coverts are grayish-white with darker spots. The legs and feet are dark brown.
The female red crossbill is olive-green above and yellowish-green below. The forehead, crown, nape, and back are mottled with dusky brown. The tail and wings are brown. The throat of most types is gray or yellowish with gray along the sides.
Juveniles and first-year birds are paler than adult females with dark stripes. Some may display wing bars, which are thinner (especially the upper wing bar) than those of the white-winged crossbill, and may have a buffy tinge rather than a pure white colour. First-year males resemble adults, but they show patches of red with yellow or brown feather tips.