Red crossbills are small passerines in the finch family of Fringillidae, in Eurasia called the common crossbill. They have very specialized, crossed bills and their wings are long and pointed. Males are brick-red and have black wings, while females are greenish-yellow, also with black wings. Juveniles are streaked brown. Red crossbills have very different sizes of bills, correlating with different food and habitat preferences, as well as distinct flight calls, these characteristics splitting Red crossbills into eight separate types. It is likely that in the future the species will be classified as multiple species.
The Red crossbill inhabits North America, southern Alaska, Newfoundland, the northern United States, Central America, North Carolina, Northern Eurasia, northern Africa, the Philippines and south-eastern Asia. Resident within its breeding range, depending on food resources, it may move southwards. This bird lives in coniferous forests, pines or spruces.
Red crossbills usually are found year round in small flocks. They typically climb about in mature conifers, their bills being used to grab cones and branches. They also sometimes land on deciduous trees to forage for aphids. With their bills adapted for getting seeds out of cones, they begin at the bottom and spiral upwards on a cone, prying each scale open and taking out seeds with their tongues. Often they will feed in flocks. These diurnal birds do not migrate but outside the breeding season they range widely searching for good conifer seed crops. They adapt well to cold weather and appear to move as a response to the availability of cone crops. Mass movements occur most often in fall, when the conifer cones ripen. Such movements may involve thousands of individuals and may result in invasions by wandering populations of new regions. Red crossbills make different flight calls, each type of sound uttered by birds of different bill shape. Birds in a flock keep in contact by means of a distinctive flight call, allowing the isolation of different groups.
Red crossbills are monogamous, seeming to stay in pairs during the year. Pairs will form within flocks. A male sings from a perch and makes display flights to attract a female. During the breeding season males are aggressive towards one another. Courtship involves the male feeding the female and the pair grabbing one another by the bill (called billing). The breeding cycle is linked more closely to food availability than to season, and the birds can breed almost any time of the year, even in mid-winter if the source of seeds is abundant. One or two broods are produced per season, depending on the range. The female constructs the nest, on a high horizontal branch in a conifer. 3 to 4 eggs are laid, pale blue-green and spotted with lavender and brown. Incubation is by the female, for about 12 to 16 days, while the male feeds her by regurgitation. The male feeds the altricial chicks for five days, then both parents feed the young. At about 18 to 22 days old they leave the nest, and are fed by their parents for another month. They may become mature before they have their adult plumage, as soon as 100 days old.
Red crossbills currently are widespread and common in their ranges, but these birds depend on mature forests for food. Their populations in most areas seem to be stable, but where deforestation is rapid, there have been some declines.
According to the IUCN Red List, the global Red crossbill population size is estimated at 90-180 million mature individuals. The breeding population in Europe is 5,800,000-13,000,000 pairs, equating to 17,400,000-39 million individuals. National population estimates are: in China: 100-100,000 breeding pairs, with 50-10,000 birds on migration and fewer than 1,000 individuals wintering; in Korea: 100-100,000 breeding pairs, with 50-10,000 birds on migration and fewer than 1,000 wintering birds; in Japan: perhaps 100-100,000 breeding pairs, with 50-10,000 birds on migration and fewer than 1,000 wintering birds; and in Russia: 10,000-100,000 breeding pairs plus 1,000-10,000 birds on migration. Overall, currently Red crossbills are classified as Least Concern (LC) and their numbers today are stable.