|Male Red Crossbill|
|Female Red Crossbill|
The Red Crossbill (Loxia curvirostra) is a small passerine bird in the finch family Fringillidae, also known as the Common Crossbill in Eurasia. Crossbills have distinctive mandibles, crossed at the tips, which enable them to extract seeds from conifer cones and other fruits.
Adults are often brightly coloured, with red or orange males and green or yellow females, but there is wide variation in colour, beak size and shape, and call types, leading to different classifications of variants, some of which have been named as subspecies.
Crossbills are characterized by the mandibles crossing at their tips, which gives the group its English name. Using their crossed mandibles for leverage, crossbills are able to efficiently separate the scales of conifer cones and extract the seeds on which they feed. Adult males tend to be red or orange in colour, and females green or yellow, but there is much variation.
In North America, nine distinct Red Crossbill variants (referred to as call types) differing in vocalizations as well as beak size and shape are recognized. Each call type evolved to specialize on different species of conifer. 
Breeding and irruption 
The Red Crossbill breeds in the spruce forests of North America, as well as Europe and Asia. Some populations breed in pine forests in certain areas of all three continents, and in North America, also in Douglas-fir. It nests in conifers, laying 3–5 eggs.
This crossbill is mainly resident, but often irrupts south when its food source fails. This species forms flocks outside the breeding season, often mixed with other crossbills. The first known irruption, recorded by the chronicler Matthew Paris, was in 1254; the next appears to have been in 1593. The engraver Thomas Bewick wrote that "It sometimes is met with in great numbers in this country, but its visits are not regular", adding that many hundreds arrived in 1821. Bewick then cites Matthew Paris as writing "In 1254, in the fruit season, certain wonderful birds, which had never before been seen in England, appeared, chiefly in the orchards. They were a little bigger than Larks, and eat the pippins of the apples [pomorum grana] but no other part of them... They had the parts of the beak crossed [cancellatas] by which they divided the apples as with a forceps or knife. The parts of the apples which they left were as if they had been infected with poison." Bewick further records an account by Sir Roger Twysden for the Additions to the Addittamenta of Matt. Paris "that in the apple season of 1593, an immense multitude of unknown birds came into England ... swallowing nothing but the pippins, [granella ipsa sive acinos] and for the purpose of dividing the apple, their beaks were admirably adapted by nature, for they turn back, and strike one point upon the other, so as to show ... the transverse sickles, one turned past the other."
Taxonomy and systematics 
This species is difficult to separate from Parrot Crossbill and Scottish Crossbill, both of which breed within its Eurasian range, as plumage distinctions from those two species are negligible, though the head and bill are smaller than in either of the other species. Care is needed in identification, especially in Eurasia, where the glip or chup call is probably the best indicator. The identification problem is less severe in North America, where only Red Crossbill and White-winged Crossbill occur. However, there has been debate as to whether different call types should be considered separate species. For example, the South Hills Crossbill, occurring in the South Hills and Albion Mountains in Idaho has been described as a new species (Loxia sinesciuris) because it shows a very low degree of hybridization with the Red Crossbill. There are also genetic differences between the call type populations. Nevertheless, few ornithologists have chosen to give these forms species status.
Some large-billed, pine-feeding populations currently assigned to this species in the Mediterranean area may possibly be better referred to either Parrot Crossbill or to new species in their own right, but more research is needed. These include Balearic Crossbill L. curvirostra balearica and North African Crossbill L. curvirostra poliogyna, feeding primarily on Aleppo Pine (Pinus halepensis); Cyprus Crossbill L. curvirostra guillemardi, feeding primarily on European black pine (Pinus nigra); and an as-yet unidentified crossbill with a Parrot Crossbill-size bill feeding primarily on Bosnian Pine (Pinus heldreichii) in the Balkans. These populations also differ on plumage, with the Balearic, North African and Cyprus races having yellower males, and the Balkan type having deep purple-pink males; this however merely reflects the differing anthocyanin content of the cones they feed on, as these pigments are transferred to the feathers.
|Distinct Eurasian Common Crossbill
based on calls
|The Sound Approach's
list based on calls
|Balearic Crossbill, Loxia curvirostra balearica||Aleppo Pine, Pinus halepensis|
|North African Crossbill, Loxia c. poliogyna||Aleppo Pine, Pinus halepensis||3E|
|Corsican Crossbill, Loxia c. corsicana||European Black Pine, Pinus nigra|
|Cyprus Crossbill, Loxia c. guillemardi||European Black Pine, Pinus nigra||5D|
|Crimean Crossbill, Loxia c. mariae||European Black Pine, Pinus nigra?|
|Luzon Crossbill, Loxia c. luzoniensis||Khasi Pine, Pinus kesiya|
|Annam Crossbill, Loxia c. meridionalis||Khasi Pine, Pinus kesiya|
|Altai Crossbill, Loxia c. altaiensis||Spruces|
|Tien Shan Crossbill, Loxia c. tianschanica||Schrenk's Spruce, Picea schrenkiana|
|Himalayan Crossbill, Loxia c. himalayensis||Himalayan Hemlock, Tsuga dumosa|
|Japanese Crossbill, Loxia c. japonica|
|Other Eurasian Crossbills|
|1A||'British Crossbill'||Type E - flight call "Chip"|
|1B||'Parakeet Crossbill'||Type X - flight call "Cheep"|
|2B||'Wandering Crossbill'||Type A - flight call "Keep"|
|Parrot Crossbill, Loxia ptyopsittacus||2D|
|Scottish Crossbill, Loxia scotica||3C|
|'Bohemian Crossbill'||Type B - flight call "Weet"|
|4E||'Glip Crossbill'||Type C - flight call "Glip"|
|'Phantom Crossbill'||Type D - flight call "Jip"|
|'Scarce Crossbill'||Type F - flight call "Trip"|
|North American Red Crossbill subspecies based on biometrics||Jeff Groth's list
|Recorded on tree species
(Jeff Groth call types)
|Newfoundland Crossbill, Loxia c. percna||Type 8||Black Spruce, Picea mariana|
|Lesser Crossbill, Loxia c. minor||Type 3||Western Hemlock, Tsuga heterophylla|
|Sitka Crossbill, Loxia c. sitkensis (probably a junior synonym of L. c. minor)||Type 3||ditto|
|Loxia c. neogaea||Type 1||Tsuga species, Picea glauca, Pinus strobus|
|Loxia c. neogaea||Type 4||Douglas Fir, Pseudotsuga menziesii|
|Rocky Mountain Crossbill, Loxia c. benti||Types 2, 7||Type 2: Rocky Mountains Ponderosa Pine Pinus ponderosa scopulorum in west, various Pinus species in east; Type 7: possibly a general diet|
|Sierra Crossbill, Loxia c. grinnelli||Type 2, 7||ditto|
|Bendire Crossbill, Loxia c. bendirei||Type 2, 7||ditto|
|Mexican Crossbill, Loxia c. stricklandi||Type 6||Pine species in section Trifoliae|
|Central American Crossbill, Loxia c. mesamericana|
|South Hills Crossbill (described as Loxia sinesciuris in 2009)||Type 9||Isolated population of Lodgepole Pine, Pinus contorta latifolia|
- Benkman, C. W.; Parchman, T. L. and E. Mezquida (2010). "Patterns of coevolution in the adaptive radiation of crossbills.". Annals of the New York Academy of Sciences 106.
- Benkman, C. W. (2003). "Divergent selection drives the adaptive radiation of crossbills". Evolution 57 (5).
- Perry, Richard Wildlife in Britain and Ireland Croom Helm Ltd. London 1978 pp. 134–5.
- Bewick, Thomas (1847 (revised edition)). A History of British Birds, volume I, Land Birds. pp. 234–235.
- Benkman, Craig W.; Smith, Julie W.; Keenan, Patrick C.; Parchman, Thomas L.; Santisteban, Leonard (2009). "A New Species of the Red Crossbill (Fringillidae: Loxia) From Idaho". The Condor 111 (1): 169–176. doi:10.1525/cond.2009.080042.
- Parchman, T. L.; C. W. Benkman (2006). "Patterns of genetic variation in the adaptive radiation of New World crossbills (Aves: Loxia)". Molecular Ecology 15.
- Introduction to vocalizations of crossbills in north-western Europe
- Bangs, Outram (12 October 1932). "Birds of Western China Obtained by the Kelly–Roosevelt Expedition". Field Museum of Natural History Publication, Zoological Series 18 (11): 343–379.
- BirdLife International (2004). "Loxia curvirostra". IUCN Red List of Threatened Species. Version 2006. International Union for Conservation of Nature. Retrieved 12 May 2006.
- Ageing and sexing (PDF; 2.9 MB) by Javier Blasco-Zumeta & Gerd-Michael Heinze