Adults are often brightly coloured, with red or orange males and green or yellow females, but there is wide variation in beak size and shape, and call types, leading to different classifications of variants, some of which have been named as subspecies.

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.

The distinctive crossed mandibles eliminate most other species, but this feature is shared by the similar two-barred crossbill, with which it overlaps considerably in range.

[3][5] Red crossbill irruptions in the British Isles occur very infrequently, and were remarked upon by writers dating back to the 13th century.

[6] The engraver Thomas Bewick wrote that "It sometimes is met with in great numbers in this country, but its visits are not regular",[7] 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.

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.

"[7] The red crossbill was formally described by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae under the binomial name Loxia curvirostra.

[3][14] The different populations of red crossbill are referred to as "types" because they are differentiated by a phenotypic feature (call note) and not all of them have yet been proven to be genetically distinct.