Red (Common) Crossbill
The crossbills are birds in the finch family Fringillidae. The four species are all in the genus Loxia. These birds are characterised by the mandibles crossing at their tips, which gives the group its English name.
These are specialist feeders on conifer cones, and the unusual bill shape is an adaptation to assist the extraction of the seeds from the cone. These are birds typically found in higher northern hemisphere latitudes, where, of course, their food source grows. They will erupt out of the breeding range when the cone crop fails.
Crossbills breed very early in the year, often in winter months, to take advantage of maximum cone supplies.
Adult males tend to be red or orange in colour, and females green or yellow, but there is much variation.
These species are difficult to separate, and care is needed even with Two-barred Crossbill, the easiest. The other three species are identified by differences in head shape and bill size, and are the subject of much taxonomic speculation, with some scientists suggesting that two or all three are conspecific.
The identification problem is least severe in North America, where only Red and White-winged occur, and worst in in the Scottish Highlands, where three species breed, and Two-barred is also a possible vagrant.
Work on vocalisation in North America suggest that there are eight or nine populations of Red Crossbill, which do not interbreed, in that continent alone, although few ornithologists seem inclined to give these forms species status.