Crossbill

The crossbills are birds in the finch family Fringillidae. The one to five (or possibly many more) species are all in the genus Loxia. These birds are characterized by the mandibles crossing at their tips, which gives the group its English name.

These birds are specialist feeders with a diet of conifer cones, and the unusual bill shape is an adaptation to assist the extraction of the seeds from the cone. They are typically found in higher northern hemisphere latitudes, where their food sources grow. They will leave their breeding range when the cone crop fails.

The different species are each adapted to specializing in feeding on different conifer species, with the bill shape optimized for opening that specific type of conifer. They can utilize other conifers than their preferred, and often need to do so when their preferred species has a crop failure, but are less efficient in their feeding (not enough to prevent survival, but probably enough to reduce breeding success).

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 color, 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 subtle 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 (possibly) worst in the Scottish Highlands, where three ‘species’ breed, and Two-barred is also a possible vagrant.

Work on vocalization in North America suggest that there are eight or nine discrete populations of Red Crossbill in that continent alone, which do not interbreed and are (like the named species) adapted to specialize on different conifer species. Few ornithologists yet seem inclined to give these forms species status though. Preliminary investigations in Europe and Asia suggest an equal, if not greater, complexity, with several different call types identified; these call types as different from each other as from the named species Scottish and Parrot Crossbills – suggesting either that they are valid species, or else that the Scottish and Parrot may not be.

Genetic research on their DNA has so far failed to reveal any difference between any of the crossbills (including the morphologically distinct Two-barred), with variation between individuals greater than any difference between the taxa. One suggestion is that limited interbreeding between the different types prevents significant genetic differentiation, and also enables each type to maintain a degree of morphological plasticity, which may be necessary to enable them to feed on different conifers when their preferred food species has a crop failure.