There are currently four 4 species in Loxia. They are the white-winged crossbill (L. leucoptera), parrot crossbill (L. pityopsittacus), Scottish crossbill (L. scotica), and the red, or common crossbill (L. curvirostra). The last three are virtually identical to each other in plumage (white-winged crossbills have distinctive wingbars and have slightly different overall coloration).
The "species" curvirostra is a grab bag of about 20 different forms, or subspecies. These forms vary more extensively in bill and body size than most other geographically variable "species" of birds. Some forms, such as ones on some Mediterranean islands, are almost as large as the parrot crossbill; others are far smaller, such as the North American Type 3 population and others in China, the Himalayas, and the Philippines. The underlying hypothesis used in this guide is that these forms are full species in their own right.
Crossbills are found primarily in association with conifer trees that have cones on them; they use their unusual bills to pull out tiny pine nuts from between cone scales. New green cones that are just about to turn brown (or ripe brown cones laden with seeds) are preferred.
All kinds of crossbills forage on spruces and hemlocks, which have small cones. Some big-billed crossbills can handle large, tough cones such as those of ponderosa pine. Other kinds of pine have cones too large and tough (or seeds with husks too thick) for even the biggest-billed crossbills to use.
For many kinds of conifer trees, the production of cones does not occur every year. Whole forests may have good cone crops one year, then become unproductive for several years. Thus, unlike most birds, which have defined annual patterns of migration, crossbills wander like nomads over vast areas in a search for ripe cones. If they don't find cones, they eat insects or other seeds.
Birdwatchers know that if crossbills can be found in a place one year, chances are that there won't be crossbills there again until years later, when the local conifers produce another crop. Their surprising and unpredictable movements make crossbills extremely exciting for birdwatchers.