breed and the two-barred is also a possible vagrant. Work on vocalization in North America suggests 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 specialise in different conifer species. While several ornithologists seem inclined to give these forms species status, no division of the American red crossbills has yet occurred. Preliminary investigations in Europe and Asia suggest an equal, if not greater, complexity, with several different call types identified; these call types being as different from each other as from the named species of the parrot and Scottish crossbills - suggesting either that they are valid species, or else that the parrot and Scottish crossbills may not be. Genetic research on their DNA 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. This led to the suggestion that limited interbreeding between the different types prevented significant genetic differentiation, and enabled 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. Research in Scotland, however, has shown that the parrot and Scottish crossbills are reproductively isolated from each other and also from the red crossbill, despite irruption of that species into their ranges, and the diagnostic calls and bill dimensions have not been lost. They are, therefore, good species. Currently accepted species and their preferred food sources are: Originally, the chestnut-backed sparrow-lark (Eremopterix leucotis) was also classified as belonging within the genus Loxia. Feeding behavior The different species specialise in feeding on different conifer species, with the bill shape optimised for opening that species of conifer. This is achieved by inserting the bill between the conifer cone scales and twisting the lower mandible towards the side to which it crosses, enabling the bird to extract the seed at the bottom of the scale with its tongue. The mechanism by which the bill-crossing (which usually, but not always, occurs in a 1:1 frequency of left-crossing or right-crossing morphs) is developed, and what determines the direction has hitherto withstood all attempts to resolve it. It is very probable that there is a genetic basis underlying the phenomenon (young birds whose bills are still straight will give a