Common Crossbill

The Common Crossbill (Loxia curvirostra) is a small passerine bird in the finch family Fringillidae. It breeds in the spruce forests of North America, where it is known as Red Crossbill, as well as Europe and Asia; some populations (possibly different species) breed in pine forests in certain areas of all three continents, and in North America, also in Douglas-fir. It nests in conifers, laying 3–5 eggs. Adult males tend to be red or orange in colour, and females green or yellow, but there is much variation.

This crossbill is mainly resident, but will regularly irrupt south if its food source fails. This species will form flocks outside the breeding season, often mixed with other crossbills.

Crossbills are characterized by the mandibles crossing at their tips, which gives the group its English name. 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. In North America, nine distinct Red Crossbill variants (referred to as call types) differing in vocalizations as well as beak size and shape are recognized. Each call type evolved to specialize on different species of conifer.

This species is difficult to separate from Parrot Crossbill and Scottish Crossbill, both of which breed within its Eurasian range. Plumage distinctions from Parrot and Scottish Crossbills are negligible. The head and bill are smaller than in either of the other species. Care is needed to identify this species. The glip or chup call is probably the best indicator. The identification problem is less severe in North America, where only Red Crossbill and White-winged Crossbill occur. However, there has been debate as to whether different call types should be considered separate species. For example, the South Hills Crossbill, occurring in the South Hills and Albion Mountains in Idaho has recently been described as a new species (Loxia sinesciuris)because it shows a very low degree of hybridization with the Red Crossbill. Additionally, recent genetic evidence indicates that there are genetic differences between the call types. Nevertheless, few ornithologists seem inclined to give these forms species status at present.

Some large-billed, pine-feeding populations currently assigned to this species in the Mediterranean area may possibly be better referred to either Parrot Crossbill or alternatively to new species in their own right, but as yet, research into them is still at a very early stage. These include Balearic Crossbill L. curvirostra balearica and North African Crossbill L. curvirostra poliogyna, feeding primarily on Aleppo Pine (Pinus halepensis); Cyprus Crossbill L. curvirostra guillemardi, feeding primarily on European black pine (Pinus nigra); and an as-yet unidentified crossbill with a Parrot Crossbill-size bill feeding primarily on Bosnian Pine (Pinus heldreichii) in the Balkans. These populations also differ on plumage, with the Balearic, North African and Cyprus races having yellower males, and the Balkan type having deep purple-pink males; this however merely reflects the differing anthocyanin content of the cones they feed on, as these pigments are transferred to the feathers.