crossbill - Feeding behavior

A finch native to the N hemisphere, especially N regions; inhabits coniferous forests. The tips of its bill cross over, an adaptation for extracting seeds from pine cones. (Genus: Loxia, 4 species. Family: Fringillidae.)

iCrossbills
Red (Common) Crossbill
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Aves Order: Passeriformes Family: Fringillidae Genus: Loxia (but see "Systematics and evolution" below) Linnaeus, 1758
Species
Loxia pytyopsittacus Loxia scotia Loxia curvirostra Loxia leucoptera Loxia megaplaga

The crossbills are birds in the finch family Fringillidae.

Feeding behavior

The different species are each adapted to specialising in feeding on different conifer species, with the bill shape optimised for opening that species of conifer.

It is very probable that there is a genetic basis underlying the phenomenon (young birds whose bills are still straight will give a cone-opening behavior if their bills are gently pressed, and the crossing develops before the birds are fledged and feeding independently), but at least in the Red Crossbill (the only species which has been somewhat thoroughly researched regarding this question) there is no straightforward mechanism of heritability.

While the direction of crossing seems to be the result of at least 3 genetic factors working together in a case of epistasis and most probably autosomal, it is not clear whether the 1:1 frequency of both morphs in most cases is the result of genetics or environmental selection: populations that feed on cones without removing or twisting them will likely show a 1:1 morph distribution no matter what the genetic basis may be, as the fitness of each morph is inversely proportional to its frequency in the population due to the fact that such birds can only access the cone with the lower mandible tip pointing towards it to successfully extract seeds, and thus a too high number of birds of one morph will result in the food availability for each bird decreasing (Edelaar et al, 2005).

The species of crossbills are difficult to separate, and care is needed even with Two-barred/Hispaniolan Crossbill, the easiest. The other 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 the previously held assumption that the Parrot and Scottish Crossbills and possibly the Hispaniolan and Two-barred Crossbill are conspecific.

Work on vocalisation 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 specialise on different conifer species.

Species and their preferred food sources are:

Parrot Crossbill, Loxia pytyopsittacus Scots pine Pinus sylvestris Scottish Crossbill, Loxia scotica (often treated as a race of Parrot Crossbill) Scots pine Pinus sylvestris and Larch Larix species (particularly plantations of L. decidua) Red Crossbill or Common Crossbill, Loxia curvirostra Spruce Picea species; some populations (distinct species?) on various Pine Pinus species and (in western North America) Douglas-fir Two-barred Crossbill or White-winged Crossbill, Loxia leucoptera Larch Larix species, particularly L.