Loxia
| Crossbill | |
|---|---|
 |
|
| Red crossbill or common crossbill (Loxia curvirostra) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Order: | Passeriformes |
| Family: | Fringillidae |
| Genus: |
Loxia Linnaeus, 1758 |
| Species | |
|
Loxia pytyopsittacus |
|
Loxia pytyopsittacus
Loxia scotia
Loxia curvirostra
Loxia leucoptera
Loxia megaplaga
The crossbill is a genus, Loxia, of birds in the finch family (Fringillidae), with three to five (or possibly many more) species. These birds are characterised by the mandibles with crossed tips, which gives the group its English name. Adult males tend to be red or orange in colour, and females green or yellow, but there is much variation.
The genus name Loxia is from the Ancient Greek loxos, "crosswise".
Crossbills are specialist feeders on conifer cones, and the unusual bill shape is an adaptation which enables them to extract seeds from cones. These birds are typically found in higher northern hemisphere latitudes, where their food sources grow. They erupt out of the breeding range when the cone crop fails. Crossbills breed very early in the year, often in winter months, to take advantage of maximum cone supplies.
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 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: the fitness of each morph is inversely proportional to its frequency in the population. 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 of this morph decreasing.
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Wikipedia