   #copyright

Crossbill

2007 Schools Wikipedia Selection. Related subjects: Birds

                          iCrossbills
   Red (Common) Crossbill
   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. The three to
   five (or possibly many more) species are all currently classified in
   the genus Loxia, but see below. These birds are characterised by the
   mandibles crossing at their tips, which gives the group its English
   name.

   These are specialist feeders on conifer cones, and the unusual bill
   shape is an adaptation to assist the extraction of the seeds from the
   cone. These are birds typically found in higher northern hemisphere
   latitudes, where their food sources grows. They will 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.

   Adult males tend to be red or orange in colour, and females green or
   yellow, but there is much variation.

Feeding behaviour

   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. 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 an 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 behaviour 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).

   They can utilise other conifers to their preferred, and often need to
   do so when their preferred species has a crop failure, but are less
   efficient in their feeding (not enough to prevent survival, but
   probably enough to reduce breeding success).

Systematics and evolution

   Analysis of mitochondrial cytochrome b sequence data (Arnaiz-Villena et
   al., 2001) indicates that the crossbills and redpolls share a common
   ancestor and only diverging during the Tortonian (c. 8 mya, Late
   Miocene). They suggest that the crossbills might be included in the
   genus Carduelis, but given that the adaptations of the crossbills
   represent a unique evolutionary path, it seems more appropriate to
   split up the genus Carduelis as it was already done during most of the
   20th century.

   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.

   The identification problem is least severe in North America, where only
   Red and White-winged occur, and (possibly) worst in the Scottish
   Highlands, where three 'species' breed, and Two-barred is also a
   possible vagrant.

   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. Few ornithologists yet seem
   inclined to give these forms species status though. Preliminary
   investigations in Europe and Asia suggest an equal, if not greater,
   complexity, with several different call types identified; these call
   types as different from each other as from the named species Scottish
   and Parrot Crossbills - suggesting either that they are valid species,
   or else that the Scottish and Parrot may not be.

   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. sibirica, L. gmelinii, L.
            laricina and (in North America) also Hemlock Tsuga
     * Hispaniolan Crossbill, Loxia megaplaga (previously treated as a
       race of Two-barred Crossbill)
          + Hispaniolan Pine Pinus occidentalis

   Retrieved from " http://en.wikipedia.org/wiki/Crossbill"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
