The Bird and its Unusual Bill

The Crossbill and the Unusual Bill of this Bird

The Common or Red Crossbill, Loxia Curvirostra

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The Crossbill is the Loxia genus of birds which is part of the Fringillidae (FINCH) family.  The name comes from the unusual quality that they have, whereby the horny sheaths of the upper and lower part of the bill cross each other slantwise (this peculiarity has, however, been many times observed as a deformity in other groups of birds, and especially in Crows.  Such cases may be well compared to the anomaly often seen in Rabbits wherein the front teeth reach an disproportionate size).  This slanted crossing of the bill also inspired the scientific name Loxia, derived from the greek loksos meaning oblique, which was given to this genus of birds by Gesner and carried on by Linnaeus.

When first seen, the crossed bill almost looks like a physical defect or mutation similar to that mentioned above in rabbits. Ornitholigists and writers in the past have often referred to the bill is this manner.  The special animosity of De Buffon on this point may perhaps be explained by the existence of a mediaeval legend (of which, however, be it said, he takes no notice), best known to English readers by Longfellow's pretty version of Mosen's poem, to the effect that the bird acquired its peculiarly shaped bill and colour of its feathers in recognition of the pity it bestowed on the suffering Jesus at the crucifixion, and the Crossbill's attempts to pull the nails from the cross, resulting in Jesus' blood spurting out over the bird's breast and staining it for all time.  Schwenckfeld in 1603 (Theriotropheum Silesiae, pp. 253, 254) gave the fable in the Latin verses of Johannes Major, which have been reprinted in Notes and Queries (series 5, vii. page 505)).

Rather than a defect though, the crossed bill is a wonderful adaption to the habits of these birds.  This design makes it possible for the seeds of fir cones from coniferous trees to be picked out with very little difficulty.  These seeds represent the Crossbill's main food although apple pips are also a favoured treat.  These birds will also feed happily on insects and other fruit seeds in the winter, at which time they will travel in any direction on the compass to visit deciduous forests, gardens, flat plains and groves in search of food.

It appears that the first description of the Crossbill's method of using its bill was by Townson in 1799 in his book "Tracts on Natural History", although the account was quite concise.   A more detailled explanation was provided by Yarrell in 1829 (Zoological Journal, pages 459 to 465), in which he described how the muscles and jaws are so masterfully capable of tearing apart cones, or fruit like apples.

The moveable joint between the quadrate bone and the mandible enables a large amount of sideways movement.  With the special way in which this bird's muscles move the mandible, as soon as the Crossbill opens its beak the mandible's point becomes located directly opposite the point of the maxilla.  The bird then inserts its bill into the fir cone or in the fleshy fruit, and then opens its bill a little further.  This additional widening of the bill, and a forceful sideways movement of the mandible whilst inside the fir cone or fruit results in the seed being torn away.  Then, just at the right time, the tongue shoots out to obtain the seed, and almost as soon as the tongue has darted out, it is quickly retracted into the Crossbill's mouth with the treasured seed itself.

The breeding season for the Crossbill seems to be more variable than other small birds, but normally eggs are laid at the end of February or beginning of March.  The nest is large and rougly-made of of spruce sticks and lichen, and is built in the coniferous trees which form the bird's main habitat.  Around four eggs are normally laid, which are similar in appearance to, but bigger than, those of the GREENFINCH.
Next...The Common or Red Crossbill, Loxia Curvirostra.

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