The Tribune - Spectrum



Sunday, April 16, 2000
Nature


They use their bills as tools
By Nutan Shukla

AS the name suggests, crossbills have been given this name because of their crossed bills, which are one of the strangest bills in the bird world, and also the most important characteristic feature of the genus Loxia, to which they belong. Both mandibles of the bill cross over at the tip, like pairs of bent pliers, and are used to open cones of various conifer trees, to reach the seeds underneath.

Crossbills use their bills to open conesThere are four species of crossbills which are found in Asia, North Africa, North and Central America and Europe. They are stout birds. The males are brick-red while females are yellow-green. They live in coniferous forests.

Crossbills, like other finches, breed whenever food is abundant. This is the reason these birds breed in different forests in different time of the year. For example, they breed in spruce forests in late autumn to winter, in spring it is the turn of the pine forests and in larch jungles breeding takes place mainly in late summer or early autumn. In the forests of mixed variety of trees these birds may breed for 10 months of the year, including winters when hardly any other bird nests. One such example was seen in Moscow where crossbill nests were found in the month of February when the air temperature was -19C, but inside the nest eggs were being incubated by the female at a temperature of 38C.

  Apart from parents crossbill nestlings, too, have the ability to survive in severe cold. They can sustain themselves in air temperature down to -36C. They often face such situations when parents have to leave them alone for finding food and they have to deal with the cold all by themselves. Whenever, nestlings are left alone they become torpid, but soon recover when brooded by the parents under whom the temperature may be as high as 55C.

Nestlings of these birds do not have crossed bills when they are born instead their mandibles are straight, like other birds, but by the fourteenth day the upper mandible starts coming longer and the crossing over of the two mandibles takes place after four weeks so that the parents can continue to feed them. As far as the crossing of the bill is concerned it is only the horny sheath of the bill which is bent and curved and not the underlying bones.

The crossed bill of the bird may look awkward, but in reality it is one of the most efficient tools in the world. This specialised bill is employed by the bird in opening unripe, green cones which are tightly closed and are very difficult to open. After holding the cone firmly with one of its large and powerful feet, the tip of the downward-curving, longer upper mandible is wedged between two of the cone scales so that the curve of the lower mandible rests on the outside of the cone. After this the bird twists its head to force the scales apart. While doing this the bird simultaneously moves the two mandibles sideways, wing special jaw muscles.

This bird has a long, protrusible tongue with a special cartilaginous cutting edge. Once the scales have been removed from the cone, the birds insert its specialised tongue inside and detaches the unripe, firmly anchored seed with the help of the tongue.

In goldfinch, a North American bird of the genus Carduelis, the female incubates eggs for long periods of time during which the male feeds his mate. As the male approaches the nest area, the female begins to beg without even seeing him. The fact is, the female listens to the flight call that the male produces during its ‘bounding’ flight. The female recognises its mate’s call because mated pairs have common calls, which are different from the calls of other pairs.

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