Kwong Wai Chong sent in images of a Common Myna (Acridotheres tristis) seen with its partner foraging at the beach. The myna has a cross bills, a deformed condition (above left). The condition has not apparently affected the bird’s feeding ability (above right).
According to Craves (1994), deformed bills may have the mandibles crossed, either mandible curved upwards or downwards or even elongated. Causes may be genetic, development, injury or disease. And depending on the severity of the deformity, the bird may or may not survive as a result of feeding difficulties.
Natural cross bills are seen in certain finches popularly known as Crossbill (Loxia spp.) of the family Fringillidae. Here, the mandibles may cross either to the left or right. The bill in Crossbills is adapted for opening conifer cones, enabling the bird to get at the seeds.
Craves, J.A. 1994. Passerines with deformed bills. North American Bird Bander, 19(1):14-18.