White-winged Becard (Pachyramphus polychopterus) Science Article 1
Bill deformities have been recorded for several bird species around the world. Craves (1994) presented a revision on bill deformities in passerines (Aves: Passeres), which was updated by the Rouge River Bird Observatory (University of Michigan-Dearborn 2004). Bill abnormalities are known to occur in low frequency in bird populations (less than 0.5%) and may be caused by injury, poor nutrition, genetic or developmental diseases, and chemical pollutants (Pomeroy 1962, Craves 1994).
Marcelo Ferreira de Vasconcelos and Marcos Rodrigues, Revista Brasileira de Ornitologia 14 (2) 165-166