Free-ranging passerine birds banded at the Mai Po Nature Reserve, Hong Kong, were affected with a skin disease characterized by crusty, proliferative lesions on legs, feet and beaks. Based on retrospective examination of 1990 to 1992 banding records, 83 of 16,353 birds and 5 of 161 species banded at Mai Po were identified as having these lesions. Eurasian tree sparrows (Passer montanus) were most commonly affected (58 of 411 birds) although 19 of 428 black-faced buntings (Emberiza spodocephala), 1 of 43 little buntings (Emberiza pusilla), 3 of 92 spotted doves (Streptopelia chinensis), and 2 of 616 eastern great reed warblers (Acrocephalus orientalis) also had clinical signs of infestation. Adult Eurasian tree sparrows were affected more often than juveniles and males were affected more often than females. Lesions on juveniles usually were on the feet while on adults lesions also were found on legs and beak. The effects of this parasitic infestation on body weight of wild Eurasian tree sparrows was not significant.