Historical and geographical patterns in Knemidocoptes mite infestations in Southern California birds

Abstract

We investigated the causes of toe and foot loss and other deformities long observed in urban Brewer’s Blackbirds (Euphagus cyanocephalus) in southern California. Histopathologic evaluation showed that afflicted individuals suffered from infestations of mites compatible with Knemidokoptes spp. (scaly-leg mites). We developed a case definition based on gross lesions in confirmed cases and the scientific literature to search two large ornithological collections for specimens exhibiting these lesions. In evaluating specimens among seven species of the family Icteridae, we found 34 specimens in the two collections with lesions consistent with Knemidokoptes spp. Species afflicted included the Red-winged Blackbird (Agelaius phoeniceus; 12 of 978 specimens), Brewer’s Blackbird (10/337 specimens), Tricolored Blackbird (A. tricolor; 4/101 specimens), Brown-headed Cowbird (Molothrus ater; 4/828 specimens), and Great-tailed Grackle (Quiscalus mexicanus; 4/224 specimens). The earliest cluster of California specimens dated to 1962. Fourteen of the 34 specimens exhibiting the condition were collected since 1999. No specimens of the Yellow-headed Blackbird (Xanthocephalus xanthocephalus; 0 of 214 specimens) or Western Meadowlark (Sturnella neglecta; 0/278) were found with the condition.