No abstract

Sub-lingual oral fistulas are a consistently observed lesion affecting the New Zealand stitchbird (hihi: Notiomystis cincta). This lesion, which has not been reported in other species, is usually only recognized when the tongue protrudes below the bird's mandible from a hole in the oral-cavity floor. In this study, we surveyed the prevalence of oral fistulas in a free-living population of stitchbirds on Tiritiri Matangi Island in 2002, 2003 and 2005. Between surveys, individuals with a fistula were caught and the progress of their lesion was monitored. The majority of birds with a fistula had a small localized lesion alongside the edge of the mandible without the tongue protruding. Oral fistulas were generally not associated with any reduction in the bird's condition or productivity, but if the tongue consistently deviated through the fistula it affected nectar-feeding efficiency. No fistulas were found in nestlings, but 9% to 10% of adult birds had some form of oral fistula, suggesting that it developed after fledging. Repeated measurement of birds showed that the size of the fistulas did not progress beyond the formation of the initial hole unless the tongue protruded. This protrusion resulted in continuous rubbing and erosion of the oral cavity floor and, ultimately, the mandible itself. Histopathology confirmed that fistulas occur in the thinnest part of the floor of the oral cavity, at the attachment point of the skin to the mandible. Despite long-term monitoring of this population, the formation of an oral fistula has never been observed and its aetiology remains elusive.