High mite burdens in an island population of Cape Wagtails Motacilla capensis: release from predation pressure?

Abstract

Cape Wagtails Motacilla capensis have been identified as a species susceptible to infection by the mite Knemidokoptes jamaicensis , but the processes influencing infection rates and prevalence have not been studied. We assessed the mite infection level of 117 Cape Wagtails captured during the 2005 breeding season on Dassen Island, South Africa, analysed mite infection as  functions of morphometrics and fat, and compared prevalence to other sites. Knemidokoptic mite infection was found in 42% of wagtails, which was more than double the prevalences reported for conspecifics on the mainland, and high  compared to other host species. Mite infection of birds captured on the island was explored graphically against the morphometrics of individuals using cubic splines. Formal statistics were then applied using generalised linear mixed models, with observation and unique ring number as random effects within a two-level hierarchical mixed binomial model, and the fixed explanatory morphometric data being natural log-transformed. Morphometric data supported  the idea that larger individuals were more likely to exhibit signs of mite infection. Possible contributing factors to this high percentage of mite-infected wagtails are discussed, with one possibility being that the low levels of predation on the island allow larger individuals to carry the cost of mite infection.

OSTRICH 2012, 83(2): 85–89