AUTHOR BLOG: How the ‘Mitey’ Have Fallen: Impacts of Burrowing Skin Mites on Reproduction of an Urban Raptor

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Black Sparrowhawks displaying symptoms of mite infection.

Julia L. van Velden

Linked paper: Negative effect of mite (Knemidokoptes) infection on reproductive output in an African raptor by J.L. van Velden, A. Koeslag, O. Curtis, T. Gous, and A. Amar, The Auk: Ornithological Advances 134:3, July 2017.

Parasites were once considered to be one of the less important factors that limit or regulate animal populations, with the impacts of predators and resource limitation previously receiving far more attention. This lack of attention probably stemmed from the mistaken belief that most parasites have evolved not to harm their hosts too much, because if their host dies, they lose the resource they depend on. We now know, however, that parasites can often strongly affect both a host’s reproduction and survival rates. Our new study published in The Auk: Ornithological Advances adds to this knowledge for a relatively understudied parasite in a wild raptor population.

Knemidokoptes mites are a genus of microscopic skin mites which burrow into the skin of birds and cause the “scaly leg” and “scaly face” conditions that are frequently seen in caged and domestic birds. They also occur in some wild species, particularly passerines. However, these parasites have rarely been recorded on raptors, except on captive birds. Additionally, almost no research has been carried out to investigate the impacts of these parasites on species’ fitness. Our study explored the symptoms of infection and the impact these mites have on the breeding performance of a wild population of Black Sparrowhawks in Cape Town, at the southernmost tip of South Africa.

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Knemidokoptes skin mites.

Black Sparrowhawks are a recent colonist to this mostly urban area, and urban living may come with associated changes in exposure to parasites and pathogens. Our study population has been closely monitored since 2001 and has grown steadily over the years, with the population now containing around 50 breeding pairs each year. In 2007, we started to notice birds in the population with strange symptoms, namely balding heads and scaly lesions on their legs. These birds appeared to be agitated and in poor condition. Post-mortem investigations revealed that, in all cases, birds with these symptoms were infected with the burrowing skin mite (Knemidokoptes spp.). We found that in some years, up to 5% of the Cape Peninsula population was infected, which represents a highly novel finding for a wild population of raptors.

Comparing between the sexes, we also found that mite infection was more frequent for males than females. Higher parasitic infection of males has been found for several other studies in different parasites and may be the result of fundamental biological and behavioural differences. In our population, we suspect that Black Sparrowhawks may become infected by these mites from their prey, possibly domestic chickens, which are known to frequently be infected by Knemidokoptes mites. Like most Accipiters, Black Sparrowhawks pluck their prey before consumption, which may mean they have greater exposure to this parasite than other raptor species, and the fact that males are responsible for hunting throughout the breeding season may explain the male bias in infection.

Most importantly, we found that Black Sparrowhawks that were infected with these mites had considerably reduced breeding success. We compared breeding performance between infected and non-infected birds and also between birds pre- and post-infection. These analyses showed that infection reduced breeding performance by over 50%. This could be because adults become too agitated to incubate or hunt effectively following infection.

We also investigated if this infection was present anywhere else in South Africa and found four hotspots of infection. Three of the infection sites were cities, and thus infection by this mite may be associated with urbanization levels and the additional stresses this may incur. Other research has, however, not yet detected any negative effect of urbanisation on this species’ health.

Our study, the first on Knemidokoptes mites within a wild population of raptors, therefore suggests that this parasite could play a role in limiting the breeding performance of infected populations. Although Black Sparrowhawks are not a species of conservation concern, this study provides important information on the negative role such parasites can play in their host’s reproductive success, which will be important if this infection is found to occur in an endangered raptor species.

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