High ectoparasite loads in hole‐nesting birds – a nestbox bias?

All studies of interactions between fleas and hole‐nesting birds so far have been done in nestboxes. This study, which is the first study of flea infestation of nests of hole‐nesters in natural holes, was carried out in the Białowieża National Park (E Poland) and demonstrated that, in contrast to those studies, both prevalence (below 10%) and intensity of infestation nests of Parus palustris and Ficedula flycatchers were very low, an order of magnitude lower than elsewhere. However, in the same forest, flycatchers breeding in nestboxes, and tits in natural holes within the nestbox plot, had significantly higher infestation rates, approaching those found in other studies. These observations indicate that the high flea loads reported for tits and flycatchers may be a product of biased sampling – exclusive collection of data in nestbox areas – rather than reflect their species‐specific characteristics. The data from natural holes undermine the widespread assumption that data on flea loads in nestboxes can be treated as representative, and generalised to areas without nestboxes.