The feather holes on the barn swallow Hirundo rustica and other small passerines are probably caused by Brueelia spp. lice.

Barn swallows Hirundo rustica often have characteristic feather holes on wing and tail feathers. During the past 15 yr, several influential papers have been based on the assumption that these holes were chewed by the louse Machaerilaemus malleus. We gathered feather-hole data from barn swallows and other passerines at 2 sites in Hungary and correlated the presence of holes with louse infestations and, more specifically, with the occurrence of M. malleus versus other species of avian lice. The shape of frequency distribution of holes was left-biased, and this bias was more pronounced in large swallow colonies that in a random sample, in accordance with the view that the causative agent of the 'feather hole symptom' is a contagious macroparasite. However, both intra- and interspecific comparisons suggest that the causative agent of the symptom had probably been misidentified. The occurrence of Brueelia spp. 'wing lice' provides the best fit to the distribution and abundance of feather holes, both in barn swallows and across some other small passerines. This identification error does not challenge the results of the former evolutionary-ecological studies based on this model system, although it has important implications from the viewpoint of louse biology.