The relationship of louse infestation and clutch size in the Common Kestrel (Falco tinnunculus)

SUMMARY Background: Lice are common ectoparasites of birds, completing their entire life cycle in the plumage. Transmission – in most cases – requires physical contact between hosts. Lice may benefit from choosing host individuals with better survival prospects and dispersal chances. Bodily contacts between parents and offsprings provide a good opportunity for host selection. The Common Kestrel is a widespread, small-bodied raptor of the Palearctic region, with well-known breeding biology and ectoparasite fauna, making it suitable to study the ecological correlates of vertical transmission of lice. Objectives: The aim of our study was to investigate how the sex, maturity and clutch size of Common Kestrel nestlings affect the abundance of their lice. Materials and Methods: Field work was carried out in Körös-Maros National Park Directorate (Hungary). The ectoparasites were collected from nestlings (n = 54) with dust-ruffling. The effect of the host’s sex, wing length and clutch size on the abundance of their lice was analysed using negative binomial mixed models. Results and Discussion: In case of Colpocephalum subzerafae, none of the investigated variables had a significant effect. Contrarily, clutch size had a significant effect on Degeeriella rufa abundance. In small clutches (3–4 nestlings) the mean abundance of D. rufa (15.5 95% C.I.: 8–30) is 3.7 times higher, than in large (5–6 nestlings) clutches (4.2 95% C.I.: 2.3–7.4). We discuss two non-exclusive explanations of this pattern. First, parental quality is known to affect the clutch size. If low quality parents are also more heavily infested with lice, this could explain the higher louse load of their nestlings. Alternatively, according to the dilution hypothesis, long life-cycle ectoparasites (such as lice), that are incapable to significantly raise their subpopulation size till the fledging of the chicks, disperse among the nestlings, resulting in lower per nestling louse counts in larger clutches. According to our results both the breeding parameters and the quality of the parents may affect their offsprings’ ectoparasite load