Ectoparasites exposure affects early growth and mouth colour in nestlings of a cavity-nesting raptor

Nest-dwelling ectoparasites are a major source of stress for growing nestling birds, as they can negatively impact on growth and physiology, with consequences on pre-fledging mortality. Ectoparasites are expected to also affect chromatic traits involved in parent-offspring communication, because their expression requires the same resources used by nestlings for parasites defence and/or it is linked to individual state. However, the effects of ectoparasites on growth, condition and expression of begging components are poorly known, especially in raptors. Here, we examined the effects of an experimental ectoparasites removal on growth (body mass and forearm length) and expression of begging traits (size and colour of flange, cere and gape) in lesser kestrel (Falco naumanni) nestlings. Our manipulation was very effective in removing ectoparasites and parasite load had several detrimental effects on early growth and expression of visual signals directed to parents. Deparasitized nestlings grew faster in terms of body mass and forearm length, as well as mouth width and cere length. In addition, the red chromaticity of cere decreased with age, with deparasitized nestlings showing a faster decrease concomitantly with the increase in body growth. Such negative effects disappeared in later nestling stages and ectoparasites did not affect pre-fledging survival. Our results showed that visual components of begging have the potential to reveal individual condition during early nestling stages, above and beyond ectoparasite infection. They are also consistent with previous evidence that ectoparasites weakly affect fitness in colonial and cavity-nesting avian hosts, which have evolved greater resistance due to prolonged host-parasite interactions. Significance statement Ectoparasites are considered a major stressor for nestling birds, as they can negatively affect their growth, survival but also traits involved in parent-offspring communication. We experimentally show that ectoparasite infection has only little and transient negative effects on the early growth of lesser kestrel nestlings. However, nestlings recover from such a slower growth without paying any cost in terms of final body size and pre-fledging survival. In addition, we reveal that mouth colour does not mirror the level of ectoparasite infestation, but it is a reliable signal of nestlings' individual general condition. Therefore, ectoparasites only weakly affect fitness in this species, likely because of the intense host-parasite interaction, which is typical of colonial and cavity-nesting species.