Nests of cavity‐nesting birds usually harbor some species of haematophagous ectoparasites that feed on the incubating adults and nestlings. Given the negative impact of ectoparasites on nestlings there will be selection on hosts to reduce parasite infestations through behavioural means. We have experimentally reduced the abundance of all ectoparasites in nests of pied flycatchers Ficedula hypoleuca to explore both whether there are changes in the frequency and duration of putative anti‐parasite behaviours by tending adults, as well as whether such anti‐parasite behaviours are able to compensate for the deleterious effects that parasites may have on nestlings. Heat treatment of nests substantially decreased the density of ectoparasites, and thereby positively affected nestling growth. The frequency and intensity of female grooming and nest sanitation behaviours during the incubation and nestling periods decreased as a consequence of the experimental reduction of ectoparasite infestation. Although nestlings begged more intensely in infested nests, the experiment had no significant effect on parental provisioning effort. Reduction of parasites resulted in larger nestlings shortly before fledging and increased fledging success. This study shows a clear effect of a complete natural nest ectoparasite fauna on parental behaviour at the nest and nestling growth in a cavity‐nesting bird. Although ectoparasites induce anti‐parasite behaviours in females, these behaviours are not able to fully remove parasite's deleterious effects on nestling growth and survival.

Despite many studies of how male characteristics affect paternity in predominantly monogamous birds, relatively little attention has been given to the traits of females that may influence extra‐pair paternity (EPP). However, the occurrence of EPP may be the result of behavioural interactions in which both male and female traits are important for determining the outcome. If EPP is driven mainly by female choice of extra‐pair sires, older, more experienced or larger females would be better able to evade mate guarding tactics and more capable of selecting extra‐pair mates and resisting unwanted suitors. This would be especially noticeable in females paired with unattractive mates. On the other hand, if EPP is driven mainly by male pursuit, we should expect that young, inexperienced or small females would be more exposed to coercive male approaches independently of social mate traits. In a study of an Iberian population of the pied flycatcher Ficedula hypoleuca, we found that EPP affected 38% of the broods and 17% of the nestlings. These values are relatively high, allowing a relatively large number of affected within‐pair mates to be included. We found that EPP is related to both female and male traits although not to any interaction between male and female traits. EPP was higher at nests tended by both younger and short‐winged females and by browner males. Older females may be more experienced and dominant while long‐winged females may be faster fliers, these traits enabling them to avoid extra‐pair copulations, while brown males are less aggressive towards male intruders. In our study population, EPP appears to be caused by male pursuit, which in some cases may overwhelm female attempts to avoid extra‐pair copulations and their social partner's ability to prevent them.

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16Nest-dwelling ectoparasites may result in costs for nestlings of cavity nesters in terms of 17 compromised growth and condition before fledging. The reduction or elimination of nest 18 ectoparasites to study their effects on avian hosts can be conducted through physical methods 19 like heat-treatment or through chemical methods using insecticides. Pyrethroids are the most 20 frequently used, although some studies have shown that they may compromise the 21 development and future survival of birds. In this study conducted in central Spain we 22 analyzed the differences between a group of fumigated Pied Flycatcher Ficedula hypoleuca 23 nests and a heat-treated group, both rendered ectoparasite free by treatments. We also 24 compared these ectoparasite free nests with a control group with natural ectoparasite loads. 25Our aim was to test the possible effects of a pyrethroid-based insecticide on reproductive 26 success, parental care behaviours and body condition of adult females and nestlings. We also 27 determined the effects of treatment on a biochemical biomarker, the total glutathione (tGSH) 28 level, involved in detoxification of xenobiotics and considered as the most important 29 intracellular antioxidant. Although behavioural variables were not affected by treatment, 30 results showed lighter 3-day old chicks and shorter tarsi and wings in nestlings shortly before 31 fledging in fumigated nests, together with depletion in tGSH levels in both female and 32 nestlings. Fumigation with pyrethroids may introduce in ectoparasite-load reduction 33