Experimental pyrethroid treatment underestimates the effects of ectoparasites in cavity-nesting birds due to toxicity

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