Experimental ectoparasite removal has a sex‐specific effect on nestling telomere length

Parasites are a strong selective force that can influence fitness‐related traits. The length of chromosome‐capping telomeres can be used to assess the long‐term costs of parasitism, as telomere loss accelerates in response to environmental stressors and often precedes poorer survival prospects. Here, we explored the sex‐specific effects of ectoparasite removal on morphology and telomere length in nestling tree swallows (Tachycineta bicolor). To do so, we experimentally removed blow fly (Protocalliphora spp.) larvae from nests using Permethrin, a broad‐spectrum insecticide. Compared to water‐treated controls, insecticide treatment of nests had a sex‐biased effect on blood telomere length: ectoparasite removal resulted in significantly longer telomeres in males but not females. While this treatment did not influence nestling body mass, it was associated with reduced feather development regardless of sex. This may reflect a relaxed pressure to fledge quickly in the absence of parasites, or alternatively, could be a negative side effect of permethrin on morphology. Exploring robust sex‐specific telomere dynamics in response to early‐life environmental pressures such as parasitism will shed light on sexual dimorphism in adult life histories and aging. Experimental removal of ectoparasites from the nests of tree swallows led to longer telomeres in male, but not female nestlings by 12 days old. This suggests that male physiology may be more sensitive to parasitism in this species, which could contribute to sex variation in life history and aging.