Tree swallows are obligate secondary cavity nesters that host a wide variety of ectoparasites, including mites, lice, fleas, and flies (Figure 1 ). These ectoparasites feed on blood, skin, and feathers (Janovy et al., 1997 ; Rendell & Verbeek, 1996 ) and can negatively impact offspring physiology, immune function, and survival (López‐Arrabé et al., 2015 (link); Martínez‐de La Puente et al., 2011 (link); Merino & Potti, 1995 ; Saino et al., 1998 ). Blow flies (Protocalliphora) have been found to infest 65.9% of tree swallow nests in northeastern US (Roby et al., 1992 ), with loads ranging from 4 to 54 parasites per nest (Grab et al., 2019 (link)). Previous work shows that blow flies feed on nestling blood and can cause anemia, hyperglycemia, and increased metabolic rates in avian hosts (De Simone et al., 2018 ; Grab et al., 2019 (link); Pryor & Casto, 2015 (link); Sun et al., 2020 (link)). One broad‐spectrum insecticide commonly used to remove ectoparasites is permethrin, which attacks the nervous system of larval and adult insects (Edwards, 2006 ). Permethrin treatment is effective against blow flies in nests of tree swallows (De Simone et al., 2018 ; Grab et al., 2019 (link)) and other bird species (Bulgarella et al., 2020 ) and also decreases the abundance of other ectoparasites like fleas and mites (Harriman et al., 2014 ; Pap et al., 2005 ; Pryor & Casto, 2017 (link)).
Any nest with a known hatching day was selected for our experiment, and then randomly assigned to one of two treatment groups, one in which ectoparasites were eliminated via the application of the insecticide permethrin (Permectrin II ©, diluted to 1% with distilled water), the other in which nests were treated with water as a control. In both treatment groups, nests were sprayed on day 0 and again on day 4. To do this, nestlings were temporarily removed from the nest, the bottom and sides of the nest were sprayed thoroughly (to minimize direct contact with nestlings), and the nestlings were returned once the nest had completely dried approximately 5 min later. To the extent possible, permethrin‐ and water‐treated nests were paired by hatch date to avoid the confounding effects that date has on many aspects of tree swallow reproduction (Winkler et al., 2020 ). The final number of nests in our study (n = 16 control, n = 16 insecticide) was less than the initial number sprayed due to brood loss.
Any nest with a known hatching day was selected for our experiment, and then randomly assigned to one of two treatment groups, one in which ectoparasites were eliminated via the application of the insecticide permethrin (Permectrin II ©, diluted to 1% with distilled water), the other in which nests were treated with water as a control. In both treatment groups, nests were sprayed on day 0 and again on day 4. To do this, nestlings were temporarily removed from the nest, the bottom and sides of the nest were sprayed thoroughly (to minimize direct contact with nestlings), and the nestlings were returned once the nest had completely dried approximately 5 min later. To the extent possible, permethrin‐ and water‐treated nests were paired by hatch date to avoid the confounding effects that date has on many aspects of tree swallow reproduction (Winkler et al., 2020 ). The final number of nests in our study (n = 16 control, n = 16 insecticide) was less than the initial number sprayed due to brood loss.
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