Birds host a vast diversity of feather symbionts of different kingdoms, including animals (e.g., lice, mites), fungi, and bacteria. Feather mites (Acariformes: Astigmata: Analgoidea and Pterolichoidea), the most abundant animal ectosymbionts of birds, are permanent inhabitants of the pterosphere (ptero feather in Greek; Labrador et al. 2020), and the ones studied here are easily spotted as small (ca. 0.5 mm) dots on the surface of flight feathers. They are highly host specific symbionts (Do~na et al. 2018), and they seem to be commensals or even mutualists of birds by taking detritus and microorganisms such as fungi and bacteria from feathers, some of which are keratinophilic and therefore can damage the feathers (Blanco and Tella 1997, Galv an 2012, Do~na et al. 2019). However, many basic questions remain to be answered, such as the moments and the places where feather mites eat. Indeed, we wondered whether this might be partly because feather mites have been studied mainly during the day, when (most) birds fly, rather than during the night when mites seem to move more freely on the wings, according to two old anecdotal reports (Dubinin 1951, McClure 1989). To investigate the night ecology of feather mites, we initially spent a whole night observing them on two individual birds. At that point, we were unaware of how it would change our understanding of the pterosphere.

The authors thank Heather Proctor for mite identification, Sarah E. Bush, Dale Clayton, and Kevin Johnson for sharing information about louse ecology, and Bernardo Toledo for helping to organize mite datasets. Funding was provided by the Ministry of Economy and Competitiveness CGL2015-69650-P project to RJ and DS; European Commission H2020-MSCAIF-2019 program (id 886532) to JD; and La Caixa-Severo Ochoa International PhD Program 2016 to ML. All authors read and approved the final manuscript. The study was conducted in compliance with the current laws of the Spanish Government, and all applicable national guidelines for the care and use of animals were followed. Endangered species were not included in this study and the methods used were considered of mild severity for the birds. Birds' manipulation was done under a permit granted by Consejer~ia de Agricultura, Pesca y Desarrollo Rural de la Junta de Andaluc~ia, and permit from the Parque Natural Sierra de Aracena y Picos de Aroche. The study protocol was approved by the Direcci~on General de Gesti~on del Medio Natural de la Consejer~ia de Medio Ambiente y Ordenaci~on del Territorio, and the Subcomit~e de Bio~etica (CSIC). Bird ringing was done by RJ under the Spanish Ministry ringing license number 300111.

Ministry of Economy and Competitiveness CGL2015-69650-P

European Commission H2020-MSCAIF-2019 program 886532