Factors affecting the presence and abundance of generalist ectoparasites in nests of three sympatric hole-nesting bird species Alejandro CANTARERO 1 , Jimena LÓPEZ-ARRABÉ 1 , Víctor RODRÍGUEZ-GARCÍA 2 , Sonia GONZÁLEZ-BRAOJOS 1 , Rafael RUIZ-DE-CASTAÑEDA 1 , Alberto J. REDONDO 3 & Juan MORENO 1 1 Department of Evolutionary Ecology, National Museum of Natural Sciences — CSIC, José Gutiérrez Abascal 2, 28006 Madrid, SPAIN, e-mail: alexcc@mncn.csic.es 2 Faculty of Biology, University of Salamanca, Campus Miguel de Unamuno, 37007 Salamanca, SPAIN 3 Department of Zoology, University of Córdoba, Campus Universitario de Rabanales (Edificio C-1), Carretera Nacional IV, Km 395A, 14071 Córdoba, SPAIN Cantarero A., López-Arrabé J., Rodríguez-García V., González-Braojos S., Ruiz-de-Castañeda R., Redondo A. J., Moreno J. 2013. Factors affecting the presence and abundance of generalist ectoparasites in nests of three sympatric hole-nesting bird species. Acta Ornithol. 48: 39–54. DOI 10.3161/000164513X669982 Abstract. Nesting cavities constitute micro-environments very likely to be colonized by ectoparasites which feed on blood of the incubating female and the nestlings. Given the negative impact of ectoparasites on nestlings there will be selection on hosts to minimize ectoparasite loads through behavioural defenses. We have addressed the implications of ectoparasitism in three sympatric avian cavity-nesters, namely Pied Flycatchers Ficedula hypoleuca, Blue Tits Cyanistes caeruleus and Nuthatches Sitta europaea, to explore if differences in prevalence and abundance of generalist ectopara- sites (blowflies, fleas and mites) can be related to interspecific differences in their nest size, nest composition and cavi- ty microclimate. Furthermore, we have aimed at detecting if interspecific variation in the incidence and intensity of anti-parasite behaviours is a consequence of the abundance of ectoparasites. Differences in nest composition among host species appear not to be the main factor explaining ectoparasite loads, while nest size, breeding phenology, brood size and nest-cavity micro-climate may affect them in different ways for each host-parasite association. Behavioural defenses against parasites are exhibited by all host species but are more intense in the host species with the highest infestation levels (Blue Tits). This study shows different sources of variation in associations between three sympatric avian cavity-nesters and their generalist ectoparasites. Key words: anti-parasite behaviour, blowflies, mites, ectoparasites, grooming, nest composition, nest sanitation, nest humidity, Pied Flycatcher, Blue Tit, Nuthatch Received — March 2013, accepted — June 2013 ACTA ORNITHOLOGICA Vol. 48 (2013) No. 1 INTRODUCTION Hole-nesting has been usually associated with selective pressures arising from the thermal envi- ronment and the impact of nest predation (Hansell 2000). Nesting cavities offer conditions of relatively constant temperature and humidity as well as protection from rain, solar radiation and predators. Nesting cavities constitute micro-envi- ronments very likely to be colonized by bacteria, decomposers and detritivores due to the presence of faeces and food remains of breeding birds, and by ectoparasites that feed on blood, skin and feathers of avian hosts (Collias & Collias 1984, Mazgajski 2007b). Nest ectoparasites feeding on the blood of nestlings and adults constitute an important selective force affecting avian life history evolution as they remove nutritional and energy resources from hosts that could otherwise be used for growth, maintenance or reproduction (Møller 1993, 1997). They may also induce costly immune, inflammatory responses (Møller et al. 2005, Owen et al. 2009) and physiological stress (Martínez-de la Puente et al. 2011). Accordingly, ectoparasite presence and abundance in nesting cavities may have constituted an important evolu- tionary factor modulating adaptations of hole- nesting birds (Heeb et al. 2000, Tripet et al. 2002a). There is mixed evidence concerning the impact of ectoparasites on reproductive success in altricial cavity-nesting birds. While some experi- mental studies have found deleterious effects on nestling growth and survival (Richner et al. 1993, Heeb et al. 1998, Tomás et al. 2008), other have