Drivers and consequences of nest ectoparasite pressure in tit nestlings

Ectoparasites impose significant costs to their hosts and modulate their life-history traits. We evaluated the prevalence and abundance of louse flies, blowflies, fleas and mites in great tits (<i>Parus major</i>) and blue tits (<i>Cyanistes caeruleus</i>) breeding in nest boxes in Central Portugal during two consecutive breeding seasons and assessed: (a) the potential physiological consequences of infestation for nestlings; (b) how nest box re-use and presence of anthropogenic materials in nests affected the ectoparasite abundance; (c) how host reproductive parameters were related to ectoparasitism; and (d) how different nest-dwelling arthropod groups, including ectoparasites, and their diversity correlated. Tit nestlings reared in nests with more blowflies showed symptoms of anaemia, such as lower haemoglobin levels and high erythrocyte maturation index, and tended to grow less. Nestlings from nests with higher number of obligatory parasitic mites had increased polychromasia, and blue tits tended to have lower probability to fledge. Great tit nestlings from nests with fleas also had increased polychromasia compared with those from non-infested nests. Nest box re-use increased the probability of infestation by louse flies and obligatory parasitic mites. In both tit species, broods that were reared later in the season had higher abundance of blowflies and obligatory parasitic mites in their nests. In great tit nests, anthropogenic materials were negatively correlated with flea abundance, and positively correlated with the abundance of Histeridae coleopterans. In great and blue tit nests, obligatory parasitic mites were less abundant when nests showed a higher abundance of Staphylinidae coleopterans and Collembola. Overall, this study shows strong negative effects of nest ectoparasite pressure, particularly blowflies and obligatory parasitic mites, on physiological and fitness measures of hole nesting birds.