Experimental manipulation of nest temperature and relative humidity reduces ectoparasites and affects body condition of Blue Tits (Cyanistes caeruleus)

Many models predict changes in the distribution and incidence of diseases associated with climate change. However, studies exploring the effect of microclimatic changes on host–parasite relationships are scarce. In this study, we increased temperature or humidity in Eurasian Blue Tit Cyanistes caeruleus nestboxes during the breeding season, to analyse the effect of each variable on the abundance of ectoparasites and, also, on the body condition of the hosts. Temperature and humidity were experimentally increased on average by about 2°C and 15%, respectively. The abundance of blowfly Protocalliphora azurea pupae was significantly reduced in nests with increased temperature compared with control nests and those with increased humidity, and was also significantly reduced in nests with increased humidity compared with control nests. The abundance of mites Dermanyssus spp. was significantly reduced in nests with increased humidity. However, there was no significant effect of the experiment on the abundance of flea Ceratophyllus gallinae larvae, biting midges (Culicoides spp.) and blackflies (Simuliidae). The body condition of nestlings was lower in nests with increased humidity and slightly lower in heated nests. However, the body condition of adults was not significantly affected by the experiment. In addition, blowfly pupae and biting midge abundance were negatively related to the body condition of the nestlings, and biting midge abundance was also negatively related to the body condition of the adults. Overall, an increase in temperature and humidity reduces the abundance of ectoparasites, which suggests that these parasites are sensitive to sudden changes in the microclimate in which they develop. Moreover, these fluctuations negatively affect the body condition of Blue Tit nestlings despite the concurrent decrease of parasites in nests. In other words, the expected benefit to the nestlings from the reduction in parasites does not occur because the microclimate also adversely affects nestlings, probably affecting their heat exchange with the nest environment.