Bats and other mammals perform a wide range of avoidance and defence strategies in response to parasites or the threat of being parasitized (Marshall, 1981; Lehane, 2005; Reinhardt and Siva-Jothy, 2007). This includes behavioural, morphological and physiological traits. Hosts may shift their activity phases in response to parasitism, they may avoid infested sites (see e.g., Brown and Brown, 1986 for a bird example) or roost and forage at sites which are unfavourable for the parasites. Some animals, mainly insects, when parasitised have been shown to choose warm areas in order to rid themselves of pathogenic bacteria and fungi (‘behavioural fever’ — Adamo, 1998; Blanford et al., 1998). For endotherms, Lehane (2005) lists examples of mammals choosing sites whose wind and other microclimatic conditions deter attack or feeding by bloodsucking ectoparasites. Many ectoparasites settle close to their hosts presumably in order to reduce travel costs to the hosts. Therefore, thermoregulating endotherms may be able to roost at sites with elevated external temperatures that ectothermic parasites may not be able to tolerate. It appears unknown whether bats or other mammals choose such hightemperature sites in order to prevent a settlement of blood-sucking ectoparasites in their close proximity. However, such host behaviour may not be unlikely, given that many blood-sucking insects harbour microbes that assist in blood digestion (Buchner, 1965; Usinger, 1966) and that these microbes are very sensitive to the exposure of high temperatures (e.g., Buchner, 1965; Chang, 1974). Several ectoparasites are also sensitive to low humidity values. Whilst they can survive dry conditions they are unlikely to reproduce (e.g., Benoit et al., 2007). If host defence occurs by choosing sites that are unfavourable to parasites, temperature or humidity differences can be expected: a) between the site of host and of parasite activity and b) between thermal conditions found to be optimal and those actually observed in the field.