Temperate-zone bat species store fat in autumn prior to hibernation. The likelihood of surviving until the next breeding season largely depends on the resources accumulated. In addition, autumn is also a period of mating behaviour known as swarming, which is energetically expensive and may affect the body condition of bats in different ways. Our goal was to determine how swarming activity, male reproductive status, sex ratio and mean temperature influence resource deposition in six bat species. Variations in the body condition index (BCI) of bats during swarming activity mainly depended on the date of capture and, to a lesser degree, on the sex; however, it was highly species-specific and linked to the time of entering hibernation. Despite a promiscuous mating system, both species and sex significantly affected resource accumulation. Among the analysed behavioural factors, swarming activity explained a small part of BCI variation. In the foliage gleaners Myotis bechsteinii, M. nattereri and Plecotus auritus, an increase in male participation (higher sex ratio) was correlated with a higher BCI, while in Myotis daubentonii and Barbastella barbastellus, it led to a lower BCI in females. In turn, epididymal distension explained a significant part of BCI variation only in two species (Myotis daubentonii and M. myotis); in both with a negative correlation. Mean temperature explained BCI variation to the highest degree in M. bechsteinii (lower temperature implied lower body mass), but in the other gleaners, an opposite and weaker effect was found. Our findings indicate that the pattern of fat accumulation during swarming is modulated by distinct species-specific and sex-specific thermoregulatory strategies (energy-saving mechanisms) rather than by differences in mating behaviour or foraging strategies.