Mangroves as maternity roosts for a colony of the rare east-coast free-tailed bat (Mormopterus norfolkensis) in south-eastern Australia

Abstract

Context Maternity roosts of insectivorous bats (where females raise young) are critical to the conservation of threatened species as roost quality can influence reproductive success. Additionally, threatened species may have specialised requirements or unusual behaviour, which may be overlooked without targeted investigation. Aims To explore which factors influence the roost selection of Mormopterus norfolkensis, by comparing day roosts, identified via radio-tracking, with environmental variables collected at tree, patch and landscape scales. Methods We collected a range of variables describing maternity roosts and surrounding patches, including internal measurements of hollows and microclimate. Additionally, we derived landscape-scale variables using a geographic information system. We then explored which variables best explained roost occurrence using logistic regression. Key results Nineteen lactating females and two male M. norfolkensis were tracked to 40 roost trees, mostly grey mangrove, Avicennia marina subsp. australasica. Lactating females were found to be faithful to two patches of mangrove forest close to where they were captured, regularly switched roosts and roosted in hollows singularly or in small groups. The attributes of mangrove patches, especially a high proportion of hollows, better predicted roosting by lactating females than roost-tree or landscape characteristics. Additionally, although the microclimate of roost hollows was not significantly different from ambient mangrove conditions, the mangrove forest was slightly more stable and had higher humidity than did other nearby habitats. Conclusions Contrary to predictions, maternity roosting group sizes were relatively small, indicating that bats were not deriving thermoregulatory benefits from communal living. However, we suggest that lactating females may benefit from the operation of a fission–fusion society among the colony as a whole. Additionally, the mature mangrove forest could offer unique roosting opportunities for bats because they support high densities of hollow-bearing trees, a stable microclimate and potentially low abundances of predators and competitors. Implications This is one of few international bat–mangrove studies and it illustrates that threatened species can behave unexpectedly and may be overlooked in conservation strategies that are based largely on anecdotal observations. We encourage further research into the value of mangrove forests to terrestrial fauna globally.