Fast recognition and usage of new passageways leading to a maternity roost of Rhinolophus ferrumequinum (Chiroptera: Rhinolophidae)

The last Austrian maternity roost of the greater horseshoe bat (Rhinolophus ferrumequinum) which is located in the loft of Eggenberg Palace in Graz, Styria, was accessible through two chimneys until 2019 when two ducts became available for the maternity colony as additional passageways. The recognition of the new passageways and the usage of all four passageways were studied in the years 2019–2023 by using infrared light barriers (ChiroTEC). The new passageways were recognised by the bats within 33 and 36 days, respectively after the spring arrival of the first bat in the maternity roost. Over the study period, the numbers of flight movements decreased at the chimneys and increased at duct 2. Nevertheless, over the study period, i.e. 5 years after installation of the ducts, chimney 1 remained the main passageway. In addition to the usual flights through the passageways, we observed high numbers of short and successively repeated in and out flights at the entrances of the passageways. These pendulum flights trigger the light barriers, and a large proportion of registrations can be attributed to them. It therefore should be concluded that the numbers of registrations obtained at roosts of the greater (and lesser) horseshoe bats must not be used for determining the numbers of individuals living in the roost.