Wind energy is an important source of bat mortality worldwide. Extensive mortality data were gathered by a provincial surveillance program carried out at wind farms in lowland Mediterranean areas of the province of Cadiz, Andalusia (South Spain) between 2005 and 2016. As many as 2,371 bat fatalities were found. We determined the patterns of fatality in this sample and tested the quality of the surveillance program used. The data generally support previous observations regarding the seasonal timing, the species affected and the sex and age of fatalities. However, contrary to studies made at higher latitudes, fatalities mostly (95%) affected sedentary species and occurred on wind farms in flat landscape used for farming and livestock rearing. The current operational surveillance programme did not focus on bats initially, and, consequently, we urge that an improvement should consider a standardized search methodology including bats, verification of species identity, precise data collection, and mandatory and regular scavenger-removal and search-efficiency trials, in order to obtain correctly adjusted fatality estimates that can be used for efficient mitigation or compensatory measures.