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Bat arousals during hibernation are related to rises in environmental temperature, body water loss and increasing body heat. Therefore, bats either hibernate in cold places or migrate to areas with mild winters to find water and insects to intake. During winter, insects are relatively abundant in wetlands with mild climates when low temperatures hamper insect activity in other places. However, the role of wetlands to sustain winter bat activity has never been fully assessed. To further understand bat behaviour during hibernation, we evaluated how the weather influenced hibernating bats, assessed the temperature threshold that increased bat arousals, and discussed how winter temperatures could affect bat activity under future climate change scenarios. The effects of weather and landscape composition on winter bat activity were assessed by acoustically sampling four different habitats (wetlands, rice paddies, urban areas and salt marshes) in the Ebro Delta (Spain). Our results show one of the highest winter bat foraging activities ever reported, with significantly higher activity in wetlands and urban areas. Most importantly, we found a substantial increase in bat activity triggered when nocturnal temperatures reached ca. 11 °C. By contrasting historical weather datasets, we show that, since the 1940s, there has been an increase by ca. 1.5 °C in winter maximum temperatures and a 180% increase in the number of nights with mean temperatures above 11 °C in the Ebro Delta. Temperature trends suggest that in 60-80 years, winter months will reach average temperatures of 11 °C (except maybe in January), which suggest a potential coming interruption or disappearance of bat hibernation in coastal Mediterranean habitats. This study highlights the significant role of wetlands in bat conservation under a climate change scenario as these humid areas represent one of the few remaining winter foraging habitats.

Habitat loss and forest fragmentation are currently substantially reducing the availability of natural bat roosts worldwide. However, since bat populations have been recognized as important ecosystem service providers, bat boxes have become one of the most popular measures employed to protect them. Evidence exists that in arid regions bat boxes act as ecological traps due to abrupt rises in summer temperatures. Several reported mortality events highlight the lack of appropriate guidelines in temperate areas, which might be putting bat conservation in jeopardy. We aimed to explore which bat box features might cause mortality so we compare temperatures in bat boxes, modelling the influence of the orientation and model, and quantifying the risk of extreme heat episodes. A total of 797 overheating events were recorded during a bat-box monitoring program in Catalunya (2014–2015). In 2016, we compared temperatures in up to 15 models side-by-side in the Ebro Delta Natural Park. Bat-box model and orientation clearly affected the number of overheating events. Black-coloured and south-facing boxes recorded the highest temperatures, including the most popular models, with temperatures over 40 °C. The number of overheating events was clearly dependent on the bat-box model. For example, a new model manufactured from rice chaff did not experience any overheating event in contrast with the 2F model that recorded over 50. We do not recommend the use of black boxes in south-facing sites in warm areas. In fact, observed internal temperature gradients suggest that complex boxes may help counteract the effects of overheating. We highlight the importance of taking microclimate characteristics into consideration when setting up a bat-box scheme.