Eptesicus nilssonii (Keyserling & Blasius, 1839)
Vespertilionidae

Least Concern

Description

The smallest member of the Eptesicus genus in Europe, measuring between 54 and 64 mm in total length, with a wingspan of 240 to 280 mm and a forearm of 37 to 44 mm. The last caudal vertebrae protrude about 4 mm from the uropatagium. The fur is dark brown, almost black, on the dorsal side with some yellowish-golden hues on the back and neck. The face, ears, and wings are dark brownish-black. Young individuals have an even darker fur with silver tints. The ventral side is yellowish-brown or beige.

Distribution

It is the northernmost bat species and the only known one that breeds above the Arctic Circle. It has a broad Palaearctic distribution ranging from France, northern Italy, and Norway, across northern and central Europe and Asia, to Japan (where it is found only on the island of Hokkaido). In Eastern Europe, it is found in the Carpathians up to northern Ukraine and the Caucasus. There are occasional records in the United Kingdom, the Balkans, and the French Pyrenees.

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Roosts and phenology

Maternity colonies typically consist of 20-50 individuals, occasionally up to 150 females. During the breeding season they may switch between several roosts. These roosts are usually in crevices, human structures like roofs or other fissures, trees, and occasionally in bat boxes. Males are usually solitary during the summer. Depending on the latitude and altitude of populations, births occur from mid-June to late July. Poor weather in summer can delay births by up to a month. Each female gives birth to one or two pups, which begin to fly after three weeks. Juvenile females often return to the maternity roost where they were born the following year and may give birth in their second year of life.

After the breeding season, adult individuals typically move about 30 km. Although it is considered a mainly sedentary species, occasional records in the North Sea and the Faroe Islands suggest occasional migratory movements. Movements of 100 to 450 km have been recorded up to four times. In winter, solitary individuals or small groups are found in mines, bunkers, and other underground shelters. Gravelly areas are believed to be important hibernation shelters.

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Habitat and diet

It is a species typical of boreal and mountain forests. Therefore, in the northern part of its distribution, it is more common at lower altitudes, while in the south, it is almost exclusive to higher and mountainous areas above 2,000 m in altitude. Its habitat typically includes moist coniferous forests, deciduous forests, moors, and meadows.

Nearly half of its diet consists of small dipterans (usually midges). The other half mainly includes beetles, lepidopterans (especially moths of the genus Hepialus), and, in some areas, hemipterans. It feeds almost exclusively on flying insects, typically species that gather in swarms. It usually hunts along vegetation edges but also in open areas up to 50 m high and around artificial lights such as street lamps. Normally, the foraging area is limited, around 20 ha, although in a colony in Germany, individuals fed in an area between 524-732 ha around the roost. This species has been observed hunting at temperatures just above the freezing point. They are quite territorial in their hunting areas, with females seen expelling individuals with whom they share a roost from their feeding territories.