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Eptesicus serotinus

Description and identification

The serotine bat is one of the largest of the British bats. It has dark brown fur which gets lighter towards the tips. Generally fur is paler on the underside of the body and they have dark brown to black ears and muzzle. They have broad wings and a leisurely, highly manoeuvrable flight, often with occasional short glides or steep descents when hunting for prey.

The echolocation calls of serotine bat when heard on a bat detector are often described as loud ‘smacks’ or irregular hand-clapping. The echolocation calls range from 15 to 65 kHz with a distinct peak at 25 to 30 kHz.

Status and distribution

This is a species with a strong southern bias within the UK. It is generally found south of the line from the Wash in the east to Anglesey in the west of the UK, although it is largely found across the southern counties of England. Nationwide, there has been a decrease of approximately 25% in the population of serotines since 1999, the baseline recording year, mostly attributed to the loss of feeding habitat where large insects such as chafer beetles can be found.62 As the serotine roosts almost entirely in buildings, like several other species, it has been subjected to adverse effects from development and the use of toxic chemicals in remedial timber treatment.

The serotine is a very scarce bat in Nottinghamshire, with only a handful of records in the past three decades of recording. Since the first record in Nottinghamshire in 1986, there have only ever been four other records of this species, all in the northern half of the county. All of these have been acquired by modern (time expansion or full spectrum) bat detectors, which makes identification more reliable.

The first record came from a house in Dunkirk in November 1986, where an injured individual was recovered during construction work. The bat was taken into care by the Nottinghamshire Bat Group, but after the bat initially appeared to recover it then sadly died. With the recent occurrence of records of this species from Mansfield and north-west Nottinghamshire, as well as in neighbouring Derbyshire, it is hoped that further research will discover a local resident population.

Roosts

Historically, the serotine is presumed to have been reliant upon tree cavities within mature specimen hardwood trees. However, as most of these have been removed for their timber value or on health and safety grounds, the serotine is now almost exclusively found roosting in buildings. The species prefers small cavities and crevices, often higher up on old traditional buildings, such as the apex of high gables and in cavity walls.

Maternity colonies for this species are small and rarely include more than 30 females. Over the summer period the males typically roost singly or in small groups, away from the nursing mothers.Serotines sometimes roost in the same building as pipistrelles or long-eared bats and they have also been known to associate with Natterer’s bat, whiskered bat and noctule.

Over the winter months these bats are largely unrecorded and their exact roost locations are unknown. It is considered likely that the species hibernate in very similar places to the much smaller pipistrelle bats; tucked away within cavity walls and deep crevices within buildings, and perhaps some caves.

Food and foraging

Serotines hunt in relatively open landscapes; hay meadows, parkland, cemeteries, village greens, golf courses and playing fields are their usual haunts. They will regularly travel and hunt along woodland edges, tree lines, around single trees and areas of still water, taking prey as high as the tree canopy or down at ground level.

The serotine bat mainly preys upon beetles including chafers and dung beetles, but will also eat flies and moths57. As a result, one of their most important foraging habitats is cattle pasture, particularly where the cattle are not treated with ivermectin (insecticides given orally to animals to treat internal parasites) as the use of these treatments can kill dung beetles for months following treatment. The modern trend to house cattle and to rear them inside in sheds rather than in fields also denies the bats foraging potential as the dung is not deposited onto the pasture for the dung beetles to exploit. It is also reported that serotines sometimes forage around white streetlights and sewage treatment works to hunt the insects drawn to these features.