The Serotine (Eptesicus serotinus) is one a species that has yet to be confirmed within Cornwall.  Many reports of Serotines have on investigation turned out to be Noctule roosts.   Serotiens are found mainly south of a line from The Wash to parts of South Wales. It is found over most of Europe where it is declining in abundance in some areas but may be increasing its range northwards. Serotines are not believed to be particularly migratory although movements of up to 330 km  (200 miles) are recorded in eastern Europe.

The Serotine has probably declined due to loss of feeding habitat where large insects such as chafers can be found. As it roosts almost entirely in buildings it is subject to the effects of building work and the use of toxic chemicals in remedial timber treatment.

Flight & ultrasound

The Serotine bat  is one of Britain's largest bat species and usually one of the first to appear in the evening, often emerging in good light. The broad wings and a leisurely, highly manoeuvrable flapping flight with occasional short glides or steep descents is distinctive. It flies at about tree-top height (to about 10 m) often close to vegetation, and will sometimes flop, wings outstretched, onto the foliage to catch large insects. The Serotine will feed around street lamps and even catch prey from the ground.

Most of the food is caught within 2 km of the roost although they may forage up to 6 km. Having caught their favourite prey (a large beetle), a Serotine will cruise around slowly, chewing and dropping the wing cases and legs. Sometimes they will take the prey to a feeding perch.

The echolocation calls of Serotine bats range from 15 - 65 kHz and peak around 27 kHz. On a heterodyne bat detector an irregular hand-clapping sound is heard which has been described as "having the rhythm of a very poor jazz drummer".

Serotine calls can be confused with those of the Noctule or Leisler's: 

Serotine's do not make the chip-chop 2-part call of the Noctule but just a "chop" that is loudest at about 27 kHz. If a "chip-chop" sound is heard with a heterodyne detector set to 25 kHz Serotines can be discounted. However, Noctules can drop the first part of their call when flying in a more cluttered environment. Serotines are found more frequently at the edges of landscape features such as treelines or hedgerows whereas Noctules prefer a very open environment. 

Leisler's has a loud call that is similar to the Noctule ("chip-chop") but with many more "chips" than "chops". The Leisler's "chop" is normally heard best above 20 kHz (at around 25 kHz) as opposed to the Noctule's "chop" which is generally heard best below 20 kHz.

Breeding

Maternity colonies consist almost exclusively of female bats and start to build up in May. Numbers in smaller maternity colonies are often stable from the end of May.

A colony usually remains at a single roost site during the breeding season but some, particularly larger colonies, change roosts. Females normally give birth to a single young in early July, though births as late as mid-August have been recorded. The baby is occasionally carried by its mother for the first few days. At 3 weeks the young are able to make their first flight and at 6 weeks can forage for themselves. The colony usually disperses by early September but a few bats may remain until early October.

The males probably remain solitary or in small groups but are occasionally found with females in spring or autumn. Mating normally takes place in the autumn but almost nothing is known of the mating behaviour. Males and females reach sexual maturity a year after their birth.

Summer roosts

Serotines roost mainly in buildings with high gables and cavity walls such as many built around 1900. They can be found in much older buildings, and often occur in churches, but are less frequently found in modern buildings. The access to the the roost is usually at or near the gable apex or the lower eaves. The Serotine is one of the most building-orientated species and is hardly ever found in trees, which presumably provided the original natural roost sites.

They roost hidden in crevices around chimneys, in cavity walls, between felt or boarding and tiles or slates, beneath floorboards and sometimes in the open roof space at the ridge ends or occasionally elsewhere along the ridge. Droppings are often present in large amounts at the gable ends or around a chimney base, although some long established colonies show no obvious signs of occupation where the roost is in a cavity wall. The point of access is not well marked, though sometimes it is slightly discoloured and there are likely to be a few droppings underneath.

Once the colony has built up in late spring there may be much squeaking before the bats emerge at night. Most of the colony emerges in the first 10 minutes and all will have left within about 40 minutes. In spring the bats return after about 30 minutes and groups of bats will circle around the roost before entering. As the season progresses, some may return to the roost in the middle of the night while others spend more time away from the roost. There may be a secondary peak of activity around dawn.

The roost building is sometimes shared with Pipistrelles or Long-eared bats, and Serotines have also been known to associate with Natterer's, Whiskered and Noctule bats.

Winter roosts

Very few Serotines are found in winter but it is likely that most hibernate in buildings. It is possible that at least par of the summer colony may remain in the same building for some, if not all, of the winter period.

Hibernating Serotines have been found inside cavity walls and disused chimneys. Very rarely they have been found in the coldest parts of caves, either in roof crevices or in accumulations of boulders.