Serotine

GB: Vulnerable

England: Vulnerable

Scotland: NA

Wales: Vulnerable

Global: Least concern

Head & body length: 58mm - 80mm

Forearm length: 48mm - 55mm

Wingspan: 320mm - 380mm

Weight: 15g - 35g

Colour: Fur dark brown above, pale underneath; face and ears black.

Serotines mainly hunt within 2km of the roost but they may forage up to six kilometres. Having caught a large beetle, a serotine will fly around slowly, chewing its prey and dropping the wing cases and legs; sometimes it will take the prey to a feeding perch.

It flies at about tree-top height (to about 10 m) often close to vegetation, and will sometimes flop, wings outstretched, on to the foliage to catch large insects. The serotine will feed around street lamps and even catch prey from the ground.

Serotines roost mainly in buildings with high gables and cavity walls. They can be found in much older buildings and churches, but are less often found in modern buildings. The access to the roost is usually at or near the gable apex or the lower eaves. The serotine is one of the most building-oriented species and is hardly ever found in trees.

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

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.

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

In spring, serotines mainly feed on flies and moths; in summer, it particularly favours chafers and dung beetles.

Serotine maternity colonies consist almost exclusively of female bats and start to build up in May. A colony usually remains at a single roost site during the breeding season, although larger colonies sometimes change roosts.

Female serotines 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 they can forage for themselves. The colony usually disperses by early September, but a few bats may remain in the roost until early October.

Serotine 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 of serotine. Males and females reach sexual maturity a year after their birth.

The echolocation calls of serotine bats range from 15 to 65kHz and peak at 25 to 30kHz. On a bat detector a sound like irregular hand-clapping is heard.

The serotine is one of our less common species, occurring mainly south of a line drawn from The Wash to parts of South Wales.

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