The Noctule bat (Nyctalus noctula) is one of the larges  British species and is usually  the first bat to appear in the evening, sometimes even before sunset. Adults generally have short, sleek, golden evenly coloured fur. Juveniles, newly moulted adults and some females are a dull chocolate brown colour. They have broad brown ears and a distinctive mushroom-shaped tragus.

In continental Europe the Noctule is a well known migrant but apart from a few found in Orkney, Shetland and on North Sea oil rigs (well outside the normal range) such movements have not been recorded in Britain. It is still a relatively common species in much of England, Wales and up to the south west of Scotland, but has become scarce in some areas of intensive agriculture. The Noctule is absent from Ireland. It is common throughout much of Europe but scarce in the south west (southern France, Iberia).

The Noctule bat has declined in Britain due to modern agricultural practices resulting in the loss of suitable feeding habitats (such as permanent pasture and woodland edge/hedgerows rich in invertebrate fauna). The heavy management and loss of suitable trees for roosting may also have contributed to this decline.

                      Video of Noctules  emerging, shot at long-range using infra-red to avoid disturbance.

Video copyright: Steve Marshall

Flight & ultrasound

Noctules have a characteristic powerful, straight flight on narrow pointed wings. They fly in the open, often well above tree top level with repeated steep dives when chasing insects. Noctule bats can fly at 50 kph (30mph).

Most food is caught on the wing and eaten in flight but occasionally prey is taken from the ground and in suburban areas. Noctules are attracted to street lamps to feed on moths. During spring Noctules will feed mainly on smaller insects such as midges, changing their diet to take beetles and moths later in the season. They forage mainly at dusk for up to 2 hours and for about half an hour at dawn.

Noctule ultrasound can be heard by some adults and children. The calls range from 20 - 45 kHz and the best frequency to listen at is around 20 kHz. On a heterodyne bat detector a characteristic 'chip chop' with occasional clicks can be heard when feeding. However, in a cluttered environment they may drop the first part of their 2-part call. Their call is very loud with a slow and irregular repetition rate.

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

Serotine's do not make the chip-chop 2-part call 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. Also, 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.

Droppings below a nursery roost

Breeding

During the summer male Noctules are solitary or form small bachelor groups. A single male establishes a mating roost during late summer, usually in a tree hole, for several weeks and defends his roost against other sexually mature males. He emits a series of shrill mating calls from the roost entrance or during flight and produces a strong odour, attracting a harem of 4 or 5, but sometimes up to 20, females which stay with him for 1 or 2 days.

In April Noctule bats begin to form mixed sex colonies and can be found in tree holes, buildings and bat boxes. Such colonies often break up in late spring and smaller maternity colonies are found in trees, rarely in buildings and bridges. The young are born in late June and July. Females usually have one young but twins are regularly recorded. For 3 to 4 weeks the young are suckled solely on their mother's milk and are fully weaned and able to forage for themselves within 6 weeks.

The maternity colonies frequently change roost, mothers carrying the smaller young between roosts during lactation. The young are left in crèches while the mothers go off to feed.

Some females become sexually mature in their first autumn but many do not mate until their second year. Males participate in mating from the end of their first year.

Summer roosts

Noctule bats are primarily tree dwellers and live mainly in rot holes and woodpecker holes. They occur rarely in buildings but will use a wide variety including modern houses. Within buildings they roost in gaps in large ridge tiles, behind hanging tiles, between the ceiling and floor boarding, above large soffits, between the tiles and ceiling of a converted attic. Sometimes they roost in the lining of tall disused industrial chimneys and other hollow walls including cavities in bridges.

Most Noctule roosts in buildings are only gathering roosts, the colonies moving off at the end of May and early June. The bats produce loud characteristic metallic chirping sounds so that Noctule colonies can be heard up to 200 - 300 m away on hot days.

It seems that they are very selective about their tree hole roosts, preferring large uncluttered woodpecker holes high up in trees in less dense areas of woodland close to the woodland edge. Studies suggest that they move roosts frequently so that the number of parasites that build up is kept to a minimum; especially in juveniles.

Winter roosts

Noctule bats hibernate mainly in trees or rock fissures and hollows, but have also been found in bat boxes, buildings and other artificial structures in winter. They sometimes form large mixed sex winter aggregations of up to 1,000 bats in mainland Europe but the group sizes are smaller in the UK.

Noctules have been observed feeding at any time in winter if conditions are suitable but most can survive  successfully without feeding for nearly 4 months and can tolerate external temperatures as low as -16ºC.

In Great Britain, all bats are fully protected under Schedule 5 of the Wildlife and Countryside Act (1981) as amended, and by the Conservation (Natural Habitats etc) Regulations (1994). An agreement on the Conservation of Bats in Europe (EUROBATS) under the auspices of the Bonn Convention, also known as the Convention on Migratory Species (CMS) is in force, and all European bats are listed under Appendix II of the CMS .