TECHNIQUE

Release of Casualty Insectivorous Bats (Wildlife Casualty Management)

Summary Information
Type of technique Health & Management / UK Wildlife Casualty Management / Techniques:
Synonyms and Keywords N.B. This information should be read in association with Wildlife Casualty Release which contains background information together with links to the Electronic Library and Organisations (UK Contacts). The related Species pages contain similar linkages.
Description This page has been prepared for the "UK Wildlife: First Aid and Care" Wildpro module, and is designed for the needs of the following species: Bechstein's bat - Myotis bechsteinii, Brandt's bat - Myotis brandtii, Brown long-eared bat - Plecotus auritus, Daubenton's bat - Myotis daubentonii, Greater horseshoe bat - Rhinolophus ferrumequinum, Grey long-eared bat - Plecotus austriacus, Large mouse-eared bat - Myotis myotis, Lesser horseshoe bat - Rhinolophus hipposideros, Noctule - Nyctalus noctula, Lesser noctule - Nyctalus leiseri, Nathusius' pipistrelle - Pipistrellus nathusii, Natterer's bat - Myotis nattereri, Northern bat - Eptesicus nilssoni, Particoloured bat - Vespertilio murinus, Pipistrelle - Pipistrellus pipistrellus, Serotine - Eptesicus serotinus, Western barbastelle - Barbastella barbastellus, Whiskered bat - Myotis mystacinus.

The UK bat species are from the families Rhinolophidae, Vespertilionidae.

  • Knowledge of the natural history of the relevant bat species and the location of local colonies are important for successful release. The local Bat Group a licensed bat worker or the Bat Conservation Trust  should be contacted for assistance if the exact origin of the bat is not known or it has been in captivity for more than a short time.

Pre-release:

  • No pre-release preparation is required if these animals have been in care for a short period of time.
  • Avoid unnecessary handling and taming.
  • Keep in an environment as close to that of the wild as is possible.
  • If releasing in the winter, ensure body weight is sufficient for hibernation.
  • (B168.7.w7)

Release assessment/criteria:

  • Released mammals:

    • Must be able to recognise, catch, manipulate, consume and digest their natural diet.
    • Must be capable of normal locomotion (movement) and have sufficient fitness for sustained activity.
    • Must have adequate sensory ability (sight, smell, hearing, touch).
    • Must be of an appropriate weight for the age, sex, and time of year for the species. 
    • Must have a satisfactory hair coat.
    • Must show appropriate wariness of humans and domestic animals.
    • (P19.1.w10, P24.233.w11, J35.147.w1, D27)
  • Appropriate health checks should be carried out prior to release. A careful assessment (risk analysis) must be made as to the risks of released animals introducing novel pathogens (disease agents) into the wild population/environment.
    • These pathogens may have been acquired from domestic animals, other wildlife casualties or humans whilst the animal was in captivity.
    • The health checks should be designed to minimise the risk that pathogens posing a threat to wild populations of this or other species will be introduced into the environment when the animal is released.
    • (V.w5, V.w6, P28.2000.w1, J15.20.w3)
  • Bats must be able to fly strongly prior to release (continuously for many minutes, as shown by testing within a large room, corridor or flight cage);
    • Provision of padding on the floor may be recommended for initial flying sessions.
    • Exercise flights should be offered at least ten minutes after feeding.(B230)
  • (B168.7.w7, P19.1.w7, B224)
  • Flight must be sustainable (some workers recommend 5-10 minutes (B229), others recommend 10-20 minutes (B230)) and should finish with a controlled landing and roosting at an appropriate high point of the room or enclosure. (B229, B230, V.w26)
  • Exercise flights of 10-20 minutes should be provided for two days for every week that a bat has been in captivity before it is released.(B230)
  • Must be of good body weight for the species and time of year, and in good condition at the time of release.(B168.7.w7)
  • Problems making a bat unsuitable for release include:
    • Decreased flight ability.
    • Permanent injuries to thumbs and toes hindering roosting, grooming and food manipulation.
    • Tooth loss affecting feeding ability.
    • Back or leg injuries.
    • (B230)

Selecting a release site:

Hard release:

  • Release at or near site of origin, i.e. in familiar territory. (B168.7.w7, P19.1.w7, D27)
  • Release back into the original colony if known; otherwise into suitable habitat for the species.(B230)
  • Knowledge of the natural history of the relevant bat species and the location of local colonies are important for successful release. The local Bat Group should be contacted via the Bat Conservation Trust for assistance if the exact origin of the bat is not known or it has been in captivity for more than a short time.

Soft release:

  • Release in suitable habitat, close to a known colony.

Timing of release:

Hard release:

  • Release in fine weather: avoid high winds, heavy rain, severe frost. (B168.7.w7,P19.1.w7).
  • Bats generally hibernate in the winter and should not normally be released at this time of year. However in some circumstance it may be appropriate e.g. following short term care of a bat which has been found during a period of mild weather and is of a sufficient body weight for hibernation.
  • If releasing in the winter:
    • Release just before sunset (usually less cold).
    • Do not release in extreme weather conditions.
    • (B168.7.w7)
  • Suggested optimal times for bat release include:
    • Release at/after dusk. (B168.7.w7, B151, P19.1.w7)
    • Release just before sunset.(B229)

Type of release:

Hard release:

Suggested techniques for hard release include:

  • Check that predators such as cats and dogs, as well as children, are not present.(B229)
  • May be placed on a tree trunk about 6 feet / 2 metres above ground level, with a clear area around for take off. (B229)
  • Move away sufficiently to avoid interfering with take off.(B229)
  • If the bat does not fly within a few minutes and is left still on the trunk, return the following morning and take back in if it has not flown.(B229)
  • May be released by placing the bat within a bat box with an open exit and positioning the box at a suitable release site (B230)
    • Release from a box does not need to be at dusk, as the bat will remain in the box until dusk.
    • Check the following morning that the bat has not remained in the box.(B230)
    • Experience is required to determine whether a bat found in a release bat box the day after release has left, fed and returned or (more likely) has not left the box and requires a further time in captivity prior to release. (B230)
  • May be released by holding the bat above the releaser's head in a natural roosting position and allowing it to fly away.(B230)
    • The bat should be allowed to warm up from torpor before release e.g. by being held briefly within cupped hands.
    • A strong light should be available to check that the bat has not fallen to the floor close by after leaving the hand.(B230)
  • If releasing after a prolonged period in captivity (e.g. two months or longer) release next to an active roost in the area of origin.(B168.7.w7)

Soft release:

  • This method should be used if hand reared bats are to be released.
  • From a release box:
    • Place a large release box at the release site, set about 2 metres above ground level.
    • Keep the bats in this box, providing food and water.
    • After 3-4 days leave open a small door at one end of the box.
    • Continue providing food until it is clear the bats are no longer returning.
    • N.B. bats may return after considerable lengths of time (months).
    • The same box, left in place, may be used for subsequent releases.
    • (B151)
  • From a flight cage:
    • The use of a pre-release flight cage with roosting boxes and a door which may be opened to provide access to the outside world may be useful for soft release of hand-reared bats.(B229)

(D24, B151, B168.7.w7, B229, B230, P19.1.w7)

Appropriate Use (?)
  • Soft release is particularly important for hand reared animals, particularly of species which need to learn about their surroundings (e.g. food sources) and/or learn survival skills such as hunting.
  • Soft release is also suitable for animals which have been in care for prolonged periods.
  • Soft release may compensate for difficulties of newly released animals finding food and shelter, particularly in a new environment and/or at a time of reduced physical fitness/stamina.
  • Hard release is most appropriate for animals which have been held in captivity for only a short time, for adult animals and for animals being released back into their own territory.
  • Survival is most likely if a bat is released within familiar surroundings, i.e. in the area from which it came.(B229)
  • It was considered that release as soon as possible (preferably within 20 days) was important for successful release (P19.1.w7); more recently evidence has indicated that bats held for prolonged periods can successfully return to their original foraging areas and roost.(B168.7.w7)
  • The individual animal must, at the the time of release, be healthy, have a reasonable level of fitness and be able to fend for itself in the wild.
Notes
  • Nocturnal species should be released at night.
  • Release should preferably take place during a period of fine weather.
Complications/ Limitations / Risk
  • Hand reared bats are not suitable for hard release.
  • Hard release may also be inappropriate for adults which have been maintained in captivity for prolonged periods and/or are being released at a site distant from their original location.
  • Opinions differ as to whether hand-reared insectivorous bats should ever be released. 
    • Some workers suggest that hand-reared bats should not be released (B168.7.w7);
    • It has been suggested that hand-reared bats would not learn to echolocate properly and catch insects sufficiently and that they would not find suitable roosts nor integrate into the wild population.(B168.7.w7)
    • It has been shown that hand-reared bats can catch insects within the confines of an aviary; time in such an aviary sufficient for the bat to learn to catch insects proficiently and not require supplementary feeding is essential.
    • Survival of a released hand reared bat at least seven months after release has been demonstrated.(B230)
    • Bats should not be released until they are catching insects in an aviary sufficiently well to maintain their body weight without provision of supplementary mealworms. (B230)
    • Exposure to other bats of the same species (e.g. permanently disabled adults) during the rearing period may be important.(B229)
    • Placing an appropriate aviary near a known bat colony could provide exposure of a juvenile to its own species.(B229)
  • Released animals may be at risk of contracting disease if there is an ongoing disease problem in the wild population at the time of release.
  • The wild population may be at risk from novel pathogens (disease agents) carried by a rehabilitated animal. These pathogens may pose the greatest threat to free-ranging populations if the animal is to be released at a site distant from its original location therefore increasing the likelihood of spread of disease. It is important to remember that the casualty wild animal may have acquired disease from domestic animals, other wildlife casualties or humans whilst in captivity. 
Equipment / Chemicals required and Suppliers
  • Suitable container for transport to the release site. See: Accommodation of Casualty Insectivorous bats
  • Pre-release flight cage (aviary) for pre-release flight training / soft release.
  • Suitable bat box from which to release.
Expertise level / Ease of Use
  • Knowledge of the natural history of the relevant bat species and the location of local colonies are important for successful release. The local The local Bat Group a  licensed bat worker or the Bat Conservation Trust should be contacted for assistance if the exact origin of the bat is not known or it has been in captivity for more than a short time.
  • Experience is required to determine whether a bat found in a release bat box the day after release has left, fed and returned or (more likely) has not left the box and requires a further time in captivity prior to release.(B230)
Cost/ Availability
  • Soft release may involve a considerable time investment.
  • Soft release may be expensive in terms of construction of appropriate temporary accommodation at the release site.
  • Costs of appropriate health screening.
Legal and Ethical Considerations
  • The potential risks to the individual being released and to the wild population into which it is being released (also to domestic animals) must be considered before release is undertaken.
  • The potential risk to humans and pets from habituated/tame individuals must be considered.
  • An offence may be committed under Section 1 of the Abandonment of Animals Act 1960 if a released rehabilitated animal does not have a reasonable chance of survival (i.e. a chance similar to its non-rehabilitated peers). 
    • This may include release at an unsuitable site, in the wrong territory, unfit, not having learned to hunt, at the wrong time of year etc. 

    (J35.147.w1, B156.21.w21, B223, W5.Jan01).

  • (J35.147.w1, B156.21.w21, B223, W5.Jan01).
  • If hand-reared bats are to be released, the bat should be marked to allow future identification if found dead or alive. This may give valuable information about the survival or otherwise of such bats; a specific licence may be required for some forms of marking. (B168.1.w1, B168.6.w6).
Author Debra Bourne
Referee Becki Lawson and Suzanne Boardman
References

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