TECHNIQUE

Release of Casualty Birds of Prey (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: Accipiter gentilis - Northern goshawk, Accipiter nisus - Eurasian sparrowhawk, Aquila chrysaetos - Golden eagle, Buteo buteo - Common buzzard, Buteo lagopus - Rough-legged buzzard, Circus aeruginosus - Western marsh harrier, Circus cyaneus - Northern harrier, Circus pygargus - Montagu's harrier, Haliaeetus albicilla - White-tailed eagle, Milvus milvus - Red kite, Pernis apivorus - European honey buzzard, Pandion haliaetus - Osprey, Falco columbarius - Merlin, Falco peregrinus - Peregrine falcon, Falco subbuteo - Hobby, Falco tinnunculus - Common kestrel, Athene noctua - Little owl, Strix aluco - Tawny owl, Asio otus - Long-eared owl, Asio flammeus - Short-eared owl, Nyctea scandiaca - Snowy owl, Tyto alba - Barn owl.

These species are from the families Accipitridae, Falconidae, Strigidae, Tytonidae.

  • Assistance of experienced personnel should be obtained for release of birds of prey; a local falconry or bird of prey expert should be consulted for advice.

Pre-release:

  • Falconry techniques may be used:
    • Falconry techniques may be useful for conditioning adults following prolonged rehabilitation.
    • Falconry techniques may be useful for training and conditioning juveniles, so that they come to associate hunting with feeding.
    • Flying on a creance (the cord which secures the hawk in training) may be useful for improving condition/ flight capability prior to release.
    • (J23.23.w2)
  • Training a bird to fly to a hackboard, lure or kite is required for some soft release methods.(D53)
  • Placing birds in an aviary or building at the proposed release site and with a good view over the surrounding area for about four to six weeks before release may be used for soft release from such an aviary or building. (D53)

Release assessment/criteria:

  • 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)
  • Birds must be able to walk, fly, see, feed and preen normally on release and have sufficient fitness for sustained flight.
  • A bird with a damaged bill which has not fully mended is not suitable for release. 
  • Birds of prey must be able to fly well and hunt efficiently.(J3.106.w1)
  • There are differences in opinion regarding the exact requirements for release of raptors. However the principle points are that following release the bird must be physically able to pursue and catch prey and must be behaviourally suited for life in the wild.
  • In general, the bird must have a high degree of flying ability. 
  • Raptors with permanent damage to the wing limiting joint movement or the movement of the radius over the ulna (synostosis) should not be released.(J3.106.w1)
  • However some authors have indicated that raptors with some degree of carpal and elbow joint restriction and some restriction of movement (sliding) of the radius over the ulna may still be able to fly effectively. (J3.106.w3)
  • Wing injuries are more likely to preclude release of aerial hunters and migratory raptor species, which are completely reliant on precise, rapid or prolonged flight, than sedentary perch hunters or scavengers.(J3.106.w1)
  • Adults with flying and hunting experience are more likely to adapt effectively to some flight impairment than are inexperienced juveniles.(J3.106.w1)
  • Flying on a creance (a length of line to which the bird is attached while the other end is wound round a stick held in the hand so that the bird cannot fly away) may be useful for assessing flight capabilities, particularly following treatment of severe injuries.(J3.106.w2, J3.106.w3, J23.23.w2)
  • Birds must have adequate plumage with normal waterproofing/weatherproofing, and must be acclimatised to outside environmental conditions at the time of release.
  • Release is dependant on satisfactory plumage condition; imping may be used to repair flight or tail feathers prior to release.(J23.23.w2)
  • If feathers cannot be repaired it may be necessary to leave the bird in a seclusion aviary (solid sides) until it moults out the damaged feathers before release is possible.(J23.23.w2)
  • Birds must display appropriate behaviour, interacting with others of their own species as normal and showing appropriate wariness of humans (neither imprinted nor habituated).
  • Birds must be of an appropriate weight for the age, sex, and time of year for the species. 
  • Detailed care records noting the weight, feeding/food intake, fitness and behaviour of the bird are extremely helpful for assessment of release suitability.
  • (P19.1.w10, P24.233.w11, P24.335.w21, B156.15.w15, B203, D27)
  • It is important to remember that some of these species are migratory or disperse following breeding.
    • Individuals of migratory species released during or just before migration must have sufficient body (fat) reserves for migration.
    • The ability to sustain flight for long periods is particularly important for migratory species.

Selecting a release site:

  • Birds which have been in care for a short time should be released where they were found or at the nearest safe point to their site of origin. (D27)
  • The release site must provide habitat meeting the nutritional, biological and behavioural needs of the bird being released and must be in the known distribution of the species. (P24.233.w11)
  • Appropriate prey must be available
  • The habitat/terrain must be appropriate for the hunting technique of the species.
  • Birds should not be released into a territory containing residents of the same species;
    • This may not be possible for the commoner species.
  • The relevant landowners and managers must agree to the release on their land and be supportive.
  • Releasing birds repeatedly from one location should be avoided because local suitable territories are likely to become saturated.
  • (J3.106.w1, J23.23.w2, D53)
  • Corvid populations in the area, which may mob any released bird of prey, must be considered.(D53)
  • The different habitat requirements of some species at different time of year must be considered.(J3.106.w3)

Timing of release:

  • Release in fine weather.
  • Release during the day (diurnal birds) or at dusk (owls).
  • (P19.1.w12, D24)
  • Release in winter (poor food availability) or spring/breeding season (intense territoriality in wild birds) is probably inappropriate (except for birds which have only been in captivity for a short time) as competition for territory is great and all local territories will already be occupied.(J3.106.w3, J23.23.w2)
  • Migratory species must be released during the period when that species is normally present in the UK.

Type of release:

Hard release:

  • This is most suitable for adults which have been in care for a short period of time.
  • Release takes place back at the point of origin.
  • (P19.1.w12, D24)

Soft release:

  • A variety of soft release techniques have been developed for use with birds of prey:
  • Traditional "hacking back", a process using falconry techniques which allows the bird to develop hunting skills while food continues to be provided, may be most appropriate for the release of hand-reared birds.
    • This is best used for a brood or group of birds which have not yet started to branch or fledge.
    • It is less successful for a single bird.
    • If the chicks have already started to branch or fledge they may leave before associating the hack site with food and starve before learning to hunt.
    • The chicks are placed in an artificial nest site similar to sites used in the wild by their species.
    • Food is provided daily by means of e.g. a chute or pipe which prevents the birds associating the food with the human carer; food is ideally deposited at night for diurnal species and in the day for nocturnal species.
    • The birds fledge at the hack site and start exploring the surrounding area at increasing distances.
    • Once the birds have been flying for two weeks the food provided may be reduced to encourage them to hunt.
    • Food must be provided over an extended period following release.
    • (D53, J23.23.w2)
    • For barn owls, chicks are placed in a suitable nest box in an appropriate building at 4-6 weeks old and fed each night.(D55)
    • Chicks are likely to start fledging 3-5 weeks later but return to the box as this is where food is provided.
    • Food provision should continue until it is no longer taken; it is suggested that food provided should not be reduced until the chicks stop eating all that is provided. (D55)
  • Provision of food initially may also be appropriate for adults which have undergone a prolonged rehabilitation process and are being released away from their original territory. (P19.1.w12, D24, J23.23.w2)
  • Aviary hack:
    • Most suitable for birds which are too old for traditional hacking back and for scavengers such as kites and buzzards.
    • A suitable aviary is erected at the release site and the bird kept in this for a period prior to release.
    • The aviary should have a good view over the surrounding area, and an area of seclusion as a refuge if the bird(s) is/are frightened.
    • One side of the aviary is solid; the carer approaches from this side and leaves food via a hatch or pipe.
    • After 4-6 weeks one side of the aviary or part or all of the roof is opened (preferably remotely using a long line) quietly to allow the birds to fly out.
    • This must be done carefully to avoid frightening the birds.
    • Food may be reduced at the period just prior to release. The subsequent provision of food will then encourage them to stay at the release site.
    • Food is provided daily in or on top of the aviary, with a reduction in food after two weeks to encourage hunting/searching for carrion.
    • (J23.23.w2, D53)
    • Suitable for the release of owls.(B203)
    • Suitable for the release of kestrels.(B203)
  • Hack board:
    • May be used for one or several birds at a time.
    • Birds may be trained to take food from an elevated board at a particular location.
    • Falconry techniques are used to train the bird to fly to the board for food, initially only a distance of about two metres, but increasing to about 50 metres, on a line. The food is tied to the board so the bird has to remain on the board to eat.
    • Daily flying to the board is continued until the bird's behaviour indicates it is comfortable in the area.
    • Prior to release the bird is taught to find food hidden under a three-sided shelter on the hackboard (less vulnerable to scavengers such as corvids).
    • The bird is then released but food provided daily, hidden in the shelter and no longer tied down.
    • After a few weeks the amount of food provided is reduced to encourage the bird(s) to hunt.
    • (D53)
  • Lure hack:
    • This is most useful for mature birds with hunting experience but which have suffered major physical trauma or for other reasons need to build up muscle.
    • The bird is taught to fly to the lure and is then flown to the lure daily to build up fitness.
    • The bird is released from the site at which the daily flights have taken place and the carer returns and offers food by calling with the lure daily.
    • Calling with the lure should be continued for at least two weeks even if the bird is not seen.
  • (D53)
  • Kite hack:
    • The bird to be released is taught to fly to a kite to which food is attached by a special device such that the food is released when the bird strikes at it.
    • The kite is flown higher each day to reach several hundred feet (check on restrictions on height of kite flying to ensure the kite does not interfere with aviation air space).
    • The bird is then released in the area in which it has been flown and the kite is put up daily.
    • (D53)
  • Full falconry hack:
    • Not suitable for some species.
    • Involves full falconry training and hunting the bird by flying it at natural quarry.
    • This allows the carer to fully assess the bird's fitness and hunting ability and the bird to become familiar with the territory in which it is flown.
    • The bird is released, once it is taking quarry regularly, where it has been flown.
    • (D53)
  • "Long release" of barn owls (Tyto alba - Barn owl):
    • This involves introducing a pair of captive-bred, unrelated barn owls into a suitable building with appropriate surrounding habitat.
    • The birds are confined to the building (but preferably have views onto the surrounding area) for up to six months.
    • Food is provided every night on a rat-proof table.
    • Once the owls breed and the chicks are half grown the adults are allowed access to the outside world, with the opening first unblocked at dusk.
    • Food is provided as before and the amount eaten is used as one method of monitoring, along with e.g. inspection of regurgitated pellets to see if the owls have been eating other prey.
    • (D55)
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. (J3.106.w1)
  • 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
  • Diurnal species should be released in the morning, giving them a full day to explore and look for food and shelter before nightfall.
  • Nocturnal species should be released at night.
  • Release should preferably take place during a period of fine weather.
  • Preferably have all birds ringed before release - contact the British Trust for Ornithology for details of local licensed bird ringers.(B156.15.w15, P19.1.w12, J3.106.w2)
Complications/ Limitations / Risk
  • Hard release is least appropriate for juveniles which have been hand reared, particularly species for which learning about their environment and/or social skills are important.
  • 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.
  • Juveniles which were taken into care before gaining independence from their parents and learning to hunt are likely to have a reduced chance of survival following release in comparison to birds which were adult when taken into care. 
  • 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
Expertise level / Ease of Use
  • Knowledge of the natural history of the species concerned is required for correct decision making regarding a suitable release site.
  • Considerable expertise and experience with falconry techniques is required for many release methods. 
  • Assistance of experienced personnel should be obtained; a local falconry or bird of prey expert should be consulted for advice.
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)
  • Barn owl (Tyto alba - Barn owl)  is listed in schedule 9 of the Wildlife and Countryside Act 1981 and release of captive bred birds without a licence is prohibited under Section 14 of that Act. A general licence (DETR general licence number WLF 100100, which came into force 7th January 1997) allows the release of wild barn owls which have been taken into care temporarily. A licence must be obtained from DETR (now DEFRA) in order to release captive bred barn owls. (P19.1.w12, W5.w1.Jan01, D53)
  • In Britain bird ringing must be undertaken by someone with a British Trust for Ornithology (BTO) ringing licence. (B118.20.w20, P19.1.w12)
Author Debra Bourne
Referee Becki Lawson and Suzanne Boardman
References

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