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Introduction and General Information

Various methods may be used for release of animals in reintroduction/conservation translocation programmes. These include hard release, soft release, fostering or cross-fostering. For some species, migration training may be part of the release.
  • It is important to consider the life history strategy of the species (r-selected or K-selected) in deciding how the release should be conducted. (J54.13.w1)
  • Stressors, including e.g. handling stress should be identified and reduced as much as possible. Which factors are most stressful vary between species and may even vary between e.g. males and females within the same species. (J471.3.w1)
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Transportation

Unless the animals to be reintroduced are reared from birth/hatching at the release site, transportation to the release site will be required.

NOTE: Transportation can be stressful, particularly when previously free-living rather than captive-bred animals are being used in the release, but stress is also likely for captive-bred individuals.

It is important to ensure that:

  • Transportation containers are of an appropriate size and shape. 
    • If commercial air travel is involved, IATA Regulations must be followed.
  • Appropriate social groupings are transported together. 
    • Depending on the species, age and sex of the individuals being transported, it may be appropriate for animals to be in individual crates, mother and offspring together, or larger social groups.
  • The risks of injury, including injury by conspecifics, are minimised.
  • The risks of escape are minimised.
  • Adequate ventilation and appropriate temperatures are provided.
  • Adequate and appropriate food and water are provided.
  • Exposure to humans, dogs, loud noises and other stressors are minimised.
  • Exposure to diseases is avoided.
  • Veterinary personnel are present during the transportation, if possible, to monitor and if necessary to carry out any required treatment or sedation/tranquillisation.

(P17.62.w6, P17.62.w7)

  • It is important to ensure that transport containers used are appropriate for the species. (B709.41.w41)
    • For New Zealand Black Robins Petroica traversi, it was found that the robins could cope with 3-8 hours of travel including heat, noise and vibration from car and boat engines, and movements associated with walking and scrambling, if the box was designed appropriately. In the case of this species, a small box with a perch, holding one bird, well ventilated, dark towards the top but with light falling on water and live food at the bottom of the cage was appropriate. If the boxes were periodically put down and left alone for a few minutes, the birds fed themselves and did not lose weight or become dehydrated. In contrast, when a box designed for a different bird species was used, weight loss occurred during transportation, the birds appeared in poor condition at release, and survival was reduced. (B709.41.w41)
Crane Considerations
  • Transport cranes in cool weather. (B115.11D.w16)
  • Ensure that transportation is as quick and efficient as possible. (B115.11D.w16)
  • The risk of injury to cranes is minimised if each crane is transported in its own transport box.
  • The box needs to be tall enough to enable the crane to stand upright, with its neck in a relaxed position.
    • For sandhill cranes, a box  60 x 90 cm x120 cm high has been used. (B115.11D.w16)
  • Cardboard boxes have been used successfully. for commercial airline flights, wooden boxes may be needed. (B115.11D.w16)
  • A soft top on the inside of the crate is advantageous to reduce the risk of the crane injuring the top of its head.
  • The design of the crate should minimise the risk that the crane can get its bill or toes caught in the fabric of the crate.
  • Non-slip flooring is important.
    • Carpet covered with a layer of wood shavings has been used successfully. (B115.11D.w16)
  • Grus canadensis - Sandhill cranes transported in a truck at 74+ days old for three days became "weak and wobbly" during the trip, with one sitting down for most of the last two days. Most of the cranes appeared to recover within two days after arrival, but the weakest bird took nine days to recover sufficiently to begin flight training. Additionally, the back and scapular feathers of the birds later showed a board band of fault bar, suggesting physiological stress associated with the journey. (P87.7.w14)
  • If cranes are transported as a group (not in individual boxes) there is a risk of the lowest-ranking bird being picked on by the other birds. This can lead to understandable reluctance of the bird to enter the transport vehicle. (P87.8.w16)
  • Grus grus - Common crane in The Great Crane Project, UK were transported early in the morning to avoid potential heat later in the day, and to avoid rush-hour traffic on the motorway part of the journey. Transport cranes were used, each carrying two cranes in separate compartments. (D449)
Egg transportation
  • Transport cases have been developed for egg transport, using hot water bottles as a source of heat, a thermometer with the readable section outside the case, a moistened sponge to maintain appropriate humidity, and polyethylene foam inserts, with shaped holes of the appropriate size for the eggs, to provide  soft, shock-absorbent individual egg compartments. Frequent monitoring is required, with refilling of hot water bottles if the case loses too much heat, or temporary opening if it gets too hot (remembering that late-incubation embryos can produce quite a lot of heat). When transporting eggs late in incubation extra care is needed to check if any eggs are very close to hatching and, if necessary, cut extra space in the foam to provide more room. (J59.9.w1)
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Hard Release

Hard release is most suitable for adult or subadult (self-sufficient) animals or social groups which have been translocated from a free-living situation and are being released into similar habitat at a different location. 
  • As with rehabilitated animals, it is sensible to release animals at a time of day which gives them the maximum time to orientate themselves and find suitable resting sites: morning for diurnal animals, evening for nocturnal animals. (P24.233.w11)
  • Hard-released animals may be more likely to disperse from the release site and less likely to make use of supplementary resources (such as food) provided for them, compared with soft-released individuals. (J182.31.w1)
  • For wild-born animals being translocated, it has been suggested that the period before release should be minimised, being "just long enough to allow them to recover from the stress of capture and transport and start to adapt to the environment outside the release pen." (J334.73.w1)
Crane Considerations
Standard hard release

Hard release involves release of cranes, of whatever age, into the release area after no or minimal time to acclimate to the area and with no support given following release. Several release studies involving Grus canadensis - Sandhill cranes  indicated that hard releases are not appropriate for captive-reared cranes, leading to poor survival (high mortality). (B115.11D.w16, P87.6.w5) However,Grus leucogeranus - Siberian cranes of ages from juveniles to three years of age have appeared to adapt quickly when released after only one or a few days of acclimation at the release site, although there is a lack of data regarding longer-term survival. (N48.5.w2, N48.6.w2, N48.6.w3, N48.11.w2)

  • Hard release of 12 immature Florida sandhill cranes (Grus canadensis - Sandhill crane) near Lake Okeechobee, Florida, USA, resulted in loss of all the cranes.  (P87.1975.w6)
  • Five-month old Grus canadensis - Sandhill cranes hard-released in Florida in 1971 showed poor survival. (14 of 14 released in September were dead by December). (P87.1978.w2)
  • One female Grus canadensis - Sandhill crane parent-reared in captivity and released on Paynes Prairie in August 1976 survived, began associating with wild sandhill cranes in November and eventually developed typical wild crane behaviour. (P87.1978.w2)
  • Hard release of 11 parent-reared greater sandhill cranes (five one-year-olds, two two-year-olds, four three-year-olds) at Grays Lake Refuge, Idaho, USA in June 1980 resulted in survival of seven cranes to autumn migration, but only one arrived on the wintering grounds and that one was not located again after part way through the spring migration in 1981. (P87.1981.w8)
  • Parent-reared (in captivity) greater sandhill cranes (Grus canadensis - Sandhill crane), 19 one-year-old and two two-year olds, were held for a few days at a pen at Grays Lake NWR and released. Generally they stayed in their groups initially, left the pen area within two weeks and wandered up to 3km away, establishing use areas. Nine of the birds survived to migration. One was killed by an eagle a few days after release, three were found dead 1-3 months after release and eight were missing, presumed dead. Released cranes generally stayed within their release groups or on the periphery of wild flocks and did not develop evident long-term associations with wild cranes. They tended to show submissive behaviour and to lose agonistic encounters with wild cranes to a greater extent than did wild juveniles. Of the nine birds which migrated, two arrived in the San Luis Valley, Colorado (traditional migration stop). Another was seen in Utah, bout 100 km southwest of the migration route. Four apparently survived their first winter and migrated north again in spring. (P87.1985.w7)
  • Isolation-reared Grus leucogeranus - Siberian crane were released onto an island in the Big White Lake, Belozersky Refuge, Russia in August 2001, where Eurasian cranes roost. (N48.1.w1)
  • One isolation-reared and one parent-reared Grus leucogeranus - Siberian crane were released in the Astrakhan Nature Reserve, Russia. The release site was the Obzhorovo site, where wild Siberian cranes were known to be found during autumn migration. (N48.1.w2)
  • Five isolation-reared and one parent-reared Grus leucogeranus - Siberian crane chicks were briefly penned, banded then released at Kunovat River Basin, Russia, in August 2003. Food was provided in the first three days after release. (N48.5.w2)
  • Grus leucogeranus - Siberian crane chick reared for release at Oka Crane Breeding Center, Russia, were released initially into a mesh pen on Omelino Island, Beloye Lake, Belozersky Wildlife Refuge, Russia, which was opened the following day. Food was provided for the first week. (N48.6.w2)
  • Four male Grus leucogeranus - Siberian crane were released in Astakhan Nature Reserve in October 2004. (N48.6.w3)
  • Two three-year-old Grus leucogeranus - Siberian crane were released in the Kunovat Federal Wildlife Refuge after only a three-day acclimation period in a netted pen at the site. (N48.11.w2)
  • Two yearling and four juvenile Grus leucogeranus - Siberian crane were transferred from Oka Crane Breeding Centre to Obzhorovo site, Astrakhan State Nature Reserve in the Volga River Delta in September 2009. The site is traditionally used by western flyway Siberian cranes as a stopover site during migration. After two days in a pen on the site they were released. Fish and mixed food was provided in the first days post-release. In 2010, three juvenile and two yearling Grus leucogeranus - Siberian crane were released a day after arrival on the site in late September. (N48.11.w3)
  • A group of two juvenile and one 1.5 year old Grus leucogeranus - Siberian crane together with a juvenile Grus grus - Common crane were released in late August 2010 after one day in a pen on site on Omelino Island, Big White Lake, at Belozersky Wildlife Refuge, Tyumen Region. This is a staging area for Grus grus - Common crane which usually roost on the island. Crane feeders were provided around the area for use by both the released birds and wild cranes. (N48.11.w4)
  • In 2012, two yearling parent-reared Grus leucogeranus - Siberian crane  were released following a three-day adaptation period in a pen on the release site; they were released 10th June 2012 by opening one wall of the pen. (N48.12.w2)
One-by-one method

The one-by-one method involves release of cranes individually into flocks of wild conspecifics. Experience with sandhill cranes showed that cranes released individually (rather than in a group) were more likely to become socially integrated into flocks of wild cranes. Release of parent-reared sandhill cranes by this method has resulted in good survival; it also avoids the need for extensive care and training of chicks as is required for costume-rearing and training to follow an ultralight plane, and it does not require a lengthy period o acclimatisation at the release site. However, it does require the presence of a core flock of free-living cranes for the juveniles to be released into. (P87.8.w15) This method has also been used for release of costume reared Grus carunculatus - Wattled cranes. Several Grus leucogeranus - Siberian crane of various ages have been released by this method, sometimes in the vicinity of only a single free-living conspecific, with apparent success.

  • This was first suggested based on the findings following hard release of sandhill cranes. (P87.1985.w7)
  • Release of costume-reared or parent-reared juveniles into flocks of wild cranes, with only one or two released at a time, has been shown to be successful, resulting in high survival, integration, and migration. (P87.8.w1)
    • This method is dependent on the presence of wild flocks into which the crane colts can be released. (P87.8.w1)
    • This method removed the need for a lengthy acclimatisation period at the release site. (P87.8.w1)
  • Baiting of wild cranes can be used to bring them into the release area and encourage mingling with released cranes. (B115.11D.w16)
  • This method has been used for release of costume-reared Grus carunculatus - Wattled cranes into a non-breeding flock of wattled cranes. Each crane, which had had a lengthy period of rearing in a natural environment, was placed in a 3m x 3m temporary pen in 1 metre of water for 18-20 hours at the release site before release. (J445.S15.w1)
    • The first bird released was placed next to a wild flock roost site and was released  into a flock of 36 wattled cranes. (J445.S15.w1)
    • The other two birds were released together into a flock of 22 wattled cranes. (J445.S15.w1)
  • A parent-reared Grus leucogeranus - Siberian crane chick was released in Iran at a site used by a wild Siberian crane and a Grus grus - Common crane. (N48.4.w1)
  • An isolation-reared female Grus leucogeranus - Siberian crane which had been recaptured after dropping out of a migration with wild cranes in March 2004 was banded and released in the Freydoon Kenar Trapping Area (Damgah) in Iran, two days after the arrival of two individual (not paired) wild Siberian cranes in early November 2005. (N48.7.w1)
  • A female parent-reared Grus leucogeranus - Siberian crane bred 2004 was released in January 2007 in Fereydoon Kenar Damgah where a wild male was present. (N48.8.w1)
  • A male parent-reared Grus leucogeranus - Siberian crane bred 2005 was kept in Iran in a pen near Fereydoon Kenar Damgah during spring and summer 2007 and was released in November 2007, flew out of the damgah and was caught in a night net at Sooteh and released again on 17th December. (N48.9.w1)
  • A female parent-reared 2005-hatch Grus leucogeranus - Siberian crane was released in December 2008 at Fereydoon Kenar Damgah where a wild male was present. (N48.10.w1)
  • Between 1992 and 2009, 126 Grus leucogeranus - Siberian crane were released into the wild on the breeding grounds, wintering grounds or migration areas [no information regarding release type].  (P143.1.w1)
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Soft Release

Soft release involves animals being maintained in an enclosed area/pen at the release site for a period of time before release, for acclimatisation. After the animals are released, they are given variable assistance such as continued food provision at or near the release pen.
  • A period of acclimatisation allows the animals to become familiar with the local climate, surrounding landmarks and natural foods etc. in the acclimatisation pen. (B482.13.w13)
  • Soft release gives time for the individuals being released to make the necessary behavioural adjustments which will assist them in surviving in the wild, such as foraging, locomotion and social interactions. (B709.36.w36, J471.2.w2)
  • Soft release may encourage site fidelity and homing behaviour, reducing unwanted dispersal. (B708, J471.2.w2)
  • Soft release may enable animals to recover from handling and transportation to the release site, so that they have sufficient energetic reserves post-release. (J17.107.w1)
  • Soft release may enhance cohesion of social groups. (B708)
  • An additional advantage of soft release is that it acts to some extent as a further period of quarantine during which disease problems in the release candidates can be detected. (J64.12.w5)
  • Birds of prey have been reintroduced by being reared in a hack-house - an artificial nest site - with food provided, the fledgling then being able both to fly free and start hunting, but also to return to the site where food is provided. (B709.40.w40)
    • When small birds of prey are released by this method there is an increased risk of the young birds, unprotected by parents, being predated. (B709.40.w40)
    • Premature dispersal (birds getting lost before they have learned to hunt successfully) is a risk. (B709.40.w40)
  • For birds, one method of soft-release is to rear the birds in an open-topped pen and allow them to fly out as they fledge. (J59.9.w2)
    • In a study with mallard, soft release (ducklings initially released into a large, predator-proof pen, supplementary fed and allowed to fly out when fledged), showed higher survival than direct-released birds. Also, survival was higher for those released early in the season than those released later. Additionally, those released by the gentle method returned earlier in the spring, which may beneficial for nesting. (J59.9.w2)
Crane Considerations Soft-release or gentle release of cranes involves holding them in a pen at the release site for two weeks or more prior to release, continuing to provide food after release and allowing them to acclimate to the release site and their free-living situation gradually. (B115.11D.w16) This method of release has produced generally much higher higher survival than hard release in Grus canadensis - Sandhill cranes. (P87.6.w5) Actually rearing cranes at the release site enables the chicks to learn their local habitat, foods etc. (P87.8.w2)
  • Baiting of wild cranes can be used to bring them into the release area and encourage mingling with released cranes. (B115.11D.w16, P87.6.w3)
  • Mississippi sandhill cranes (Grus canadensis - Sandhill crane), parent-reared in captivity, were soft-released by being brailed and placed into a two-hectare, open-topped predator-proof pen at the release site. Food was provided in hoppers under shelters (as they had been used to previously) plus corn was scattered on the ground. The cranes started using the feeders less and foraging on the scattered corn more by about 10 days, and then also started probing the ground. Brails were removed from a few cranes at a time at one-week intervals. They gradually started leaving the pen (some sooner than others), but returning to feed. Use of the surrounding area gradually increased. Some associations were seen with wild cranes, but none permanent. All of the cranes were still alive five months after release. (P87.1981.w7)
  • Mississippi cranes parent-reared and soft-released produced 41/66 birds surviving for at least a year, with most of those which survived more than a few months becoming integrated with wild cranes. (P87.6.w5)
  • Greater sandhill cranes (Grus canadensis - Sandhill crane) were isolation/costume reared before being moved at 10 weeks old to the Necadah National Wildlife Refuge release site, where they were kept in a corral, led out during the day to forage, and allowed to fly out when they fledged. All five healthy chicks released in this way were found to interact positively with wild cranes within days of fledging and four were positively identified in Wisconsin the following spring after migration, with one being identified again in Florida in the next winter. (J54.8.w1)
  • Grus canadensis - Sandhill cranes costume-reared near the release site were soft-released into an open-topped 1.85 hectare pen. The pen was designed to keep predators out (2.42 m fence, plus 0.3 m buried below ground to stop predators digging in and two electrified wires 0.6 and 0.3 m below the top to discourage climbing) and included mudflat, shallow marsh and a small upland island. A crane costume dummy was placed in the pen (a costume similar to those worn by the caretakers, but stuffed and mounted on a metal stake) for about the first two weeks. Ear and shelled corn (maize) was placed in the pen to both provide food for the released chicks and attract wild cranes. Some of the released chicks integrated with the wild cranes and migrated with them. Others which failed to migrate were re-caught using the costume and transported to one of the staging areas used on migration; most of those then migrated with the other cranes. [1992](P87.6.w3)
    • In 1989, baiting was discontinued earlier than in 1988 and wild cranes left but the chicks remained as a group at the release location. (P87.6.w3)
    • It was found that if chicks which were less bonded to each other were released in small groups close to wild cranes they were more likely to stay with, and migrate with, those cranes. (P87.6.w3)
    • Released cranes also bonded with and followed released cranes from previous years. (P87.6.w3)
    • For 38 chicks, released 1988-1990, minimal survival at one year post-release was 84%, with a minimum return rate to Upper Michigan of 74%; the estimated return rate, excluding those found dead, missing since their first winter, or not located during the summer (individuals with non-functioning transmitters) was 97% (one bird which had been released in Wisconsin during the previous autumn migration never returned to the Upper Peninsula area of Michigan. (P87.6.w3, P95.1.w2)
    • Only two mortalities were confirmed, one predation and one apparently aflatoxicosis. (P87.6.w3)
    • Juveniles which did not correctly begin migration from their natal area with wild cranes often deviated or stopped short of the main wintering area, but they did winter more in the major normal areas in subsequent winters. (P87.6.w3, P95.1.w2)
    • Males returned more closely to the rearing area while females dispersed more, yearling females being found on average 26.6 km from the rearing/release area, compared to only 3.5 km for yearling males. The distribution of females appears to be more closely related to that of the cranes they are associating with.(P95.1.w2)
    • By two years old most of the released males and some of the females were paired with wild cranes. (P95.1.w2)
    • Captive reared birds were confirmed nesting in 1993 on Seney National Wildlife Refuge, with chicks produced in two nests. [1994](P95.1.w2)
  • Greater sandhill cranes (a migratory species) captive-reared and released in Florida in spring at about a year old showed 56% survival to a year after release; the single largest cause of mortality was predation (6/12 deaths), particularly by bobcats and two thirds of the mortality occurred during the fist six months post-release. (J40.57.w2)
  • Soft-release was used for translocation of sub-adult (two-year-old) Florida sandhill cranes to Grand Bay Wildlife Management Area, Georgia. Cranes captured using oral sedation (Alphachloralose Sedation of Cranes) were kept in an open-topped pen at the release site, monitored, provided with supplementary food and prevented from flying by means of temporary wing brails (Brailing), which were moved from one wing to the other after two weeks, to prevent wing atrophy. Release was initiated simply by removing the wing brails one evening; cranes flew out of the pen during the following day. (P87.8.w9)
  • Group-reared sandhill cranes and whooping cranes led by microlight to Bosque del Apache NWR in New Mexico were not penned on arrival. However, field corn had been knocked down to make it accessible to cranes, encouraging wild cranes to use the area, a crane decoy and an all-terrain vehicle the cranes were used to being with while foraging were left at the feeding area, and a crane decoy was used to encourage roosting on a sandbar island; the researchers attracted the released cranes to the roost each evening and to the feeding area in the morning "until they developed daily movement patterns similar to the wild cranes" (an injured individual was placed on a sandbar island and supplied with food until it was able to leave the island). After coyotes swam to the sandbar one dawn, the released cranes were encouraged to change to other roosting areas. (P87.8.w10)
  • Isolation-reared greater sandhill cranes fledged in Ontario, trucked most of the way to a South Carolina wintering site and led the final 108 km by ultralight, after arrival the birds were penned at night for four days, then left free. A costumed dummy in the pen and provission of ample food encouraged the birds to stay in the area, and they were led on several local flights for familiarisation with the area initially. (P87.8.w12)
  • Greater sandhiill cranes which had been migrated following behind a truck were released by the one-by-one technique into a flock of wild sandhill cranes; (P87.8.w13)
  • Grus americana - Whooping cranes, parent-reared in captivity or isolation (costume) reared, were soft-released on the Kissimmee Prairie area in Florida. They were confined to a pen initially by brailing one wing and were either released by two weeks or the brail was changed to the other wing for the remaining time in the pen. In 1993 and 1994, birds were released directly from this pen while in 1995 they were moved after dark to a smaller satellite pen in optimum habitat, and allowed to fly out, or walk out after pen side was removed three days later. (P87.7.w16)
  • Soft release with costumed caretakers continuing to interact with chicks after release, and careful use of dummy costumes may be advantageous in the post-release period. (P87.8.w21)
  • Grus americana - Whooping cranes costume-reared at Patuxent Wildlife Research Center, Maryland and migrated behind an ultralight from Necadah NWR, Juneux County, Wisconsin to Florida in autumn 2001 were soft-released, allowed to roam during the day but (after predation of two birds) encouraged to roost in the pen or in safe water areas at night, and provided with drinking water and supplementary food (commercial pellets). (P87.9.w6)
  • In northern China, Grus japonensis - Red-crowned crane and Grus vipio - White-naped crane chicks have been hand-reared, soft-released into the Zhalong marshes, then returned to captivity for the winter. After this, the birds have paired either with each other or with wild individuals and nested in the marshes around their rearing area. Offspring of these birds have generally been kept with their parents in captivity through their first winter then allowed to join the free-living flocks during the following year. It was noted that offspring from the tame/wild pairs were more tolerant of humans and therefore more able to live in a human-dominated landscape than were fully wild cranes. (B115.11D.w16)
  • Grus grus - Common crane in The Great Crane Project, UK in 2010 were initially released into two 30 x 30 m roofed pens (aviaries), about half into each pen. Three weeks later the two pens were joined together to give a single roofed pen, and the gates were opened to allow the birds access to the larger, fenced but un-netted two hectare enclosure around the pens. The external gates from the large enclosure onto the surrounding site were also opened at this time. (D449)
    • Decoys were placed outside the enclosure, with supplementary food. These were periodically moved, and food was put out while the cranes were roosting. (D449)
    • Costumed parents led the cranes out of the enclosure and showed them one of the external feed points. (D449)
    • Costumed parents visited the cranes initially daily, later only to fill the feeders, weekly. (D449)
    • Supplementary food was provided throughout the first winter and particularly during freezing weather, but reduced after the frosts. (D449)
  • Grus grus - Common crane in The Great Crane Project, UK in 2011 were initially released into one large aviary 60 x 30 m, since the birds had formed a single cohort before transfer to the release site. (D450)
    • The fledglings were seen practicing flying in this space.
    • After about two weeks, birds released the previous year started visiting the enclosure.
Rearing by mixed wild/tame parents
  • In Japan, wild male Grus japonensis - Red-crowned crane have been caught, wing-clipped and placed in large open breeding enclosures. Wild females have paired with these males and the resultant offspring have fledged, left the pens with the females and joined the wild flock. (B115.11D.w16, J54.13.w1, P87.1975.w6)
    • The juveniles, staying tame, have been trapped and overwintered indoors rather than being allowed to migrate, then released into the local marshes in spring, where they have bred, showed strong philopatry (whether pairing with others from the tame flock or with wild cranes) and have made use of marsh habitat close to farms, which was not being used by the fully-wild birds; this has functionally increased the breeding range. (J54.13.w1)
  • At the Khinganski Nature Reserve, northeastern Russia, rearing by mixed tame/wild parents with release of the semi-wild offspring is being attempted with Grus japonensis - Red-crowned crane and Grus vipio - White-naped crane, with the aim of making the wild population more tolerant of humans and therefore able to use available habitat which is otherwise avoided due to human activity. (B115.11D.w16)
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Fostering and Cross-Fostering

Fostering of eggs (or chicks) is a special release method which can be used in birds, in which eggs (or chicks) are placed into the nests of free-living birds release area. The chicks are reared by the host parents, with all the advantages of parent rearing.
  • In some circumstances eggs are placed into the nests of the same species, either to replace known infertile eggs or to produce a larger clutch, in which case additional food may be provided either directly or (preferably) by ensuring an abundant source of food with which the parent birds can feed the chicks; this has been carried out successfully with raptors, enabling wild pairs to rear much larger broods than normal.
  • Alternatively, eggs (or chicks) are placed into the nest of another bird species, a process known as cross-fostering. (B709.40.w40)
    • Cross-fostering can have variable results. In particular, there is a risk that cross-fostered birds will sexually imprint on the foster parents rather than on conspecifics. Whether or not this will happen is highly variable between species. (B707.12.w12)
    • For peregrine falcons, placing chicks into nests of wild prairie falcons has been effective, with the chicks being reared, fledging and then forming pairs with birds of their own species. (J54.13.w1)
    • Release of groups of incubator-hatched masked bobwhite quail together with wild adult, vasectomised male Texas bobwhite quail has improved survival of the released quail, because the males tend to shepherd the chicks, teaching them appropriate responses to predators  and taking them to food and cover. (J54.13.w1, B709.39.w39)
  • Whenever eggs are transported it is important to transport them within a container in which they will be protected from breakage. When eggs which have already been incubated are transported it is also important to transport the eggs at an appropriate and fairly stable temperature.
  • Fostering can enable the young to learn essentail behaviours and traditions. (B708)
  • Cross fostering can be used to create a new population in the location where the birds are hatched and reared, and even a new migratory route, if used with species which learn their migratory route by following their parents. This has been used in geese in which it was thought that traditional wintering grounds had become less suitable:
Crane Considerations
  • Taking eggs from the nests of wild whooping cranes is feasible without significant negative impact on the free-living whooping crane population because the cranes rarely abandon their nest after being disturbed for egg removal, and because although whooping cranes usually lay two eggs and often hatch two chicks it is rare that both chicks survive to fledging. Removing one of two fertile, developing eggs from a nest does not significantly reduce the number of chicks fledged per wild pair. (B709.25.w25, J727.6.w1)
  • Productivity of whooping cranes at Wood Buffalo National Park has been improved by placing known fertile, developing eggs into nests of pairs with consistently low hatching success with their own eggs. (J727.6.w1)
  • Further information regarding the survival of cranes following cross-fostering is provided in Post-Release Activities - Studies and Assessment
Grays Lake Experiment
  • During the period 1975 - 1988, 288 eggs of Grus americana - Whooping cranes, 215 from the Wood Buffalo National Park (WBNP), Canada, and 73 from breeding pairs at Patuxent Wildlife Research Center, Maryland, were placed into the nests of Grus canadensis - Sandhill cranes at the Grays Lake Refuge. (J727.6.w1)
  • Studies before the experiment started had found that the sandhill cranes at this location showed a similar nesting chronology to the whooping cranes at Wood Buffalo National Park; that they had a high breeding density (250 pairs nesting annually in 8,900 hectares) and high nesting success (78 - 92%);that they were unlikely to desert the nest after egg manipulation; that pesticide and heavy metal contamination appeared to be low. Additionally, the geographical range used is at the western periphery of the known historical range of whooping cranes, and there are many national or state wildlife refuges in the summering, wintering and migration route areas. (B709.25.w25, J727.6.w1)
    • Additional reasons why the experiment was thought likely to succeed were similarities in the species such as: monogamous pair-bonds; tendency to return to the same nesting territory each year; eggs of similar shape, size and colouration; same number of eggs per clutch; similar incubation periods; hatchlings similar in appearance; family units maintained for similar times, with young remaining with their parents for about nine months; omnivorous diets; and use of traditional geographical ranges, learned from the parents. (J727.6.w1)
    •  Known differences included: whooping cranes more carnivorous; whooping cranes more aquatic; ethological differences in displays and calls. It was thought that the differences in displays and calls would be sufficient to prevent hybrid pairs being formed. (B709.25.w25)
  • Of the 215 WBNP eggs, 15 were infertile or did not contain a viable embryo. Overall, 77% of the eggs from WBNP and 60% of those from PWRC hatched. Primary causes of egg failure were predation by coyotes, infertility or embryonic death. (J727.6.w1)
  • During 1975-1977, only 13 of 35 hatched chicks from eggs from WBNP reached fledging (55.6%, 36.45 and 22.2% in 1975, 1976 and 1977, compared with 61.9%, 40.0% and 34.3% for sandhill crane chicks in the same years. Out of five chicks hatched from eggs from Patuxent, four were lost at <30 days, two during a severe snowstorm and two from unknown causes while the fifth was lost at 74 days, probably to coyotes. (B709.25.w25)
  • Mortality was high in the first 30 days, with 47 chicks lost during this period, mainly due to predation by coyotes and red foxes, and to poor weather conditions (late snow storms, low (freezing) temperatures, rain, hail). In some years, drought and low water levels led to deaths of chicks from starvation. In such years chicks that survived to fledging may have been predisposed to mortality during migration, due to poor physical condition. (J727.6.w1)
  • In the first three years, conditions were poor: abnormally late nesting and unusually cold weather extending into the breeding season in 1975, drought in 1976, with poor resultant food availability, and drought with very low spring water levels in 1977. Supplementary food was provided and widely used in 1976 and 1977, and supplementary water provided in 1977 ( pumped into a stock pond, and in a water trough). (B709.25.w25)
  • In the first year, problems were seen with harassment by cattle and problems crossing barbed wire fences. Thereafter the cattle were removed from some areas and the fences modified. (B709.25.w25)
  • By 1996, only 13 of the 85 whooping cranes fledged at Grays Lake were known to still be alive. For 27 of the 72 which died, mortality was known to be due to collisions with power lines(40%), mainly in Colorado, accidents involving fences (22%), disease (18%) and avian predation (7%). Except for yearlings, survival was lower in females than in males, leading to a skewed sex ratio.
  • No permanent pairs formed between whooping cranes. (J727.6.w1)
    • During 1986 -1988, 13 translocations were carried out involving nine different birds, aiming to improve pair formation. These efforts were unsuccessful. (J727.6.w1)
Guide birds
  • An attempt was made to bond adult Grus americana - Whooping cranes which had been cross-fostered and reared by sandhill cranes as part of the Grays Lake experiment, to whooping crane chicks, in the hope that they would act as guide birds for the young whooping cranes. In 1993 a male and a female whooping crane were caught, held in a pen and introduced to chicks when the chicks were a few days old.  In 1994 a single whooping crane was similarly held and introduced to a chick. The male and female in 1993 showed some signs of possibly pair bonding while in the pen, and fed the chicks on invertebrates, but on release the male showed no interest in the chicks; while the female showed some interest, she was more interested in the male. The adults migrated and the chicks did not; they were recaptured and two died due to capture myopathy and shock respectively after transportation. In 1994, the chick was limping at the time of release and although the adult repeatedly approached it, the chick lay down, did not respond and was eventually taken in for veterinary treatment. The chick was released after rehabilitation but the adult then ignored the chick, which was harassed by sandhill cranes; the chick was later fatally injured by a coyote. (P87.7.w15)
Sandhill cranes in Florida
  • Greater sandhill crane (Grus canadensis - Sandhill crane) eggs were cross-fostered to wild Florida sandhill cranes in Florida. (J40.57.w2)
    • These showed only 39% survival from hatching to dispersing from their natal home range (and only 15% of transferred eggs resulted in dispersing young), possibly because the differences in nesting season meant that the transfers of eggs and hatching of chicks took place quite late in the Florida sandhill crane nesting season, when water levels were dropping and conditions were not optimal for nesting and chick rearing. (J40.57.w2)
Siberian cranes in Russia
  • Two Grus leucogeranus - Siberian crane eggs were placed into the nest of a pair of Grus grus - Common crane in June 2003 in the Kunovat River Basin, Russia. The pair were fond with one Siberian crane chick on 11th August 2003; the chick was caught, banded and released. (N48.5.w2)
  • On 1st and 2nd June 2011Grus leucogeranus - Siberian crane eggs were placed into nests of pairs of Grus grus - Common crane  in the forest-tundra zone on the border between Uvat and Tobol districts, Tyumen Region, Russia, where both species are known to breed. In each case, two eggs were placed into a nest with a one-egg clutch. Water testing was used to confirm that the projected hatching dates were the same. Both pairs of Grus grus - Common crane were observed to continue incubation after the addition of the eggs. (N48.12.w2)
  • In the Kunovat Wildlife Refuge, 39 eggs of Siberian cranes have been cross-fostered into nests of Grus grus - Common crane. (P143.1.w1)
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Migration considerations

Migratory species pose an extra problem in re-introduction, as they either have to be released into an area suitable for all-year use and encouraged to stay at that location (i.e. to not migrate) or they have to be enabled to migrate. Migratory species such as waterfowl learn their migratory routes, stop-over sites and wintering grounds from their parents and other conspecifics. For re-introduced birds, this can be problematic.
  • Ultralight aircraft have been used successfully to teach migration routes.
    • Costume-reared fledgling Branta canadensis - Canada goose were led by ultralight aircraft on autumn migration to wintering sites 640 to 1,312 km from the rearing area. Geese led in this way successfully navigated the return journey to their natal area in the spring, while this did not occur when geese were transported to the wintering areas by truck. (P87.7.w12)
Crane Considerations Migratory cranes require appropriate breeding grounds, migration routes, staging areas and wintering areas. Cranes develop strong bonds to their natal area with these being particularly close for male cranes. It is important for migratory cranes to be reared/fledged at the intended breeding area. Migratory routes have to be learned. This can occur by following other cranes (normally, it would be by fledglings following their own parents). Alternatively, this can be taught.
  • For the Gray's Lake experiment, it was assumed that cross-fostered whooping cranes would learn the routes by following their foster parents.
  • For migratory cranes, rearing and release at the intended breeding site is recommended, rather than the wintering site; greater sandhill cranes reared and released in Texas (wintering site) returned there for the summer despite having set off on the spring northward migration with wild cranes. (P87.6.w4)
  • Even if flying the whole migration route, and subsequently returning to the appropriate summer and wintering areas, cranes will find their own routes, no exactly follow the taught route. (P87.8.w17)
Ultralight led migrations
  • In 1995, it was demonstrated that costume-reared sandhill cranes could be trained to fly behind an ultralight aircraft. (P87.7.w12)
  • In October 1995, 20 sandhill cranes raised from nests from wild pairs were costume-reared; of these, 11 fledged sandhill cranes were led on migration by an ultralight aircraft, from Grace, Idaho, to Bosque del Apache NWR, New Mexico. Six of the cranes flew the whole distance. (P87.7.w13)
    • One stopped following, was temporarily lost and returned to the rearing area. This bird was later released in the wintering area and initally joined another of the released cranes, but then disappeared (killed or left the area). (P87.7.w13)
    • One flew some of the time but was transported by truck on some days. (P87.7.w13)
    • One had problems flying from the first day and was transported by truck from the fourth day onwards. After arrival in New Mexico it was found to have coccidiosis. (P87.7.w13)
    • Two cranes were killed in an attack by a pair of golden eagles. Other attacks were thwarted by the ultralight aircraft; the cranes also became more adept at avoiding the eagles. (P87.7.w13)
    • Within days of arriving at the wintering area, the young birds were associating with and imitating the behaviour of, wild cranes. (P87.7.w13)
    •  survived to spring and migrated north; by May 1996, two of these birds had returned to within 53 km of the rearing area in Idaho. (P87.7.w13)
  • Group-reared greater sandhill cranes and whooping cranes were led on migration from Idaho to New Mexico by ultralight. (P87.8.w10)
  • Whooping cranes costume-reared at Patuxent Wildlife Research Center, Maryland, were transferred at 47-64 days old to Necadah NWR, Juneux County, Wisconsin. Training to follow the ultralight aircraft was carried out from soon after hatching. Migration with the ultralight started 17th October with arrival at an inland site in Florida, near Chassahowitzka NWR on 3rd December and at the site of the salt marsh release pen on 5th December, 2001. Six birds migrated with the ultralight; the sevent, which had had early wing problems, was transported by truck. The cranes were then soft-released, allowed to roam during the day but (after predation of two birds) encouraged to roost in the pen or in safe water areas at night. (P87.9.w6)
  • Grus leucogeranus - Siberian crane chicks in Russia were flown behind a hang glider at stopover sites along migration. It had been intended to fly the cranes behind the hand glider along the route, but designated take-off and landing sites were flooded, therefore the cranes had to be transported and only given flights at suitable stopover points. It was considered that the chicks had had good flight training and adapted well to different weather conditions. Seven of the cranes were released along the Siberian Crane Flyway. (N48.3.w2)
  • Further information regarding the survival of cranes following these migrations is provided in Post-Release Activities - Studies and Assessment
Truck-led migrations
  • Greater sandhill cranes reared in Ontario and trucked most of the way to the designated wintering site in South Carolina showed some tendency to move northwards in spring but did not migrate properly. (P87.8.w12)
  • In October 1995, 10 greater sandhill crane chicks were trained to enter, ride in, and fly after a truck. The truck was then used to transport the chicks from Patuxent Wildlife Research Center, Maryland, USA, to northern Arizona; after five weeks of flight training there, the birds were led about 600 km south to a wintering area on the Arizona/Mexico border (Buenos Aires National Wildlife Refuge). (P87.7.w14)
    • A major hazard was power lines, with at least 120 sets of lines counted along the route. Three collisions of the juveniles with power lines occurred, one being fatal while in the others the birds were uninjured; many near-collisions occurred. It was noted that flying the cranes near power lines should be avoided when the birds were tired. (P87.7.w14)
    • Another hazard was attacks by golden eagles, which scattered the birds. (P87.7.w14)
    • Two chicks scattered by golden eagle attacks flew back to the migration start site, about 130 km away and two others were found about 55 km form the attack site in the direction of the migration start site, indicating navigation back to their release site. (P87.7.w14)
    • Cranes overheated in flight on hot days. (P87.7.w14)
    • Forests posed problems, as the trucks were out of sight while the cranes flew over the trees. (P87.7.w14)
    • "Abandonment training" might be useful: one bird showed higher attachment after being lost for five hours and it was considered that solo stranding of each chick for a few hours followed by making sure the chick "found" its caretakers, might improve attentiveness during migration. (P87.7.w14)
    • After wintering in a pen, these cranes failed to migrate north in spring, given weekly opportunities to do so, nor did they start migration back to the wintering area in the autumn; when "jump started" they did continue flying south but not along the desired route. The following spring they did start migrating north but had to be removed after landing on a golf course, were translocated a short distance to a more appropriate site, but apparently became disorientated and flew east rather than north. (P87.8.w14)
  • Greater sandhill cranes were migrated flying behind trucks in 1996. Of the 14 birds, 11 participated well in the migration (i.e. flew behind the trucks). (P87.8.w13)
    • As in 1995, a major hazard was power lines; there were two fatalities and about 15collisions not resulting in injury. (P87.8.w13)
    • Attacks by golden eagles were avoided; where these were anticipated, a truck went ahead and fired warning shots. (P87.8.w13)
    • The cranes showed considerable tenacity in following even in poor light or extreme dust. (P87.8.w13)
    • Later departure than in 1995 avoided the worst of the heat, which was advantageous. (P87.8.w13)
    • By the end of migration they were acclimated to humans sufficiently to possibly become nuisance birds. (P87.8.w13)
  • Further information regarding the survival of cranes following these migrations is provided in Post-Release Activities - Studies and Assessment
Stage-by-Stage migration
  • Costume-reared greater sandhill cranes were trailer-trained for transport and trained to roost near a plastic crane decoy and a costume-draped humanoid frame (a scare eagle), transported from the rearing site (Fish Springs National Wildlife Refuge, west-central Utah) by trailer with periodic stops about every 30 km, at many of which they were given the opportunity to make brief flights, then were released into a flock of wild sandhill cranes at the desired wintering area (Gila Bend, Arizona).
    • Repeatedly, the most subordinate bird in the group would become more difficult to catch and return to the trailer.
    • In 1998, five birds died from an unknown toxin at one of the stops. Another bird was lost to the experiment after refusing to allow recapture.
    • From the 1998 cohort, none of the birds migrated back to the rearing area in the spring. From the 1999 cohort there was some evidence of the birds having learned the route, but they then moved away from the chosen wintering area.
  • Costume-reared Grus leucogeranus - Siberian crane were transported initially on a boat with stops to fly behind the ultralight plane, then by car, with the 2,000 km journey complete as planned to the Belozersky Wildlife Refuge, Tyumen Region. It was planned that here they would join the wild Grus grus - Common crane flocks on migration. Unfortunately due to an early heavy snowfall the common cranes left early when the juveniles had had only one week to start socialisation (usually three weeks were required). One juvenile did leave with the wild cranes and flew with them for 500 km before dropping behind. The others were returned to Oka Crane Breeding Centre with the plan of releasing them in the Kunovat River Basin in 2013. (N48.12.w3)
  • Further information regarding the survival of cranes following these migrations is provided in Post-Release Activities - Studies and Assessment
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Authors & Referees

Authors Debra Bourne MA VetMB PhD MRCVS (V.w5)
Referee --

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