Action: Foster eggs or chicks of raptors with wild conspecifics
Key messagesRead our guidance on Key messages before continuing
- Ten out of 11 studies from across the world found that fostering raptor chicks to wild conspecifics had high success rates.
- A single study from the USA found that only one of six eggs fostered to wild bald eagle Haliaeetus leucocephalus nests were hatched and raised. The authors suggest that the other eggs may have been infertile.
- A replicated study from Spain found that Spanish imperial eagle Aquila adalberti chicks were no more likely to survive to fledging if they were transferred to foster nests from three chick broods (at high risk from siblicide), compared to chicks left in three-chick broods.
- A replicated study from Spain found that young (15–20 years old) Montagu's harrier Circus pygargus chicks were successfully adopted, but three older (27–29 day old) chicks were rejected.
Natural variations in reproductive output can be detrimental when populations are very small, for example if pairs fail to produce fertile eggs or some pairs repeatedly fail to raise offspring successfully. One way to minimise this problem is to foster eggs and chicks between nests. Eggs and chicks from nests with more offspring than they are likely to be able to raise can be moved to those with infertile eggs. Alternatively, if a pair produces fertile eggs or healthy chicks but consistently fails to raise chicks then it may be possible to transfer offspring to a more successful pair.
In other circumstances it may be possible to foster chicks with other species (‘cross-fostering’). Studies describing this intervention are discussed in the following section ‘Foster eggs or chicks with wild non-conspecifics (cross-fostering).’
Supporting evidence from individual studies
A replicated study in the eastern USA in 1977-80 (Wiemeyer 1981) found that, of 12 captive bred bald eagle Haliaeetus leucocephalus nestlings (ten hand-reared and two parent-reared) fostered to wild nests when they were 2.5-5 weeks old, 11 were accepted by the foster parents. The remaining nestling was killed by a foster parent shortly after being introduced to the nest. Wild nests all had either eggs (including dummy eggs designed to induce continued incubation) or nestlings, which sometimes remained in the nest and were sometimes transferred to other wild nests. All foster pairs had histories of reproductive failure. In addition, in 1978-9, six captive-produced eggs were transferred to wild nests in the Chesapeake Bay area of Virginia and Delaware. Only one of these hatched and the eaglet was successfully raised and fledged. It is not known whether the other eggs were fertile, but the authors suggest that they may have been chilled after foster parents took a long time to return to the nest after the introduction of the new eggs. A further two eaglets were removed from the wild as eggs, hand-reared (described in ‘Artificially incubate and hand-rear birds in captivity’) and returned to more successful wild nests. Both eaglets were seen in advanced stages of development and were presumed to have fledged. This study also describes several ex situ interventions, discussed in ‘Use captive breeding to increase or maintain populations’, ‘Use artificial insemination in captive breeding’ and ‘Release captive-bred individuals’.
A small study in wetlands in the Doñana National Park, southern Spain, in summer 1984 (Gonzales et al. 1986), found that an orphaned fledgling Spanish imperial eagle Aquila adalberti was successfully supported by foster parents. The orphan was found after leaving the nest (aged approximately 91 days) and initially taken into captivity before being released, with some supplementary food, into the home range of a family with two young of approximately the same age (which had left the nest but which were still dependent on parental feeding). All young were fed and chased by parents at approximately the same frequency, suggesting the foster fledgling had been accepted. This study also discusses other interventions, in ‘Add perches to electricity pylons to reduce electrocution’, ‘Bury or isolate power lines’, ‘Use signs and access restrictions to reduce disturbance at nest sites’ and ‘Remove/treat endoparasites’.
A small study at a lake in Pennsylvania, USA, in 1988 (Rymon 1990), found that two osprey Pandion halieatus chicks were successfully adopted by a breeding pair that had lost their two chicks to predation. The foster chicks were 5.5 weeks old and were placed in the nest two weeks after the original young were last seen and were accepted later that day. No information is provided on fledging or subsequent survival but the author notes that “the nestlings were fed well and protected by their foster parents throughout the nesting period”.
A replicated study in 1977-88 in wetlands in the Doñana National Park, Andalucia, Spain (Ferrer & Hiraldo 1991), found that there were no differences in survival between chicks relocated from nests were siblicide was likely to occur into nests thought to be safer (68% of 19 chicks surviving) compared to unmoved chicks (82% of 77 chicks surviving from all unmanipulated nests and 73% of 18 chicks from broods of three). Twelve of the moved chicks were from within the park and seven were moved in from outside. This study also discusses other interventions, in ‘Add perches to electricity pylons to reduce electrocution’, ‘Bury or isolate power lines’, ‘Use signs and access restrictions to reduce disturbance at nest sites’ and ‘Remove/treat endoparasites’.
A 1993 review (Cade & Jones 1993) found that 69% of 71 captive-bred Mauritius kestrel Falco punctatus chicks fostered to wild nests on Mauritius between 1986 and 1992 survived until independence. This study is discussed in more detail in ‘Use captive breeding to increase or maintain populations’, ‘Artificially incubate and hand-rear birds in captivity’ and ‘Release captive-bred individuals’.
A 1995 update (Jones et al. 1996) of the same conservation programme studied in (5), found that fostering of hand-reared nestlings had a probability of surviving to independence. A total of 331 birds were released into various sites from 1984-1985 and 1993-1994 of which 78% became independent and 61% survived their first winter. Of these, 105 young were placed into 46 different broods (5-18 day old nestlings were placed in the nests of wild pairs that had been incubating for > 2 weeks containing 1-5 other nestlings). Overall, 96 (91%) were fledged and 78 (81%) of these survived to independence. A total of 44 wild breeding pairs successfully raised at least one young to independence. The remainder of the released young were hacked in nestboxes. At the end of the 1993-1994 breeding season, the natural population had recovered to 222-286 birds (containing at least 56 breeding pairs and 40-70 non-breeding birds).
A replicated study in cereal fields in Madrid province, central Spain, in the breeding seasons of 1991-6 (Arroyo & Garcia 2002) found that 15 Montagu's harrier Circus pygargus nestlings introduced into foster nests at the age of 15–20 days were all successfully ‘adopted’, whereas three fledglings introduced at the age of 27–29 days were rejected by their intended foster parents and attacked when they begged for food. All nests already had nestlings in and all but two received a single nestling. One of these one was given two 15-20 day-old nestlings, the other two 27-29 day-olds.
A replicated study in pine forests in Slovakia in summers between 1993 and 2000 (Kornan et al. 2003) found that golden eagle Aquila chrysaetos chicks removed from nests with two chicks, hand-reared or fostered in captivity and then fostered by wild conspecifics were successfully raised 74% of the time (of 35 fostering attempts). Failures were due to siblicide, predation or unknown causes. Without fostering, second chicks in eagle nests are frequently killed by siblings. In 2000 a chick was removed from a nest with two chicks in and initially placed in the nest used by the foster parents that year. However the parents ignored the chick, so it was moved to a nest used in previous years, 690 m away. Once in the second nest, the chick was fed and cared for. The authors suggest that the second nest was more obvious and so the parent eagles could see the foster chick more easily.
A replicated 2004 study of a Mauritius kestrel Falco punctatus release programme (Nicoll et al. 2004) found no difference in survival between birds ‘hacked’ as fledglings (and provided with supplementary food until independence), those fostered to wild breeding pairs or wild-bred birds (80% for 42 fostered birds; 80% for 46 hacked birds and 75% for 284 wild-bred birds). This study is discussed in detail in ‘Release captive-bred individuals into the wild to restore or augment wild populations’.
A small study in Maharashtra, western India, in March 2003 (Pande et al. 2004) found that a Bonelli’s eagle Aquila fasciatus (also known as Hieraaetus fasciatus) nestling that was removed from its nest when 40-42 days-old was repeatedly ejected from its nest by its parents after being returned. However, when transferred to a nest 250 km away occupied by a pair with two fledglings that had already fledged, the nestling was fed by both parents and fledglings for a week until it fledged in late March.
A small study at a reservoir in southern Spain in 2005 (Muriel et al. 2006) found that a pair of ospreys Pandion haliaetus successfully raised two chicks that were fostered in an artificial nest (see ‘Provide artificial nesting sites’) when 12 and 15 days-old. The suitability of the pair as parents was tested by temporarily fostering a black kite Milvus migrans with them. Only after the kite had been fed and looked after were osprey chicks introduced. Both chicks fledged aged 53 and 55 days and left on migration 47 and 48 days after fledging. One chick was monitored with a GPS locator and reached typical wintering grounds in Senegal.
- Wiemeyer S.N. (1981) Captive propagation of bald eagles at Patuxent Wildlife Research Center and introductions into the wild. Raptor Research, 15, 68-82
- Gonzalez J.L., Heredia B., González L.M. & Alonso N. (1986) Adoption of a juvenile by breeding Spanish imperial eagles during the postfledging period. Raptor Research, 20, 77-78
- Rymon L. (1990) Osprey nestlings fostered by hacked adults two weeks after predation of their young. Journal of Raptor Research, 24, 71-72
- Ferrer M. & Hiraldo F. (1991) Evaluation of management techniques for the Spanish imperial eagle. Wildlife Society Bulletin, 19, 436-442
- Cade T.J. & Jones C.G. (1993) Progress in restoration of the Mauritius kestrel. Conservation Biology, 7, 169-175
- Jones C.G., Heck W. & Lewis R.E. (1996) The restoration of the Mauritius kestrel Falco punctatus population. Ibis, 137, S173-S180
- Arroyo B.E. & Garcia J.T. (2002) Alloparental care and kleptoparasitism in the semicolonial Montagu's harrier Circus pygargus. Ibis, 144, 676-679
- Kornan M., Majda M., Macek M. & Kornan J. (2003) An Unusual Case of Adoption of a Golden Eagle (Aquila chrysaetos) Chick in the Mala Fatra Mountains, Northwestern Slovakia. Journal of Raptor Research, 37, 259-260
- Nicoll M.A.C., Jones C.G. & Norris K. (2004) Comparison of survival rates of captive-reared and wild-bred Mauritius kestrels (Falco punctatus) in a re-introduced population. Biological Conservation, 118, 539-548
- Pande S., Pawashe A. & Pednekar B. (2004) How long is too long? A case of fostering nestling bonelli's eagles Hieraaetufsa sciatus. Journal of Raptor Research, 38, 381-382
- Muriel R., Ferrer M., Casado E. & Schmidt D. (2006) First breeding success of osprey (Pandion haliaetus) in mainland spain since 1981 using cross-fostering. Journal of Rapto Research, 40, 303-304