The current status of European populations of Osprey (Pandion haliaetus) is clearly different among regions (Saurola 1997, J. Raptor Res. 31:129–137). Although the central and northern areas of Europe support large populations in favorable conditions with increasing or stabilized trends, the situation in the Mediterranean basin is unfavorable, with few, small and isolated populations (Thibault et al. 2001, Le Balbuzard pêcheur en Corse: du martyre au symbole de la protection de la nature, Ed. Alam, Ajaccio, France). In Spain there are only two small breeding populations, each consisting of about 15–20 pairs, both in insular territories: the Canary Islands and Balearic Islands (Martí and Moral 2003, Atlas de las aves reproductoras de España, Ministerio de Medio Ambiente, SEO/BirdLife, Madrid, Spain). The species was extirpated from mainland Spain after 1981, when the last pair bred in the province of Alicante (Urios et al. 1991, Atlas de las aves nidificantes de la Comunidad Valenciana, Generalitat Valenciana, Valencia, Spain), after a continuous decline in the number of breeding pairs since the 1960s.
The Iberian Peninsula is an important passage area for migratory Ospreys traveling between Europe and wintering grounds in Africa, and a small number of northern European birds winter in southern Spain (Saurola 1997). Despite apparently suitable breeding conditions (Casado 2005, J. Raptor Res. 39:168–173), Ospreys have been unable to recolonize the region, perhaps due to natal philopatry and low breeding dispersal (Poole 1989, Ospreys: a natural and unnatural history, Cambridge Univ. Press, Cambridge, U.K.).
To accelerate the return of the Osprey to the Iberian Peninsula, a reintroduction program commenced in 2003 in the region of Andalusia (Casado 2005). Between 2003 and 2005, 42 young Ospreys were released, by means of hacking, at two locations: a reservoir in the province of Cádiz and a coastal marshland in the province of Huelva.
In 2005, after 2 yr of releasing juveniles and observing interactions between adults and released young, we recorded three nest construction attempts by different non-reintroduced Osprey pairs close to the hacking site in Cádiz. All attempts were less than 40 km from the release point and one was only 2 km away, in the same reservoir. Only one of these three pairs constructed a complete nest and made a breeding attempt, at a reservoir 30 km from the nearest release point. The nest was built on top of an inactive power pole situated over the water near the shore. After a courtship period, from February to early March, the pair started to incubate on 16 March, but 2 d later a strong wind dislodged the nest. The pair stayed one more day at the power pole, but stopped incubating.
Failed breeding attempts may reduce site or mate fidelity in subsequent breeding seasons (Newton 1979, Population ecology of raptors, Buteo Books, Vermillion, SD U.S.A.). Thus, to encourage site fidelity, we erected an artificial nest on another pylon 150 m away from the original nest 2 d after incubation stopped. The pair occupied the new platform the next day, showing pre-laying behavior until 29 March, when they began incubating a single egg. This egg was sterile or addled, because it had not hatched after more than 60 d of incubation, which was >15 d more than the maximum documented incubation period for Osprey (mean = 39 d, range 35–43 d, Poole 1989).
Because our objective was to encourage successful breeding behavior by the pair, and because we knew definitively that the natural attempt had failed, we decided to translocate two Osprey chicks from a demographically secure population to be fostered by the pair before this breeding attempt was definitively abandoned. Before proceeding, we had to ensure that the pair would be good foster parents, as they probably had no prior breeding experience. Also, collection and transportation of the young from the donor country would take several days. Consequently, we decided to use temporary cross-fostering by placing a chick of another non-endangered species in the nest.
Cross-fostering is a manipulative technique that has been employed in many bird species including several birds of prey (Bird et al. 1985, International Council for Bird Preservation, Technical Publication No. 5:433–438; Cade and Temple 1994, Ibis 137:161–172). Although not recommended as a primary method in management programs for species recovery because of potentially contradictory results (Slagsvold et al. 2002, Proc. R. Soc. Lond. 269:1449–1455), cross-fostering has proved to be a useful supplementary technique in both long term and temporary situations (Bird et al. 1985; Barclay 1987 in B.G. Pendleton et al. [Eds.], Raptor management techniques manual, Natl. Wildl. Fed., Washington, DC U.S.A.). In this case, the selected cross-fostered species was the Black Kite (Milvus migrans) which is abundant in the area, is similar in size to an Osprey, and can feed on fish. This was the first known cross-fostering attempt with Osprey and Black Kite. On 25 May we translocated one 12 d old Black Kite from a nest with three chicks to the artificial Osprey nest, leaving the single egg laid by the Ospreys as a further attraction. At first the Ospreys displayed alarm behaviors, flying over the nest and calling, but after 1.7 hr, they landed on the nest. The male returned 1 hr later with a fish and the female fed the Black Kite chick. We monitored the nest during the next days to ensure the parental behavior of the pair continued appropriately.
After the cross-fostering proved successful, we began the fostering experiment. Two Osprey chicks, 12 and 15 d old, were collected from a nest with traditionally low productivity due to human disturbance in NE Germany, which supports one of the most successful breeding populations in Europe (Schmidt 2001, Vögelwelt 122:117–128). These chicks were translocated to the nest in Cádiz on 7 June. We substituted the Osprey chicks for the Black Kite and the addled egg, and left two fish in the nest as well. The adult Ospreys landed on the nest and started to feed both chicks just 33 min after the chicks had been introduced. The Black Kite was apparently in good condition and was returned to its natal nest.
During the subsequent weeks, the Osprey nest was monitored to ensure adequate development of the chicks and to prevent human disturbance. Before fledging, the Osprey chicks were measured, weighed, and ringed, and blood samples were collected. Both were equipped with conventional VHF tail-mounted transmitters to allow us to track movements during the post-fledging period, and a satellite PTT was mounted on the larger chick so we could monitor migration. In mid-July both juveniles fledged at 53 and 55 d old, within the age range recorded in migratory populations of Osprey (Poole 1989). During the post-fledging period the young Ospreys improved their flight skills and attempted fishing, although we could not confirm any successful captures. Movements were all within 1500 m of the nest and both parents, especially the male, provided fish for both juveniles.
In early September, both young left the reservoir and started their migration to wintering grounds in Africa, 48 and 47 d after fledging. This was approximately 15 d more than the average post-fledging period recorded in other European populations although within the upper range limit (Bustamante 1995, Bird Study 42:31–36). The final destination of the bird with the PTT was a typical Osprey wintering ground in the Gambia River basin of Senegal. It reached the wintering area almost 19 d after departing its natal site, with an average speed of 148 km/d, which is within the range recorded for other Ospreys (Kjellén et al. 2001, J. Avian Biol. 32:57–67).
After the juveniles departed, the adult Ospreys stayed all winter in the vicinity of the breeding area, which is typical for other Osprey populations of southern Europe (Poole 1989, Thibault et al. 1995, J. Raptor Res. 29:204–207).
Conspecific attraction is potentially important as part of the process of habitat selection in territorial or non-colonial birds (Ahlering and Faaborg 2006, Auk 123:301–312). If the increased breeding activity recorded after the start of the reintroduction program was not coincidental, then the presence of young released Ospreys may have influenced the decision made by the non-released birds to breed at this location. This has implications for future management of Osprey breeding populations.
For conservation of threatened species, techniques like temporary cross-fostering can be appropriate and useful under certain conditions, and thus should be considered as potential tools in conservation programs. During spring 2006, a new breeding attempt by the Osprey pair in the reservoir of Cádiz was confirmed. Thus, techniques such as cross-fostering, fostering, and habitat management have contributed to the successful settlement of the first breeding pair of Ospreys in mainland Spain after an absence of 25 yr. Because of our results, cross-fostering and fostering techniques will continue being used in the Osprey reintroduction project, but only as emergency tools to ensure successful breeding attempts of those pairs with problems in the early stages of the breeding cycle.
We thank the Brandenburg State Bird Conservation Dept. for the supply of Osprey nestlings and in particular Torsten Langgemach and Paul Sömmer for making possible the collection and translocation of the chicks from Germany. We also thank Philip Whitfield, James E. Woodford, Mark Martell and an anonymous referee for reviewing early drafts of this letter. This study was supported by G.I.A.S.A. and Junta de Andalucía, and by a pre-doctoral grant F.P.U. from Ministerio de Educación y Ciencia.