Abstract. The Saker falcon (Falco cherrug) is a globally endangered species recovered in 2018 in Bulgaria. The only known active pair currently breeding in the wild in the country is made up of captive-bred and released birds, part of the first reintroduction programme for the species in Bulgaria and globally. As part of the project, sourced Saker falcons of known European origin are bred ex situ, the juveniles are parent-reared and released in the wild by means of the hacking method. The aim of this study was to evaluate the success and effectiveness of the captive breeding and rearing methods. The objective of the study was to investigate, compare and assess the results from fertilization, hatching, survival and hacking of captive bred Saker falcons registered over a 10-year period (2011-2021) at the Wildlife Rehabilitation and Breeding Centre (WRBC) Green Balkans, with a more detailed comparison of the breeding activities carried out in 2020 and 2021 (as the reintroduction programme was restarted in 2020). The results show more than threefold increase in output of laid eggs with the employment of the double clutching method in 2020 and 2021, and an overall 65% release rate of the hatched in the WRBC Saker falcons, for the investigated period.

Considered extinct as breeding species in the early 2000s, the Saker Falcon was recovered when the first active nest from the new history of the species in Bulgaria was discovered in 2018, formed of two birds that were re-introduced back in 2015. Currently, there is only one confirmed wild breeding pair in the country - the male from 2015 with a female changed in 2020, released again as a part of the programme, in 2016. This is a report on the preliminary results and analysis of the ongoing establishment phase of the re-introduction of the Saker Falcon (Falco cherrug) in Bulgaria - first ever performed for this species in the country and globally. The period studied is 2015-2020. Following the re-introduction activities started in 2011, the current phase is defined by standardised methodology and a unified approach. Analysed and presented are methods for captive breeding and hacking, the breeding performance of the falcons, the number of released individuals, data from the post-fledging dependence period and a model of population growth.

Abstract. With saker falcon populations on the mend globally, and in Bulgaria, special caution is taken to ensure the best possible care of the birds in their breeding programmes. Diets play a particularly important role as they affect the health of not only the breeding pairs, but also the young birds. With the ultimate goal for the captive-bred sakers being to reestablish viable saker falcon populations, releasing healthy falcons is paramount. As part of the ongoing saker falcon reintroduction efforts in Bulgaria, special emphasis was placed on well-rounded diets carefully tailored to the species, and the accompanying proper sanitation procedures. Outlined are best practices concerning the variety of dietary regimes for saker falcons in captivity and for released falcons based on their biology and behaviour. In captivity the meals were different for non-breeding birds, breeding birds, for nesting birds, and for birds with chicks in the nest. The hacked juvenile saker falcons had a change in diet every two weeks – starting with food similar to the one in the breeding facilities, to the more natural for the species meals. The different techniques aimed at replicating wild conditions resulted in a natural breeding behaviour for the saker falcons kept in captivity, and successful adaptation for the released juveniles. The meal preparation methods kept the risk of food-transmitted diseases to a minimum.

We collected blood samples from 44 Western Saker Falcons (Falco cherrug cherrug) and 68 Eastern Saker Falcons (Falco cherrug milvipes) in captivity at two facilities in Bulgaria to evaluate the health of individuals involved in a reintroduction project, and to gain insight into appropriate veterinary care and treatment of the birds. We compared the values of 18 biochemical parameters between the two subspecies and also between females and males. We found significant differences between the two subspecies for some of the parameters tested; however, they likely resulted from different feeding regimes at the two breeding facilities—one housing most of the Western Saker Falcons and the other all the Eastern Saker Falcons. Further tests should compare the biochemical values between birds of the two subspecies housed in the same facility. We also found significant differences in the biochemistry of female and male falcons across some parameters tested. After 20 yr without a record of Saker Falcons (Falco cherrug) nesting in Bulgaria, this globally endangered species has been successfully breeding there since 2018 as a result of an ongoing reintroduction project. By establishing baseline information on the differences in the biochemistry of Saker Falcons, our research may guide future studies of this rare species and aid veterinary care and treatment of the birds breeding in captivity.

The globally endangered saker falcon (Falco cherrug) is currently being re-introduced in Bulgaria, where the falcons are bred in captivity and released through the hacking method. We relied on the birds’ pedigree when forming the breeding pairs from 2011. In 2021-2022, we had the opportunity to evaluate our captive population via DNA tests. We performed the first genetic assessment of the sakers in the WRBC through a genome evaluation of the most important founders (n = 12) and, in 2022, we executed a microsatellite analysis on 30 saker falcons from the programme. We compared the results with the known pedigree and history of the saker falcons. The genetic tests helped to assign relatedness to some birds with missing or incomplete pedigrees, indicating the test can complement that information and lead to better management of the captive group. One pair was separated as a precaution as it was indicated by one the tests that the two birds are more closely related than expected. The research could be beneficial to other raptor captive breeding programmes dealing with a similar group composition.