Study species and area
WTE is a large bird of prey with females being larger than males. The species is Eurasian in distribution and ecologically the Old World equivalent of the New World’s Bald Eagle (Haliaeetus leucocephalus ). Our study was involved with the second phase (east Scotland) of a reintroduction initiative instigated by an initial (1975 – 1985: 82 birds) reintroduction and later reinforcement translocations (1993 – 1995: 59 birds) using birds from western Norway to the west coast of Scotland (Green et al., 1996; Evans et al., 2009; Whitfield et al., 2009a, b; Sansom et al., 2016). The core initiative allowing our study aimed to provide a further nucleus of WTE population growth on the east coast of Scotland, situated away from the previous centers of population re-establishment and subsequent expansions on the west coast.
We sampled nestlings extracted from a wild donor WTE population in Norway: every June between 2007 and 2012 WTE nestlings were collected from monitored nests in the western counties of Møre og Romsdal, Hordaland and Sogn og Fjordane. Single birds were extracted from nests that had two or three chicks which from field observations were deemed old enough (five to eight weeks old) to be suitable for translocation on plumage features and physical appearance (Helander, 1981). Extractions were coordinated with the goal of a balanced sex ratio (based on biometrics). Birds were uniquely metal banded (BTO: British Trust for Ornithology) in Norway prior to translocation, for subsequent identification.
There were extractions of 15 (2007, 2008, 2009), 19 (2010), 16 (2011) and six (2012) nestlings per annum, resulting in 86 translocated birds (85 were released as one 2009 bird died in captivity from aspergillosis, despite veterinary intervention). Of these, 55 were double sampled for biochemical and hematological parameters 2009 – 2012 and are the subject of this study.
After transport from Norway, all birds had a full clinical examination within 24 h of arrival to identify any potential abnormalities. Translocated birds were kept at a hacking facility in northeast Fife, southeast Scotland, in wooden (three sides enclosed) and wire mesh aviaries measuring approximately 3.6 x 3 x 2.7 m that had ‘nesting’ platforms covered in bark chips and soft vegetation, and at least two long perches. Two or three birds were housed in each aviary, with similarly sized birds being kept together. Birds were supplied withad libitum food involving the wide range exploited by WTE (e.g. Whitfield et al., 2013): fish, birds and mammals, with vitamin and mineral supplements (Nutrobal®, Vertak, UK) added to the food daily. Contact with humans was minimized by food being provided via a hatch in the aviaries’ back panel. Captive eagles were kept under daily observation via ‘spy holes’ in aviaries’ back panels for surveillance of their status and any signs of injury or ill-health, with a qualified veterinarian on call should the need arise: no eagles were injured because of our activities.
The first sampling was undertaken shortly after all birds were in the hacking facility. This allowed for greater efficiency in sampling all birds at the same time after extraction and transportation from Norway to Scotland (Table 1). The second sampling was taken shortly before birds’ release from captivity when they were also fitted with patagial wing tags or color-bands and a VHF telemetry tag, when biometric data (including body mass) were again taken (Table 1). Birds were aged (days since hatch date) using the method of Løseth et al. (2019): after T. Nygård pers. comm.) involving measurement of a central tail feather. This method was preferred to those in Helander (1981) and Helander et al. (2007) involving other recorded biometrics when like Løseth et al. (2019) we also found the “Nygård” method to produce more realistic and consistent results.