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The Occurrence of Reindeer Calves in the Diet of Nesting Golden Eagles in Finnmark, Northern Norway

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The Occurrence of Reindeer Calves in the Diet of Nesting Golden Eagles in Finnmark, Northern Norway Ornis Fennica 84:112–118. 2007 The occurrence of reindeer calves in the diet of nesting Golden Eagles in Finnmark, northern Norway Trond V. Johnsen, Geir H. Systad, Karl O. Jacobsen, Torgeir Nygård & Jan O. Bustnes* Johnsen, T.V., Norwegian Institute for Nature Research, The Polar Environmental Centre, N-9296 Tromsø, Norway Systad, G.H., Norwegian Institute for Nature Research, The Polar Environmental Centre, N-9296 Tromsø, Norway Jacobsen, K.O., Norwegian Institute for Nature Research, The Polar Environmental Centre, N-9296 Tromsø, Norway Nygård, T., Norwegian Institute for Nature Research, Tungasletta 2, 7485 Trondheim, Norway Bustnes, J.O., Norwegian Institute for Nature Research, The Polar Environmental Centre, N-9296 Tromsø, Norway. [email protected] (*Corresponding author) Received 12 January, revised 12 July 2007, accepted 27 July 2007 To assess the importance of semi-domesticated reindeer Rangifer tarandus calves in the diet of Golden Eagles Aquila chrysaetos, in Finnmark (northern Norway), we collected prey remains at 37 nests over six years (2001–2006). The study area was divided into 1) a fjord area, which is an important calving area for reindeer, and 2) an inland area where few reindeer give birth. 469 prey items were collected over the years. The diet of eagles was numerically dominated by birds (73% of collected prey items), especially willow/rock ptarmigan Lagopus spp. (51%), while mammals made up 27%, with mountain hare Lepus timidus as the most common species. Remains of reindeer calves were found in half of the nests studied and made up 8.5% of the collected prey items: 13.2% in the fjord area and 6.5% in the inland area. There was a higher chance of finding reindeer calves at nests in the fjord area than inland, and in nests situated in birch forest than in pine forest. The num- ber of reindeer calves in the Golden Eagle diet in Finnmark corroborates well other stud- ies from northern Fennoscandia. The importance of the Golden Eagle as a predator on reindeer can’t, however, be assessed here. 1. Introduction calves (reviewed by Watson 1997). Several stud- ies have estimated the extent to which Golden Ea- The Golden Eagle Aquila chrysaetos is one of the gles consume various ungulate calves, both wild largest avian predators in mountainous areas in and domesticated (Bleich et al. 2004, Deblinger & both Eurasia and North America. It has a diverse Alldredge 1996, Phillips et al. 1996). However, diet, but major prey groups include gallinaceous few studies have examined the extent to which birds, hares Lepus timidus and often ungulate Golden Eagles prey on semi-domesticated rein- Johnsen et al.: Occurrence of reindeer calves in Golden Eagle diet 113 deer Rangifer tarandus, which are kept by the stretches from sea level to inland Finnmark where Sámi people of northern Scandinavia and Finland. altitude varies between 50 and 600 meters. The Norberg et al. (2005, 2006) found that the Golden study area was divided into two sub areas: one near Eagle was the dominating predator of reindeer the coast in the area surrounding Porsangerfjord calves in northern Finland; i.e. in one study area (denoted as fjord area), and one inland area with the eagles accounted for 40% of the mortality woodlands surrounding the village Karasjok (de- (Norberg et al. 2005). Moreover, Tjernberg (1983) noted as inland area) (Fig. 1). The two areas are found that reindeer calves made up about 9% of about 30 km apart. In the two areas there are about the diet of nesting eagles in northern Sweden, and 50 known Golden Eagles territories (including estimated that about 0.7% of the calves born were data from 2006). Since all nests were found in taken by Golden Eagles. Franzén (1996) reported woodland, the study areas were divided into two a very low predation rate based on a vast field main habitats based on forest types in which the study, but summarized some proven attacks on nests were situated. In this case it was 1) pine Pinus reindeer by Golden Eagles. From central Norway sylvestris forest, and 2) birch Betula pubescens fo- there is some information about eagle predation in rest. reindeer herding areas. Using radio collars, The study was started in 2001 and results are Nybakk et al. (1999, 2002) found that Golden Ea- presented for the period 2001–2006. Twenty-six gles accounted for about 6% of the calf mortality. nest sites were used in this study (Fig. 1). Seven The main area for reindeer husbandry in Norway nests were visited in more than one year (i.e. two or is, however, Finnmark, the northernmost county three years). Hence there were 37 successful nest- (Fig. 1). This area has a reindeer population of ing attempts (i.e. fledged chicks) included in the about 150,000, although the number varies be- study, with an annual variation of 5–11 nests. tween years. In recent years there has been contro- Nests were visited in summer, mainly in early versy over the Golden Eagle as a predator on rein- July, and the area surrounding the nest was deer in Norway, and the aim of this study was to searched for prey remains and regurgitated pellets. examine the occurrence of reindeer calves in the All prey remains considered to be fresh, i.e. from diet of nesting Golden Eagles in an important area the sampling year, were collected and frozen for for semi-domesticated reindeer in central Finn- later determination. To assess the freshness of the mark. The eagles nest from April until July and remains we used a classification similar to Sulkava reindeer calve in May (Tveraa et al. 2003). Thus, et al. (1998), in which old remains were those in the prey remains at the nest late in the breeding which soft tissue was absent, bones were greenish season will indicate the importance of reindeer and feathers were softened. To avoid double- calves for breeding birds (Sulkava et al. 1998, counting, we made sure that the remains were not Tjernberg 1983). It does not, however, account for from the same prey individuals; for example to predation by sub-adult and other non-breeding ea- count as two individual items, we had to find two gles. left or right wings for birds, and similarly for rein- We collected prey items at nests of Golden Ea- deer calves or other mammals (feet, heads, etc.). gles over six years (2001–2006) in two areas in Our aim was to collect data from a large num- mid Finnmark (Fig. 1): 1) an area surrounding ber of nests over several years. It was, however, Porsangerfjord (fjord area), which is an important only possible to collect prey at successful nests, calving area for reindeer, and 2) a woodland, in- since very few prey remains could be found at land area, near the Finnish border, where few rein- nests that had been abandoned at an early stage. deer calves are born. Nesting success varied greatly between years (G.H. Systad et al. in prep), and pairs very often changed their nest site between years. It was thus 2. Material and methods impossible to collect information from many nests over several years. The study area was in the Alta, Porsanger and Statistical analyses were carried out using the Karasjok municipalities in Finnmark, a major area statistical package R (R core team, 2006). To test for reindeer herding in Norway (Fig. 1). The area for factors influencing the probability of having 114 ORNIS FENNICA Vol. 84, 2007 Fig. 1. The study areas in Finn- mark, northern Norway. Black cir- cles denote suc- cessful Golden Eagle nests dur- ing the study peri- od, 2001–2006. reindeer calves in the diet, we used logistic regres- cies of prey were identified, in addition to seven sion (Wald statistics) with a bivariate response groups for which species were not determined, variable (0: nests where no calves were found; 1: such as willow/rock ptarmigan (Lagopus spp.), nests where calves were found). Independent vari- unidentified ducks, wading birds and passerines, ables were area and habitat type (forest type as well as rodents (Table 1). Birds made up 73% by around the nest). number of the collected prey items. Ptarmigan In all analyses, we used the number of prey dominated the sample comprising 51% by number found at each nest as a weight variable. Unfortu- of the prey, followed by mountain hare (13%, Ta- nately, the number of nests per year was small and ble 1). Other important prey were waterfowl, such no interaction terms testing for differences be- as geese and ducks, and comprised about 10% by tween forest types in the different areas could be number of the prey remains. There were some dif- included in the models. Another potential problem ferences between the two areas, notably that the with our data set is that some nests (n = 7) occur in proportion of mammals was higher in the fjord the data in more than one year and may lead to area (38%) compared to the inland area (23%), and pseudo-replication. This may be a source of error the importance of birds was higher in the inland since some pairs may have special prey prefer- area (77% vs. 62%). ences, which again will make the estimates in our In total, 18 (48.6%) of the nests contained re- analyses more uncertain. We were, however, un- mains of reindeer calves, but calves only made up able to correct for this potential error source due to 8.5% by number of all prey items (Table 1). In the the small samples. fjord area 13.2% of the prey items were from rein- deer calves, while in the inland area it was 6.5% (Table 1).
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