Use of night-roosts

Night roosts have been described as potential hazard areas for eagles when they are situated close to power lines (Mojica et al. 2009, Rollan et al. 2010). We examined the use of night-roosts of juvenile WTEs on Smøla, and investigated whether their positions and proximity to wind tur-bines could involve potential risks. We used GPS-positions of satellite-tagged eagles to examine this question. Only positions retrieved after December 1 the first calendar year, were used, as first year birds are attached to the nest area during their first autumn as long as they are fed by their parents. A total of 33 birds gave such data. A total of 52 major night-roosts were identified by means of subjective inspection of the map (Figure 30), choosing the sites which apparently had the highest density of night positions. A night position was defined as a position obtained between 19:00 and 05:00 hrs.

Figure 30. Map of Smøla showing night positions (19:00 – 05:00) of juvenile satellite-tagged white-tailed eagles as blue dots. The wind turbines are indicated as green stars, farmed areas as yellow.

Of the 10,923 positions recorded during night-time within the Smøla municipality (blue-shaded area, Figure 30), 6,237 (57%) were located within the 52 areas judged as important night-roosts.

These had an approximate area of 15 km², out of Smøla’s total land area of 274 km²,i.e. ca. 5%.

Figure 31. The traffic of juvenile white-tailed eagles showed as lines between the nigh roosts and the daytime areas. Green lines represent movements during the morning (04:00-08:00), while purple lines represent evening movements (16:00-20:00). The main night roosts are shown as purple-shaded areas.

Although drawn as straight lines (Figure 31), the traffic in and out of the night roosts is not nec-essarily linear, the lines on the map are just connections between consecutive points in time by single birds. Lines through the SWPP area do not necessarily mean that birds have flown through it, as there are possibilities to fly around. Nevertheless, some clear patterns emerge. Some night roosts are more used than others, and some flight corridors seem evident. One night roost (the Maurdalen forest in the central island, with traffic in all directions, Figure 32), and the areas west of the main agricultural areas of Frostaheia, showing clear movement patterns to and from the shallow archipelago in the far north (Veiholmen).

In addition to the major night roosts in the central parts of the island, there is also an obvious concentration in the archipelago in the south and southwest, and on the islands to the northwest of the SWPP, in the Dyrnes area. Two forest patches close to turbines numbers 21 and 42, re-spectively, north-west and north of the SWPP area are also major roosts. There is a noticeable lack of night positions in the west and the east, as well in the Veiholmen area. These areas seem to function as almost pure day areas, probably as feeding-grounds. Many of the roosts are in for-est patches, but not all.

Figure 32. Night positions at Maurdalen, central Smøla, and lines connecting consecutive posi-tions between day and night. This is probably the most important night-roost of juvenile white-tailed eagles on Smøla, containing 512 GPS-positions. Green shaded areas are forest, dotted blue areas are bogs.

Figure 33. The main night roosts of juvenile white-tailed eagles on Smøla. The size of the red squares indicate the size of the roost shown by the number of night positions recorded (from 20 to 512), and the size of the blue circles indicates the number of bird having visited the roost at least once (from two to 15).

Some roosts were used by many birds (up to 15), and contained many positions, while some contained many positions from a smaller number of birds, and there were all kinds of variations.

The importance of the roost in the south-western archipelago, in the middle of the island, and in the northwest, are clearly shown in Figure 33. Near the SWPP, the roosts north of the power plant are noticeable; especially the one close to turbine 21 and 22 in the north-western corner (having been known for many years among ornithologists), and the one centrally in the north (close to turbine 42 and 1). Together, these have had nine casualties of eagles, and one may speculate whether these were connected to traffic in and out of the night-roosts. It is well known that utility structures pose hazards to large raptors when they are in important flight corridors (Ariza 1998, Mojica et al. 2009), and the same has been documented for wind turbines in areas of high concentrations of soaring or feeding raptors (Barrios & Rodriguez 2007, Farfan et al.

2009, Smallwood & Thelander 2008).

The WTEs will necessarily use low altitudes during their flights in and out of the night roosts. If the night roosts are close to the SWPP area, this might pose a hazard. These patterns are im-portant to take into account when planning wind-power plants, and pre-construction knowledge of such roost should be obtained during the planning process. If the plans for a stage three of the SWPP is to be realised, such data are now available. It might be more than a coincidence that nine out of 39 (23%) of the WTEs killings caused by turbines on Smøla have occurred at four turbines close to night roost.