Birds in southern Öresund in relation to the wind farm at ... - Vattenfall

The number of aerial surveys undertaken in different months and years are to be seen in
Table 1. Gaps in the coverage was due to difficult flying conditions, the aerial surveys also
being dependent on the availability of suitable aircraft in the region.
Analysis of survey data
For the five most common species (Cormorant, Long-tailed Duck, Eider, Red-breasted
Merganser and Herring Gull) comparisons of the utilization of the Lillgrund area before and
after the construction of the wind farm was analyzed using two methods: with Jacob’s index
and by comparing densities. For these analyses we used three different zones: the wind farm
area, a buffer zone of 2 km around the outer limits of the wind farm and the outer area, i.e. the
northern area in general excluding the windfarm and buffer zone. In the aerial surveys we
considered lines 1 – 9 to constitute the northern area. In the density calculations we restricted
the analysis to the parts of the survey lines that covered water deeper than three meter (see
Fig. 6). The same area divisions were used below in the calculations of Jacob’s index. The
total areas of the three different zones were: windfarm area 5 km 2 , buffer zone 29 km 2 and for
the outer area 224 km 2 , of which 20%, 15% and 15 % were actually covered by the aerial
surveys.
The Jacob’s indices show the relative use of the different zones in relation to expected use.
Densities are just a measure of the number of birds per area unit. Hence the two measures in
part describe different things and results from the two may differ. Jacob’s indices do not give
any information at all about the absolute numbers or densities of birds using a specific zone,
but instead show the proportion of birds using the zone in relation to a) the total number of
birds using the whole study area (all zones) and b) the proportion of the zone out of the
whole study area. This means that it may very well be so that densities in a specific zone can
increase at the same time as the Jacob’s index for that zone can decrease, since the latter
depends on the total number of birds registered in the whole study area.
The density calculations were based on the aerial surveys as the boat surveys cover too small
parts of the areas outside the wind farm area and the buffer zone. The total area within the
survey belt of the aerial surveys were 1.0 km 2 within the wind farm area, 4.2 km 2 for the
buffer zone and 33.9 km 2 for the areas outside these zones. The mean counts within the
survey belts within each zone were calculated for each year and season separately and the
densities were calculated from these means and the surveyed area within each zone (see
above). The means are found in Table A10 in the appendix.
If birds avoid the wind farm area or not was analyzed by calculating Jacob’s selectivity index
D (hereafter Jacob’s index, Jacobs 1974). This index was calculated for the five most
commonly recorded species during boat and air surveys, and was used for describing if the
birds avoided or were attracted to the wind farm area. Indices were calculated both for the
period before the construction of the wind farm and for the period with the farm in operation.
Separate indices were also calculated for each survey year (centered around mid-winter) in
order to look at annual variation in avoidance or preference. Avoidance or preference for the
area outside but close to the wind farm, up to 2 km away from the turbines, was also analyzed
(see Fig. 6 for delimitation of the areas used). The birds recorded in the remaining parts of the
study area covered by boat transects or the northern part of the aerial survey area were used in
the calculations of the total number of birds in the whole study area and for calculating
proportions of birds using the areas closest to the wind farm, but no indices were calculated
for these parts.
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