This report presents an analysis of recent changes in waterbird habitat utilisation around the Horns Rev 1 wind farm, with particular emphasis on Common Scoter.
Ornithological investigations of waterbird numbers and distribution in the study area around the Horns Rev 1 wind farm were initiated in 1999. As part of a demonstration programme on the environmental feasibility of offshore wind farms a total of 34 surveys of bird distributions were conducted in the period from 1999 until 2005. In late 2005 and early 2006 additional six surveys were conducted in relation to the Horns Rev 2 EIA process.
Results from the demonstration programme concluded that the distribution of divers and Common Scoter were adversely affected by the presence of the wind turbines at Horns Rev.
In late 2006 and early 2007 Vattenfall A/S maintenance crews and helicopter pilots reported increasing numbers of Common Scoters present within the wind farm site. On that background a series of four surveys of waterbird distribution in the area was programmed during January to April 2007.
Data from surveys in January, February, March and April 2007 showed that Common Scoter was the most numerous bird species in the study area, with a total of 356,635 observed birds. Herring Gulls (7,661), Eider (5,674) and diver sp. (511) were other numerous species in the area.
Common Scoters dramatically changed their distribution in the study area during the period from 1999 to 2007 for reasons other than the presence of the turbines. Therefore a comparison of distribution of this species pre- and post-construction of the wind farm, using a traditional BACI concept, was impossible. The analyses presented here thus build on data from the January to April 2004 to 2007.
During three out of four surveys in 2007 more Common Scoters than during any previous surveys were recorded within the foot print of the wind farm. On 25 January 2,112 birds, on 15 February 4,624 birds, on 3 March 1,359 and on 1 April 35 Common Scoters were encountered in the wind farm area.
Analyses of Common Scoter encounter rates in six 2x2 km grid cells within the wind farm area compared to encounter rates in 14 grid cells in the periphery of the wind farm site showed no significant difference for the three early surveys, while significantly lower encounter rates within the wind farm during a survey on 1 April. Based on the summed data set from 2007 there was no significant difference between encounter rates in the wind farm site and the periphery.
Analyses of Common Scoter cumulative distance frequency distributions in 500 m intervals from the wind farm centre point out to a radius of 6 km for each of the years between 2004 and 2007 showed that gradually higher percentages of the birds present within this radius were recorded within the wind farm site. The same pattern was found when analysing the proportion of birds within 3 km of the wind farm centre point to the total number of birds present within 6 km of the centre point, most dramatically amongst the proportion of individuals occurring within the area, which progressively increased from 10% in 2004 to 50% in the results from the survey in 2007.
We therefore conclude that Common Scoter may indeed occur in high densities between newly constructed wind turbines at sea, but this may only occur a number of years after initial construction. We still cannot exclude the explanation that this reflects changes in food supply rather than a change in the behaviour of the birds themselves.
As Common Scoters were virtually absent from Horns Rev prior to the construction of the wind farm it is difficult to judge how many birds the wind farm site would support by 2007, had the wind farm never been constructed. The use of spatial modelling tools may help elucidate whether the present found numbers of birds represent 100% of what could be expected in the absence of the wind turbines, given the nature of the habitat. Such an exercise was beyond the scope of this report.
Spatial modelled density surfaces of Common Scoter, including estimated total numbers within the study area, will be presented in a separate report for each of the four surveys conducted in 2007.
There was no sign that divers, previously concluded to avoid the area of the wind farm and its surroundings, had changed their distribution relative to the wind farm.