VULNERABILITY OF SCOTTISH SEABIRDS TO OFFSHORE WIND

3.7 Species index values for collision concern and displacement concern

Garthe and H◘ppop (2004) computed a risk index that summed the first four factor scores and divided the sum by four, and multiplied that by the sum of factor scores five and six, divided by two. This recognised that the first four factors all relate to flight ability and flight behaviour, while factors five and six relate to habitat use and susceptibility to disturbance. Thus their index combines both collision risk and disturbance/habitat loss considerations into a single score, which is potentially confusing.

An alternative approach is to score separately for collision concern and for disturbance/habitat displacement concern. We take that new approach here.

For collision risk, it seems appropriate to give a high weighting to the flight altitude (percent flying at blade height), and low weightings to manoeuverability, percent of time flying, and nocturnal flight activity (again differing from the approach in Garthe and H◘ppop 2004). So our index multiplies the percentage flying at blade height by the mean of the other three factors, and multiplies the resulting value by the conservation importance score ( Table 11). Flight altitude score ranges from 0 to 35, the mean of the other three factors ranges from 1.3 to 3.7 (within a theoretically possible range of 1 to 5), and the conservation importance score from 7 to 18 (within a theoretically possible range of 4 to 20). It would be possible to rescale these scores in some way to adjust weighting of the three components in the overall index. We considered this possibility carefully, and while there is an attraction to rescaling to give equal weight to the behavioural component and the conservation importance component, we felt that the higher weighting to flight altitude was entirely appropriate in view of the crucial importance of this in determining potential collision risk. This approach is consistent with feedback from reviewers who overwhelmingly felt that it was appropriate to upweight flight height influence in this index. Exploratory rescaling to give equal weight to flight height and conservation importance had only a small influence on the ranking of species, so the index is relatively robust to the detailed weighting given to these two components.

For disturbance/habitat displacement our index multiplies the disturbance score by the habitat loss score, and multiplies the resulting value by the conservation importance score. The disturbance score multiplied by the habitat loss score varies from 1 to 20. Thus the risk factor varies over a similar range to the variation in conservation importance score, giving these two components similar weighting in the overall index. The total has then been divided by 10, to recognise that the disturbance/displacement impact on populations is likely to be considerably less than a direct

mortality impact such as from collisions and therefore the two scales should not be compared in a quantitative way but only in terms of the species ranking within one scale ( Table 12).

Table 11. Species concern in the context of collision impacts: percent flying at blade height x 1/3(manoeuverability score + % time flying score + nocturnal flight score) x conservation importance score (ranked by index value).

Table 12. Species concern in the context of disturbance and/or displacement from habitat (Disturbance score x Habitat flexibility score x Conservation Importance score)/10 (Ranked by index value)