Falco tinnunculus

2 KESTREL ( FALCO TINNUNCULUS View in CoL )

Rather than take a mean of the flight times/ha/hr, the overall figure is obtained by dividing the total flying time by the total hahr. In this case with identical VP areas this approach will make no difference. Therefore the overall kestrel flight activity was 2.46 x 10 -7 hrs/ha/hr, amounting to 9.58 x 10 - 4 hr/hr over the whole site, taking account of the overlap of the VPs

Kestrels were present on the site throughout the observation period, amounting to 153 days and they were presumed to be able to fly for an average of 14.1 hours daylight per day, a total of 2157.3 hours

Kestrel occupancy ( n) of the wind farm area is, therefore, estimated to be 2.07 hours per year for the observation period of five months (9.58 x 10 -4 x 2157.3).

2.1.2 Number of Transits of Kestrel Through the Rotors

The size of the flight risk volume (Vw) is 4,682,040,000m 3. This is calculated by multiplying the area of the wind farm by the height over which birds were observed (120m).

The combined volume swept out by the turbine rotors (Vr) is 1,465,961.06m 3. This is calculated by multiplying the number of wind turbines (36) by̟r 2 by (d + l), where r is the rotor radius (54.65m), d is the depth of the rotor blade from front to back (4m), and l is the body length of a kestrel (0.34m).

The model assumes that use of the airspace containing the rotors is random.

The bird occupancy of the volume swept by the rotors in seconds ( b) is:

( n x 3,600) x (Vr/Vw)

= ( 2.07 x 3600) x (1,465,961.06/4,682,040,000)

= 2.33 bird-secs.

The time taken for a bird to make transit through the rotor and completely clear the rotors ( t) is (d + l)/v, where d is the depth of the rotor blade from front to back (4), l is the body length for kestrel (0.34m) and v is the speed of the bird through the rotor (10.1 ms-1) (32), = 0.43secs.

The number of bird transits through the rotors during the five month observational period is b / t = 5.42

2.1. 3 Estimating Collision Likelihood

The probability of collision depends on the size of the bird (length and wingspan), the breadth and pitch of the turbine blades, the rotation speed of the turbine, and the flight speed of the bird. To facilitate calculation, many simplifications have to be made. The bird is assumed to be of simple cruciform shape, with the wings at the halfway point between nose and tail. The turbine blade is assumed to have a width and a pitch angle (relative to the plane of the turbine), but to have no thickness.

The probability of bird collision for given bird and blade dimensions and speeds is the probability, were the bird placed anywhere at random on the line of flight, of it overlapping with a blade swathe. The calculation derives a probability of collision for a bird at a radius r from the turbine hub, and at a position along a radial line which is at an angle x from the vertical. This probability is then integrated over the entire rotor disc, assuming that the bird transit may be anywhere at random within the area of the rotor disc.

For ease of use the above calculations are laid out on an Excel spreadsheet provided by SNH. As the turbine speed varies with wind speed, an average rotation period of 3.73 seconds has been used. Pitch will also vary with wind speed, but a worst case scenario of 90 o has been used. A kestrel is assumed to travel at an average speed of 10.1 ms-1 and exhibit flapping flight (which was typical of the birds observed during the surveys). The model predicts that an average of 20.6% of kestrel flights through the rotor swept area would result in collisions. The turbines are, however, likely to be static for 20% of the time as the wind speeds are either too low (ie <4 ms-1) or too high (>25 ms-1). Collision likelihood has, therefore, been multiplied by 0.8 giving a predicted collision rate of 16.48%.

The estimated number of collisions is then calculated by multiplying the number of birds flying through the operating rotors by the probability that a bird is hit whilst flying through the rotors. The number of birds predicted to collide with the operating rotors over the observation period is 0.89 birds per year (5.42 x 16.48%). This assumes no avoiding action is taken by the birds.

In practice, birds are expected to display a high level of awareness of operational turbines. No reliable quantitative data are available to enable avoidance of turbines to be calculated, however studies in the USA have reported rates ranging between 90% and 99% for varying species. Avoidance rates are thought to lie in the upper end of the range (>98%) for many raptor species. Mortalities for kestrel at have been calculated using avoidance rates of 90%, 95%, 98% and 99% to provide an indication of potential risk (see Table 1.4 below).

This equates to a loss of a bird every 22.38 years from March to July at 95% avoidance or a bird every 111.91 years at 99% avoidance.